– 130 Powerful Instructions – Most Single-clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16 MHz
– On-chip 2-cycle Multiplier
• Nonvolatile Program and Data Memories
– 16K Bytes of In-System Self-Programmable Flash
Endurance: 1,000 Write/Erase Cycles
– Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
– Boundary-scan Capabilities According to the JTAG Standard
– Extensive On-chip Debug Support
– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
• Peripheral Features
– Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes
– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture
Mode
– Real Time Counter with Separate Oscillator
– Four PWM Channels
– 8-channel, 10-bit ADC
8 Single-ended Channels
7 Differential Channels in TQFP Package Only
2 Differential Channels with Programmable Gain at 1x, 10x, or 200x in TQFP
Package Only
– Byte-oriented 2-wire Serial Interface
– Programmable Serial USART
– Master/Slave SPI Serial Interface
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
• Special Microcontroller Features
– Power-on Reset and Programmable Brown-out Detection
– Internal Calibrated RC Oscillator
– External and Internal Interrupt Sources
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby
OverviewThe ATmega16 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced
RISC architecture. By executing powerful instructions in a single clock cycle, the
ATmega16 achieves throughputs approaching 1 MIPS per MHz allowing the system
designer to optimize power consumption versus processing speed.
Block DiagramFigure 2. Block Diagram
VCC
PA0 - PA7PC0 - PC7
GND
AVCC
AREF
PORTA DRIVERS/BUFFERS
PORTA DIGITAL INTERFACE
MUX &
ADC
PROGRAM
COUNTER
PROGRAM
FLASH
INSTRUCTION
REGISTER
INSTRUCTION
DECODER
CONTROL
LINES
AVR CPU
ADC
INTERFACE
STACK
POINTER
SRAM
GENERAL
PURPOSE
REGISTERS
X
Y
Z
ALU
STATUS
REGISTER
PORTC DRIVERS/BUFFERS
PORTC DIGITAL INTERFACE
TWI
TIMERS/
COUNTERS
INTERNAL
OSCILLATOR
WATCHDOG
TIMER
MCU CTRL.
& TIMING
INTERRUPT
UNIT
EEPROM
OSCILLATOR
OSCILLATOR
INTERNAL
CALIBRATED
OSCILLATOR
XTAL1
XTAL2
RESET
2466B–09/01
PROGRAMMING
LOGIC
+
-
PORTB DIGITAL INTERFACE
PORTB DRIVERS/BUFFERS
SPI
COMP.
INTERFACE
USART
PORTD DIGITAL INTERFACE
PORTD DRIVERS/BUFFERS
PD0 - PD7PB0 - PB7
3
The AVR core combines a rich instruction set with 32 general purpose working registers.
All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing
two independent registers to be accessed in one single instruction executed in one clock
cycle. The resulting architecture is more code efficient while achieving throughputs up to
ten times faster than conventional CISC microcontrollers.
The ATmega16 provides the following features: 16K bytes of In-System Programmable
Flash with Read-While-Write capabilities, 512 bytes EEPROM, 1K byte SRAM, 32 general-purpose I/O lines, 32 general purpose working registers, a JTAG interface for
Boundary-scan, On-chip Debugging support and programming, three flexible
timer/counters with compare modes, internal and external interrupts, a serial programmable USART, a byte oriented 2-wire Serial Interface, an 8-channel, 10-bit ADC with
optional differential input stage with programmable gain (TQFP package only), a programmable Watchdog Timer with internal oscillator, an SPI serial port, and six software
selectable power saving modes. The Idle mode stops the CPU while allowing the
SRAM, timer/counters, SPI port, and interrupt system to continue functioning. The
Power-down mode saves the register contents but freezes the oscillator, disabling all
other chip functions until the next interrupt or hardware reset. In Power-save mode, the
asynchronous timer continues to run, allowing the user to maintain a timer base while
the rest of the device is sleeping. The ADC Noise Reduction Mode stops the CPU and
all I/O modules except asynchronous timer and ADC, to minimize switching noise during
ADC conversions. In Standby mode, the crystal/resonator oscillator is running while the
rest of the device is sleeping. This allows very fast start-up combined with low-power
consumption. In Extended Standby mode, both the main oscillator and the asynchronous timer continue to run.
The device is manufactured using Atmel’s high density nonvolatile memory technology.
The On-chip ISP Flash allows the program memory to be reprogrammed in-system
through an SPI serial interface, by a conventional nonvolatile memory programmer, or
by an On-chip Boot program running on the AVR core. The boot program can use any
interface to download the application program in the Application Flash memory. Software in the Boot Flash section will continue to run while the Application Flash section is
updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU
with In-System Self-Programmable Flash on a monolithic chip, the Atmel ATmega16 is
a powerful microcontroller that provides a highly-flexible and cost-effective solution to
many embedded control applications.
The ATmega16 AVR is supported with a full suite of program and system development
tools including: C compilers, macro assemblers, program debugger/simulators, in-circuit
emulators, and evaluation kits.
Pin Descriptions
VCCDigital supply voltage.
GNDGround.
Port A (PA7..PA0)Port A serves as the analog inputs to the A/D Converter.
Port A also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used.
Port pins can provide internal pull-up resistors (selected for each bit). The Port A output
buffers have symmetrical drive characteristics with both high sink and source capability.
When pins PA0 to PA7 are used as inputs and are externally pulled low, they will source
current if the internal pull-up resistors are activated. The Port A pins are tristated when a
reset condition becomes active, even if the clock is not running.
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Port B (PB7..PB0)Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port B output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port B pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port B pins are tristated when a reset
condition becomes active, even if the clock is not running.
Port B also serves the functions of various special features of the ATmega16 as listed
on page 55.
Port C (PC7..PC0)Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port C output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port C pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port C pins are tristated when a reset
condition becomes active, even if the clock is not running. If the JTAG interface is
enabled, the pull-up resistors on pins PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.
Port C also serves the functions of the JTAG interface and other special features of the
ATmega16 as listed on page 58.
Port D (PD7..PD0)Port D is an 8-bit bidirectional I/O port with internal pull-up resistors (selected for each
bit). The Port D output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port D pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port D pins are tristated when a reset
condition becomes active, even if the clock is not running.
Port D also serves the functions of various special features of the ATmega16 as listed
on page 60.
RESET
XTAL1Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL2Output from the inverting oscillator amplifier.
AVCCThis is the supply voltage pin for Port A and the A/D Converter. It should be externally
AREFThis is the analog reference pin for the A/D Converter.
About Code
Examples
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. The minimum pulse length is given in Table
15 on page 35. Shorter pulses are not guaranteed to generate a reset.
connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.
This documentation contains simple code examples that briefly show how to use various
parts of the device. These code examples assume that the part specific header file is
included before compilation. Be aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent. Please
confirm with the C compiler documentation for more details.
2466B–09/01
5
AVR CPU Core
IntroductionThis section discusses the AVR core architecture in general. The main function of the
CPU core is to ensure correct program execution. The CPU must therefore be able to
access memories, perform calculations, control peripherals, and handle interrupts.
Architectural OverviewFigure 3. Block Diagram of the AVR MCU Architecture
Data Bus 8-bit
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Control Lines
Program
Counter
Direct Addressing
Indirect Addressing
Status
and Control
32 x 8
General
Purpose
Registrers
ALU
Data
SRAM
EEPROM
I/O Lines
Interrupt
Unit
SPI
Unit
Watchdog
Timer
Analog
Comparator
I/O Module1
I/O Module 2
I/O Module n
In order to maximize performance and parallelism, the AVR uses a Harvard architecture
– with separate memories and buses for program and data. Instructions in the program
memory are executed with a single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the program memory. This concept
enables instructions to be executed in every clock cycle. The program memory is in-system reprogrammable Flash memory.
The fast-access Register file contains 32 x 8-bit general purpose working registers with
a single clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU)
operation. In a typical ALU operation, two operands are output from the Register file, the
operation is executed, and the result is stored back in the Register file – in one clock
cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for
Data Space addressing – enabling efficient address calculations. One of the these
address pointers can also be used as an address pointer for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-register,
described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single register operations can also be executed in the ALU. After
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an arithmetic operation, the Status Register is updated to reflect information about the
result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions,
able to directly address the whole address space. Most AVR instructions have a single
16-bit word format. Every program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot program section and
the Application program section. Both sections have dedicated Lock Bits for write and
read/write protection. The SPM instruction that writes into the Application Flash memory
section must reside in the Boot program section.
During interrupts and subroutine calls, the return address program counter (PC) is
stored on the stack. The stack is effectively allocated in the general data SRAM, and
consequently the stack size is only limited by the total SRAM size and the usage of the
SRAM. All user programs must initialize the SP in the reset routine (before subroutines
or interrupts are executed). The stack pointer SP is read/write accessible in the I/O
space. The data SRAM can easily be accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional
global interrupt enable bit in the status register. All interrupts have a separate interrupt
vector in the interrupt vector table. The interrupts have priority in accordance with their
interrupt vector position. The lower the interrupt vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control
Registers, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as
the Data Space locations following those of the Register file, $20 - $5F.
ALU – Arithmetic Logic
Unit
The high-performance AVR ALU operates in direct connection with all the 32 generalpurpose working registers. Within a single clock cycle, arithmetic operations between
general-purpose registers or between a register and an immediate are executed. The
ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful multiplier
supporting both signed/unsigned multiplication and fractional format. See the “Instruc-
tion Set” section for a detailed description.
Status RegisterThe Status Register contains information about the result of the most recently executed
arithmetic instruction. This information can be used for altering program flow in order to
perform conditional operations. Note that the status register is updated after all ALU
operations, as specified in the Instruction Set Reference. This will in many cases
remove the need for using the dedicated compare instructions, resulting in faster and
more compact code.
The status register is not automatically stored when entering an interrupt routine and
restored when returning from an interrupt. This must be handled by software.
The AVR status register – SREG – is defined as:
Bit76543210
ITHSVNZCSREG
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
2466B–09/01
7
• Bit 7 - I: Global Interrupt Enable
The global interrupt enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the global
interrupt enable register is cleared, none of the interrupts are enabled independent of
the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt
has occurred, and is set by the RETI instruction to enable subsequent interrupts. The Ibit can also be set and cleared by the application with the SEI and CLI instructions, as
described in the instruction set reference.
• Bit 6 - T: Bit Copy Storage
The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T bit as source or
destination for the operated bit. A bit from a register in the Register file can be copied
into T by the BST instruction, and a bit in T can be copied into a bit in a register in the
Register file by the BLD instruction.
• Bit 5 - H: Half Carry Flag
The half carry flag H indicates a half carry in some arithmetic operations. Half carry is
useful in BCD arithmetic. See the “Instruction Set Description” for detailed information.
• Bit 4 - S: Sign Bit, S = N
The S-bit is always an exclusive or between the negative flag N and the two’s complement overflow flag V. See the “Instruction Set Description” for detailed information.
• Bit 3 - V: Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetics. See the
“Instruction Set Description” for detailed information.
• Bit 2 - N: Negative Flag
⊕ V
General Purpose
Register File
The negative flag N indicates a negative result in an arithmetic or logic operation. See
the “Instruction Set Description” for detailed information.
• Bit 1 - Z: Zero Flag
The zero flag Z indicates a zero result in an arithmetic or logic operation. See the
“Instruction Set Description” for detailed information.
• Bit 0 - C: Carry Flag
The carry flag C indicates a carry in an arithmetic or logic operation. See the “Instruction
Set Description” for detailed information.
The Register file is optimized for the AVR Enhanced RISC instruction set. In order to
achieve the required performance and flexibility, the following input/output schemes are
supported by the Register file:
•One 8-bit output operand and one 8-bit result input
•Two 8-bit output operands and one 8-bit result input
•Two 8-bit output operands and one 16-bit result input
•One 16-bit output operand and one 16-bit result input
Figure 4 shows the structure of the 32 general purpose working registers in the CPU.
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Figure 4. AVR CPU General Purpose Working Registers
70Addr.
R0 $00
R1$01
R2$02
…
R13$0D
GeneralR14$0E
PurposeR15$0F
WorkingR16$10
RegistersR17$11
…
R26$1AX-register low byte
R27$1BX-register high byte
R28$1CY-register low byte
R29$1DY-register high byte
R30$1EZ-register low byte
R31$1FZ-register high byte
Most of the instructions operating on the Register file have direct access to all registers,
and most of them are single cycle instructions.
The X-register, Y-register and
Z-register
As shown in Figure 4, each register is also assigned a data memory address, mapping
them directly into the first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory organization provides great
flexibility in access of the registers, as the X, Y, and Z pointer registers can be set to
index any register in the file.
The registers R26..R31 have some added functions to their general purpose usage.
These registers are 16-bit address pointers for indirect addressing of the Data Space.
The three indirect address registers X, Y, and Z are defined as described in Figure 5.
Figure 5. The X-, Y-, and Z-registers
15XHXL0
X - register7070
R27 ($1B)R26 ($1A)
15YHYL0
Y - register7070
R29 ($1D)R28 ($1C)
15ZHZL0
Z - register7070
R31 ($1F)R30 ($1E)
2466B–09/01
In the different addressing modes these address registers have functions as fixed displacement, automatic increment, and automatic decrement (see the Instruction Set
Reference for details).
9
Stack PointerThe stack is mainly used for storing temporary data, for storing local variables and for
storing return addresses after interrupts and subroutine calls. The stack pointer register
always points to the top of the stack. Note that the stack is implemented as growing from
higher memory locations to lower memory locations. This implies that a stack PUSH
command decreases the stack pointer.
The Stack Pointer points to the data SRAM stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the
program before any subroutine calls are executed or interrupts are enabled. The Stack
Pointer must be set to point above $60. The Stack Pointer is decremented by one when
data is pushed onto the Stack with the PUSH instruction, and it is decremented by two
when the return address is pushed onto the Stack with subroutine call or interrupt. The
Stack Pointer is incremented by one when data is popped from the Stack with the POP
instruction, and it is incremented by two when data is popped from the Stack with return
from subroutine RET or return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some
implementations of the AVR architecture is so small that only SPL is needed. In this
case, the SPH register will not be present.
Bit151413121110 9 8
SP15SP14SP13SP12SP11SP10SP9SP8SPH
SP7SP6SP5SP4SP3SP2SP1SP0SPL
76543210
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
R/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
00000000
Instruction Execution
Timing
This section describes the general access timing concepts for instruction execution. The
AVR CPU is driven by the CPU clock clk
, directly generated from the selected clock
CPU
source for the chip. No internal clock division is used.
Figure 6 shows the parallel instruction fetches and instruction executions enabled by the
Harvard architecture and the fast-access Register file concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for
functions per cost, functions per clocks, and functions per power-unit.
Figure 6. The Parallel Instruction Fetches and Instruction Executions
T1T2T3T4
clk
CPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
Figure 7 shows the internal timing concept for the Register file. In a single clock cycle an
ALU operation using two register operands is executed, and the result is stored back to
the destination register.
10
ATmega16(L)
2466B–09/01
Figure 7. Single Cycle ALU Operation
clk
CPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
ATmega16(L)
T1T2T3T4
Reset and Interrupt
Handling
The AVR provides several different interrupt sources. These interrupts and the separate
reset vector each have a separate program vector in the program memory space. All
interrupts are assigned individual enable bits which must be written logic one together
with the Global Interrupt Enable bit in the Status Register in order to enable the interrupt.
Depending on the program counter value, interrupts may be automatically disabled
when Boot Lock bits BLB02 or BLB12 are programmed. This feature improves software
security. See the section “Memory Programming” on page 253 for details.
The lowest addresses in the program memory space are by default defined as the Reset
and Interrupt vectors. The complete list of vectors is shown in “Interrupts” on page 42.
The list also determines the priority levels of the different interrupts. The lower the
address the higher is the priority level. RESET has the highest priority, and next is INT0
– the External Interrupt Request 0. The interrupt vectors can be moved to the start of the
boot Flash section by setting the IVSEL bit in the General Interrupt Control Register
(GICR). Refer to “Interrupts” on page 42 for more information. The Reset vector can
also be moved to the start of the boot Flash section by programming the BOOTRST
fuse, see “Boot Loader Support – Read-While-Write Self-Programming” on page 240.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts
are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit is
automatically set when a Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that
sets the interrupt flag. For these interrupts, the Program Counter is vectored to the
actual interrupt vector in order to execute the interrupt handling routine, and hardware
clears the corresponding interrupt flag. Interrupt flags can also be cleared by writing a
logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the
corresponding interrupt enable bit is cleared, the interrupt flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or
more interrupt conditions occur while the global interrupt enable bit is cleared, the corresponding interrupt flag(s) will be set and remembered until the global interrupt enable bit
is set, and will then be executed by order of priority.
2466B–09/01
The second type of interrupts will trigger as long as the interrupt condition is present.
These interrupts do not necessarily have interrupt flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served.
11
Note that the status register is not automatically stored when entering an interrupt routine, nor restored when returning from an interrupt routine. This must be handled by
software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately
disabled. No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be used to
avoid interrupts during the timed EEPROM write sequence..
Assembly Code Example
in r16, SREG; store SREG value
cli; disable interrupts during timed sequence
sbi EECR, EEMWE; start EEPROM write
sbi EECR, EEWE
out SREG, r16; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMWE); /* start EEPROM write */
EECR |= (1<<EEWE);
SREG = cSREG; /* restore SREG value (I-bit) */
When using the SEI instruction to enable interrupts, the instruction following SEI will be
executed before any pending interrupts, as shown in this example.
Assembly Code Example
sei; set global interrupt enable
sleep ; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
C Code Example
_SEI(); /* set global interrupt enable */
_SLEEP(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
Interrupt Response TimeThe interrupt execution response for all the enabled AVR interrupts is 4 clock cycles
minimum. After 4 clock cycles the program vector address for the actual interrupt handling routine is executed. During this 4 clock cycle period, the Program Counter is
pushed onto the Stack. The vector is normally a jump to the interrupt routine, and this
jump takes 3 clock cycles. If an interrupt occurs during execution of a multi-cycle instruction, this instruction is completed before the interrupt is served. If an interrupt occurs
when the MCU is in sleep mode, the interrupt execution response time is increased by 4
clock cycles. This increase comes in addition to the start-up time from the selected
sleep mode.
A return from an interrupt handling routine takes 4 clock cycles. During these 4 clock
cycles, the Program Counter (2 bytes) is popped back from the Stack, the Stack Pointer
is incremented by 2, and the I-bit in SREG is set.
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AVR ATmega16
Memories
In-system
Reprogrammable Flash
Program Memory
This section describes the different memories in the ATmega16. The AVR architecture
has two main memory spaces, the Data Memory and the Program Memory space. In
addition, the ATmega16 features an EEPROM Memory for data storage. All three memory spaces are linear and regular.
The ATmega16 contains 16K bytes On-chip In-System Reprogrammable Flash memory
for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 8K x 16. For software security, the Flash Program memory space is divided
into two sections, Boot Program section and Application Program section.
The Flash memory has an endurance of at least 1000 write/erase cycles. The
ATmega16 Program Counter (PC) is 13 bits wide, thus addressing the 8K program
memory locations. The operation of Boot Program section and associated Boot Lock
Bits for software protection are described in detail in “Boot Loader Support – ReadWhile-Write Self-Programming” on page 240. “Memory Programming” on page 253 contains a detailed description on Flash data serial downloading using the SPI pins or the
JTAG interface.
Constant tables can be allocated within the entire program memory address space (see
the LPM – Load Program Memory instruction description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 10.
Figure 8. Program Memory Map
Application Flash Section
Boot Flash Section
$0000
$1FFF
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13
SRAM Data MemoryFigure 9 shows how the ATmega16 SRAM Memory is organized.
The lower 1120 Data Memory locations address the Register file, the I/O Memory, and
the internal data SRAM. The first 96 locations address the Register file and I/O Memory,
and the next 1024 locations address the internal data SRAM.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In
the Register file, registers R26 to R31 feature the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base
address given by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and postincrement, the address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O registers, and the 1024 bytes of internal data SRAM in the ATmega16 are all accessible through all these addressing modes.
The Register file is described in “General Purpose Register File” on page 8.
Figure 9. Data Memory Map
Register File
R0
R1
R2
...
Data Address Space
$0000
$0001
$0002
...
R29
R30
R31
I/O Registers
$00
$01
$02
...
$3D
$3E
$3F
$001D
$001E
$001F
$0020
$0021
$0022
...
$005D
$005E
$005F
Internal SRAM
$0060
$0061
...
$045E
$045F
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Data Memory Access TimesThis section describes the general access timing concepts for internal memory access.
The internal data SRAM access is performed in two clk
10.
Figure 10. On-chip Data SRAM Access Cycles
T1T2T3
clk
CPU
Address
Data
Data
Compute Address
WR
RD
Address Valid
cycles as described in Figure
CPU
Write
Read
Memory Access Instruction
Next Instruction
2466B–09/01
15
EEPROM Data MemoryThe ATmega16 contains 512 bytes of data EEPROM memory. It is organized as a sep-
arate data space, in which single bytes can be read and written. The EEPROM has an
endurance of at least 100,000 write/erase cycles. The access between the EEPROM
and the CPU is described in the following, specifying the EEPROM Address Registers,
the EEPROM Data Register, and the EEPROM Control Register.
For a detailed description of SPI and JTAG data downloading to the EEPROM, see
page 265 and page 270, respectively.
EEPROM Read/Write AccessThe EEPROM access registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 1. A self-timing function, however, lets the user software detect when the next byte can be written. If the user code
contains instructions that write the EEPROM, some precautions must be taken. In
is likely to rise or fall slowly on power-up/down. This
CC
The EEPROM Address
Register – EEARH and EEARL
heavily filtered power supplies, V
causes the device for some period of time to run at a voltage lower than specified as
minimum for the clock frequency used. See “Preventing EEPROM Corruption” on page
20. for details on how to avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next
instruction is executed. When the EEPROM is written, the CPU is halted for two clock
cycles before the next instruction is executed.
Bit151413121110 9 8
–––––––EEAR8EEARH
EEAR7EEAR6EEAR5EEAR4EEAR3EEAR2EEAR1EEAR0EEARL
76543210
Read/WriteRRRRRRRR/W
R/WR/WR/WR/WR/WR/WR/WR/W
Initial Value0000000X
XXXXXXXX
The EEPROM Data Register –
EEDR
16
ATmega16(L)
• Bits 15..9 - Res: Reserved Bits
These bits are reserved bits in the ATmega16 and will always read as zero.
• Bits 8..0 - EEAR8..0: EEPROM Address
The EEPROM Address Registers
– EEARH and EEARL specify the EEPROM address
in the 512 bytes EEPROM space. The EEPROM data bytes are addressed linearly
between 0 and 511. The initial value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.
Bit76543210
MSBLSBEEDR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bits 7..0 - EEDR7.0: EEPROM Data
For the EEPROM write operation, the EEDR register contains the data to be written to
the EEPROM in the address given by the EEAR register. For the EEPROM read opera-
2466B–09/01
The EEPROM Control Register
– EECR
ATmega16(L)
tion, the EEDR contains the data read out from the EEPROM at the address given by
EEAR.
Bit76543 2 10
––––EERIEEEMWEEEWEEEREEECR
Read/WriteRRRRR/WR/WR/WR/W
Initial Value000000X0
• Bits 7..4 - Res: Reserved Bits
These bits are reserved bits in the ATmega16 and will always read as zero.
• Bit 3 - EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set.
Writing EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a
constant interrupt when EEWE is cleared.
• Bit 2 - EEMWE: EEPROM Master Write Enable
The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be
written. When EEMWE is set, setting EEWE within 4 clock cycles will write data to the
EEPROM at the selected address If EEMWE is zero, setting EEWE will have no effect.
When EEMWE has been written to one by software, hardware clears the bit to zero after
four clock cycles. See the description of the EEWE bit for an EEPROM write procedure.
• Bit 1 - EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When
address and data are correctly set up, the EEWE bit must be written to one to write the
value into the EEPROM. The EEMWE bit must be written to one before a logical one is
written to EEWE, otherwise no EEPROM write takes place. The following procedure
should be followed when writing the EEPROM (the order of steps 3 and 4 is not
essential):
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.
6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The
software must check that the Flash programming is completed before initiating a new
EEPROM write. Step 2 is only relevant if the software contains a boot loader allowing
the CPU to program the Flash. If the Flash is never being updated by the CPU, step 2
can be omitted. See “Boot Loader Support – Read-While-Write Self-Programming” on
page 240 for details about boot programming.
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the
EEPROM Master Write Enable will time-out. If an interrupt routine accessing the
EEPROM is interrupting another EEPROM access, the EEAR or EEDR register will be
modified, causing the interrupted EEPROM access to fail. It is recommended to have
the global interrupt flag cleared during all the steps to avoid these problems.
2466B–09/01
When the write access time has elapsed, the EEWE bit is cleared by hardware. The
user software can poll this bit and wait for a zero before writing the next byte. When
17
EEWE has been set, the CPU is halted for two cycles before the next instruction is
executed.
• Bit 0 - EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the
correct address is set up in the EEAR register, the EERE bit must be written to a logic
one to trigger the EEPROM read. The EEPROM read access takes one instruction, and
the requested data is available immediately. When the EEPROM is read, the CPU is
halted for four cycles before the next instruction is executed.
The user should poll the EEWE bit before starting the read operation. If a write operation
is in progress, it is neither possible to read the EEPROM, nor to change the EEAR
register.
The calibrated oscillator is used to time the EEPROM accesses. Table 1 lists the typical
programming time for EEPROM access from the CPU.
Table 1. EEPROM Programming Time.
Number of
Calibrated RC-
Symbol
EEPROM write (from CPU)approximately 83007.5 ms9.0 ms
oscillator Cycles
Min
Programming
Time
Max
Programming
Time
18
ATmega16(L)
2466B–09/01
ATmega16(L)
The following code examples show one assembly and one C function for writing to the
EEPROM. The examples assume that interrupts are controlled (for example by disabling interrupts globally) so that no interrupts will occur during execution of these
functions. The examples also assume that no Flash boot loader is present in the software. If such code is present, the EEPROM write function must also wait for any
ongoing SPM command to finish.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEWE
rjmp EEPROM_write
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r16) to data register
out EEDR,r16
; Write logical one to EEMWE
sbi EECR,EEMWE
; Start eeprom write by setting EEWE
sbi EECR,EEWE
ret
C Code Example
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address and data registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMWE */
EECR |= (1<<EEMWE);
/* Start eeprom write by setting EEWE */
EECR |= (1<<EEWE);
}
2466B–09/01
19
The next code examples show assembly and C functions for reading the EEPROM. The
examples assume that interrupts are controlled so that no interrupts will occur during
execution of these functions.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEWE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from data register
inr16,EEDR
ret
C Code Example
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
Preventing EEPROM
Corruption
20
ATmega16(L)
During periods of low V
the EEPROM data can be corrupted because the supply volt-
CC,
age is too low for the CPU and the EEPROM to operate properly. These issues are the
same as for board level systems using EEPROM, and the same design solutions should
be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too
low. First, a regular write sequence to the EEPROM requires a minimum voltage to
operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the
supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design
recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD). If the
detection level of the internal BOD does not match the needed detection level, an
external low V
Reset Protection circuit can be used. If a reset occurs while a write
CC
operation is in progress, the write operation will be completed provided that the
power supply voltage is sufficient.
2466B–09/01
ATmega16(L)
I/O MemoryThe I/O space definition of the ATmega16 is shown in “Register Summary” on page 290.
All ATmega16 I/Os and peripherals are placed in the I/O space. The I/O locations are
accessed by the IN and OUT instructions, transferring data between the 32 general purpose working registers and the I/O space. I/O registers within the address range $00 $1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the
value of single bits can be checked by using the SBIS and SBIC instructions. Refer to
the Instruction Set section for more details. When using the I/O specific commands IN
and OUT, the I/O addresses $00 - $3F must be used. When addressing I/O registers as
data space using LD and ST instructions, $20 must be added to these addresses.
For compatibility with future devices, reserved bits should be written to zero if accessed.
Reserved I/O memory addresses should never be written.
Some of the status flags are cleared by writing a logical one to them. Note that the CBI
and SBI instructions will operate on all bits in the I/O register, writing a one back into any
flag read as set, thus clearing the flag. The CBI and SBI instructions work with registers
$00 to $1F only.
The I/O and peripherals control registers are explained in later sections.
2466B–09/01
21
System Clock and
Clock Options
Clock Systems and their
Distribution
Figure 11 presents the principal clock systems in the AVR and their distribution. All of
the clocks need not be active at a given time. In order to reduce power consumption, the
clocks to modules not being used can be halted by using different sleep modes, as
described in “Power Management and Sleep Modes” on page 30. The clock systems
are detailed Figure 11.
Figure 11. Clock Distribution
Asynchronous
Timer/Counter
General I/O
Modules
clk
ADCCPU CoreRAM
clk
ADC
clk
I/O
ASY
AVR Clock
Control Unit
Source Clock
Clock
Multiplexer
clk
CPU
clk
FLASH
Reset Logic
Watchdog Timer
Watchdog Clock
Watchdog
Oscillator
Flash and
EEPROM
CPU Clock – clk
I/O Clock – clk
I/O
Flash Clock – clk
22
ATmega16(L)
CPU
FLASH
Timer/Counter
Oscillator
External RC
Oscillator
External Clock
Crystal
Oscillator
Low-frequency
Crystal Oscillator
Calibrated RC
Oscillator
The CPU clock is routed to parts of the system concerned with operation of the AVR
core. Examples of such modules are the General Purpose Register File, the Status Register and the data memory holding the Stack Pointer. Halting the CPU clock inhibits the
core from performing general operations and calculations.
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and
USART. The I/O clock is also used by the External Interrupt module, but note that some
external interrupts are detected by asynchronous logic, allowing such interrupts to be
detected even if the I/O clock is halted. Also note that address recognition in the TWI
module is carried out asynchronously when clk
is halted, enabling TWI address recep-
I/O
tion in all sleep modes.
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPU clock.
2466B–09/01
ATmega16(L)
Asynchronous Timer Clock –
clk
ASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked
directly from an external 32 kHz clock crystal. The dedicated clock domain allows using
this Timer/Counter as a real-time counter even when the device is in sleep mode.
ADC Clock – clk
ADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and
I/O clocks in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.
Clock SourcesThe device has the following clock source options, selectable by Flash fuse bits as
shown below. The clock from the selected source is input to the AVR clock generator,
and routed to the appropriate modules.
Table 2. Device Clocking Options Select
Device Clocking Option CKSEL3..0
External Crystal/Ceramic Resonator1111 - 1010
External Low-frequency Crystal1001
External RC Oscillator1000 - 0101
Calibrated Internal RC Oscillator0100 - 0001
External Clock0000
Note:1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the
CPU wakes up from power down or power save, the selected clock source is used to
time the start-up, ensuring stable oscillator operation before instruction execution starts.
When the CPU starts from reset, there is as an additional delay allowing the power to
reach a stable level before commencing normal operation. The watchdog oscillator is
used for timing this real-time part of the start-up time. The number of WDT oscillator
cycles used for each time-out is shown in Table 3. The frequency of the watchdog oscillator is voltage dependent as shown in “ATmega16 Typical Characteristics – Preliminary
Data” on page 289. The device is shipped with CKSEL = “0001” and SUT = “10” (1 MHz
Internal RC Oscillator, slowly rising power).
(1)
Table 3. Number of Watchdog Oscillator Cycles
Time-out (VCC = 5.0V)Time-out (VCC = 3.0V)Number of Cycles
(1)
4 ms
(1)
64 ms
Notes:1. Values are guidelines only. Actual values are TBD.
4 ms
64 ms
(1)
(1)
4K
64K
Crystal OscillatorXTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can
be configured for use as an On-chip oscillator, as shown in Figure 12. Either a quartz
crystal or a ceramic resonator may be used. The CKOPT fuse selects between two different oscillator amplifier modes. When CKOPT is programmed, the oscillator output will
oscillate will a full rail-to-rail swing on the output. This mode is suitable when operating
in a very noisy environment or when the output from XTAL2 drives a second clock
buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the
oscillator has a smaller output swing. This reduces power consumption considerably.
This mode has a limited frequency range and it can not be used to drive other clock
buffers.
23
2466B–09/01
For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and
16 MHz with CKOPT programmed. C1 and C2 should always be equal for both crystals
and resonators. The optimal value of the capacitors depends on the crystal or resonator
in use, the amount of stray capacitance, and the electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in
Table 4. For ceramic resonators, the capacitor values given by the manufacturer should
be used. For more information on how to choose capacitors and other details on oscillator operation, refer to the Multi-purpose Oscillator application note.
Figure 12. Crystal Oscillator Connections
C2
C1
XTAL2
XTAL1
GND
The oscillator can operate in three different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL3..1 as shown in
Table 4.
Table 4. Crystal Oscillator Operating Modes
Frequency Range
CKOPTCKSEL3..1
1101
11100.9 - 3.012 - 22
11113.0 - 8.012 - 22
0101, 110, 1111.0 ≤12 - 22
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. This option should not be used with crystals, only with ceramic resonators.
(2)
(MHz)
0.4 - 0.9–
(1)
Recommended Range for Capacitors
C1 and C2 for Use with Crystals (pF)
24
ATmega16(L)
2466B–09/01
ATmega16(L)
The CKSEL0 fuse together with the SUT1..0 fuses select the start-up times as shown in
Table 5.
Table 5. Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
CKSEL0SUT1..0
000 258 CK
001 258 CK
010 1K CK
011 1K CK
100 1K CK
10116K CK–
11016K CK4 ms
11116K CK64 ms
Notes: 1. These options should only be used when not operating close to the maximum fre-
quency of the device, and only if frequency stability at start-up is not important for the
application. These options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating
close to the maximum frequency of the device, and if frequency stability at start-up is
not important for the application.
Power-save
(1)
(1)
(2)
(2)
(2)
Additional Delay
from Reset
= 5.0V)Recommended Usage
(V
CC
4 ms
64 ms
–
4 ms
64 ms
Ceramic resonator, fast
rising power
Ceramic resonator, slowly
rising power
Ceramic resonator, BOD
enabled
Ceramic resonator, fast
rising power
Ceramic resonator, slowly
rising power
Crystal oscillator, BOD
enabled
Crystal oscillator, fast
rising power
Crystal oscillator, slowly
rising power
Low-frequency Crystal
Oscillator
2466B–09/01
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency
crystal oscillator must be selected by setting the CKSEL fuses to “1001”. The crystal
should be connected as shown in Figure 12. By programming the CKOPT fuse, the user
can enable internal capacitors on XTAL1 and XTAL2, thereby removing the need for
external capacitors. The internal capacitors have a nominal value of 36 pF. Refer to the
32 kHz Crystal Oscillator application note for details on oscillator operation and how to
choose appropriate values for C1 and C2.
25
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in Table 6.
Table 6. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
SUT1..0
001K CK
011K CK
1032K CK64 msStable frequency at start-up
11Reserved
Note:1. These options should only be used if frequency stability at start-up is not important
Power-save
(1)
(1)
for the application.
Additional Delay
from Reset
= 5.0V)Recommended Usage
(V
CC
4 msFast rising power or BOD enabled
64 msSlowly rising power
External RC OscillatorFor timing insensitive applications, the external RC configuration shown in Figure 13
can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should
be at least 22 pF. By programming the CKOPT fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND, thereby removing the need for an external
capacitor. For more information on oscillator operation and details on how to choose R
and C, refer to the External RC Oscillator application note.
Figure 13. External RC Configuration
CC
V
R
NC
XTAL2
XTAL1
C
GND
The oscillator can operate in four different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL3..0 as shown in
Table 7.
Table 7. External RC Oscillator Operating Modes
CKSEL3..0 Frequency Range (MHz)
0101≤ 0.9
01100.9 - 3.0
01113.0 - 8.0
10008.0 - 12.0
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in Table 8.
26
ATmega16(L)
2466B–09/01
ATmega16(L)
Table 8. Start-up Times for the External RC Oscillator Clock Selection
Calibrated Internal RC
Oscillator
Start-up Time from
Power-down and
SUT1..0
0018 CK–BOD enabled
0118 CK4 msFast rising power
1018 CK64 msSlowly rising power
116 CK
Note:1. This option should not be used when operating close to the maximum frequency of
Power-save
(1)
the device.
Additional Delay
from Reset
(VCC = 5.0V)Recommended Usage
4 msFast rising power or BOD enabled
The calibrated internal RC oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All
frequencies are nominal values at 5V and 25
°C. This clock may be selected as the sys-
tem clock by programming the CKSEL fuses as shown in Table 9. If selected, it will
operate with no external components. The CKOPT fuse should always be unprogrammed when using this clock option. During reset, hardware loads the calibration byte
into the OSCCAL register and thereby automatically calibrates the RC oscillator. At 5V,
°C and 1.0 MHz oscillator frequency selected, this calibration gives a frequency within
25
± 1% of the nominal frequency. When this oscillator is used as the chip clock, the
Watchdog Oscillator will still be used for the Watchdog Timer and for the reset time-out.
For more information on the pre-programmed calibration value, see the section “Calibra-
tion Byte” on page 255.
Note:1. The device is shipped with this option selected.
1.0
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in Table 10. XTAL1 and XTAL2 should be left unconnected (NC).
Table 10. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Start-up Time from
Power-down and
SUT1..0
006 CK–BOD enabled
016 CK4 msFast rising power
(1)
10
11Reserved
Note:1. The device is shipped with this option selected.
Power-save
6 CK64 msSlowly rising power
Additional Delay
from Reset
= 5.0V)Recommended Usage
(V
CC
2466B–09/01
27
Oscillator Calibration Register
– OSCCAL
Bit76543210
CAL7CAL6CAL5CAL4CAL3CAL2CAL1CAL0OSCCAL
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial ValueDevice Specific Calibration Value
• Bits 7..0 - CAL7..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the internal oscillator to remove process variations from the oscillator frequency. This is done automatically during chip
reset. When OSCCAL is zero, the lowest available frequency is chosen. Writing nonzero values to this register will increase the frequency of the internal oscillator. Writing
$FF to the register gives the highest available frequency. The calibrated oscillator is
used to time EEPROM and Flash access. If EEPROM or Flash is written, do not calibrate to more than 10% above the nominal frequency. Otherwise, the EEPROM or Flash
write may fail. Note that the Oscillator is intended for calibration to 1.0 MHz, 2.0 MHz,
4.0 MHz, or 8.0 MHz. Tuning to other values is not guaranteed, as indicated in Table 11.
Table 11. Internal RC Oscillator Frequency Range.
Min Frequency in Percentage of
OSCCAL Value
$0050100
$7F75150
$FF100200
Nominal Frequency (%)
Max Frequency in Percentage of
Nominal Frequency (%)
External ClockTo drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 14. To run the device on an external clock, the CKSEL fuses must be programmed to “0000”. By programming the CKOPT fuse, the user can enable an internal
36 pF capacitor between XTAL1 and GND.
Figure 14. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
28
ATmega16(L)
2466B–09/01
ATmega16(L)
When this clock source is selected, start-up times are determined by the SUT fuses as
shown in Table 12.
Table 12. Start-up Times for the External Clock Selection
Start-up Time from
Power-down and
SUT1..0
006 CK–BOD enabled
016 CK4 msFast rising power
106 CK64 msSlowly rising power
11Reserved
Power-save
Additional Delay
from Reset
= 5.0V)Recommended Usage
(V
CC
Timer/Counter OscillatorFor AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the
crystal is connected directly between the pins. No external capacitors are needed. The
oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external
clock source to TOSC1 is not recommended.
2466B–09/01
29
Power Management
and Sleep Modes
Sleep modes enable the application to shut down unused modules in the MCU, thereby
saving power. The AVR provides various sleep modes allowing the user to tailor the
power consumption to the application’s requirements.
To enter any of the six sleep modes, the SE bit in MCUCR must be written to logic one
and a SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the
MCUCR register select which sleep mode (Idle, ADC Noise Reduction, Power-down,
Power-save, Standby, or Extended Standby) will be activated by the SLEEP instruction.
See Table 13 for a summary. If an enabled interrupt occurs while the MCU is in a sleep
mode, the MCU wakes up. The MCU is then halted for four cycles in addition to the
start-up time, it executes the interrupt routine, and resumes execution from the instruction following SLEEP. The contents of the register file and SRAM are unaltered when
the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes
up and executes from the Reset vector.
Figure 11 on page 22 presents the different clock systems in the ATmega16, and their
distribution. The figure is helpful in selecting an appropriate sleep mode.
MCU Control Register –
MCUCR
The MCU Control Register contains control bits for power management.
These bits select between the six available sleep modes as shown in Table 13.
Table 13. Sleep Mode Select
SM2SM1SM0Sleep Mode
000Idle
001ADC Noise Reduction
010Power-down
011Power-save
100Reserved
101Reserved
110Standby
111Extended Standby
Note:1. Standby mode and Extended Standby mode are only available with external crystals
or resonators.
(1)
(1)
• Bit 6 - SE: Sleep Enable
30
The SE bit must be written to logic one to make the MCU enter the sleep mode when the
SLEEP instruction is executed. To avoid the MCU entering the sleep mode unless it is
the programmers purpose, it is recommended to write the Sleep Enable (SE) bit to one
just before the execution of the SLEEP instruction and to clear it immediately after waking up.
ATmega16(L)
2466B–09/01
ATmega16(L)
Idle ModeWhen the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter
Idle Mode, stopping the CPU but allowing SPI, USART, Analog Comparator, ADC, 2wire Serial Interface, Timer/Counters, Watchdog, and the interrupt system to continue
operating. This sleep mode basically halts clk
clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as
internal ones like the Timer Overflow and USART Transmit Complete interrupts. If
wake-up from the Analog Comparator interrupt is not required, the Analog Comparator
can be powered down by setting the ACD bit in the Analog Comparator Control and Status register – ACSR. This will reduce power consumption in Idle Mode. If the ADC is
enabled, a conversion starts automatically when this mode is entered.
CPU
and clk
, while allowing the other
FLASH
ADC Noise Reduction
Mode
When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter
ADC Noise Reduction Mode, stopping the CPU but allowing the ADC, the external interrupts, the 2-wire Serial Interface address watch, Timer/Counter2 and the Watchdog to
continue operating (if enabled). This sleep mode basically halts clk
, while allowing the other clocks to run.
FLASH
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is enabled, a conversion starts automatically when this mode is
entered. Apart form the ADC Conversion Complete interrupt, only an external reset, a
watchdog reset, a Brown-out Reset, a 2-wire Serial Interface address match interrupt, a
Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, an external level interrupt
on INT0 or INT1, or an external interrupt on INT2 can wake up the MCU from ADC
Noise Reduction Mode.
I/O
, clk
, and clk-
CPU
Power-down ModeWhen the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter
Power-down mode. In this mode, the external oscillator is stopped, while the external
interrupts, the 2-wire Serial Interface address watch, and the Watchdog continue operating (if enabled). Only an external reset, a watchdog reset, a Brown-out Reset, a 2-wire
Serial Interface address match interrupt, an external level interrupt on INT0 or INT1, or
an external interrupt on INT2 can wake up the MCU. This sleep mode basically halts all
generated clocks, allowing operation of asynchronous modules only.
Note that if a level triggered interrupt is used for wake-up from Power-down mode, the
changed level must be held for some time to wake up the MCU. Refer to “External Interrupts” on page 64 for details.
When waking up from Power-down mode, there is a delay from the wake-up condition
occurs until the wake-up becomes effective. This allows the clock to restart and become
stable after having been stopped. The wake-up period is defined by the same CKSEL
fuses that define the reset time-out period, as described in “Clock Sources” on page 23.
Power-save ModeWhen the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter
Power-save mode. This mode is identical to Power-down, with one exception:
If Timer/Counter2 is clocked asynchronously, i.e., the AS2 bit in ASSR is set,
Timer/Counter2 will run during sleep. The device can wake up from either Timer Overflow or Output Compare event from Timer/Counter2 if the corresponding
Timer/Counter2 interrupt enable bits are set in TIMSK, and the global interrupt enable
bit in SREG is set.
If the asynchronous timer is NOT clocked asynchronously, Power-down mode is recommended instead of Power-save mode because the contents of the registers in the
31
2466B–09/01
asynchronous timer should be considered undefined after wake-up in Power-save mode
if AS2 is 0.
This sleep mode basically halts all clocks except clk
, allowing operation only of asyn-
ASY
chronous modules, including Timer/Counter 2 if clocked asynchronously.
Standby ModeWhen the SM2..0 bits are 110 and an external crystal/resonator clock option is selected,
the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to
Power-down with the exception that the oscillator is kept running. From Standby mode,
the device wakes up in 6 clock cycles.
Extended Standby ModeWhen the SM2..0 bits are 111 and an external crystal/resonator clock option is selected,
the SLEEP instruction makes the MCU enter Extended Standby mode. This mode is
identical to Power-save mode with the exception that the oscillator is kept running. From
Extended Standby mode, the device wakes up in 6 clock cycles..
Table 14. Active Clock Domains and Wake Up Sources in the Different Sleep Modes
Active Clock domainsOscillatorsWake up sources
Sleep
Modeclk
CPU
clk
FLASH
clkIOclk
ADC
clk
Main Clock
Source Enabled
ASY
Timer Osc.
Enabled
IdleXXXXX
ADC
Noise
Redu-
XXXX
ction
INT2
INT1
INT0
(2)
(2)
XX X XXX
(3)
X
TWI
Address
Match
Timer
2
SPM /
EEPROM
ReadyADC
XXXX
Other
I/O
Power
Down
Power
Save
Standby
Extended
Standby
(2)
X
(1)
(2)
(1)
X
XX
XX
(2)
X
(2)
(3)
X
(3)
X
(3)
(3)
X
X
XX
(2)
X
XX
(2)
Notes:1. External Crystal or resonator selected as clock source.
2. If AS
2 bit in ASSR is set.
3. Only INT2 or level interrupt INT1 and INT0.
Minimizing Power
Consumption
There are several issues to consider when trying to minimize the power consumption in
an AVR controlled system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as few as possible of the device’s
functions are operating. All functions not needed should be disabled. In particular, the
following modules may need special consideration when trying to achieve the lowest
possible power consumption.
Analog to Digital ConverterIf enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should
be disabled before entering any sleep mode. When the ADC is turned off and on again,
the next conversion will be an extended conversion. Refer to “Analog to Digital Converter” on page 195 for details on ADC operation.
32
ATmega16(L)
2466B–09/01
ATmega16(L)
Analog ComparatorWhen entering Idle Mode, the Analog Comparator should be disabled if not used. When
entering ADC Noise Reduction Mode, the Analog Comparator should be disabled. In the
other sleep modes, the Analog Comparator is automatically disabled. However, if the
Analog Comparator is set up to use the Internal Voltage Reference as input, the Analog
Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled, independent of sleep mode. Refer to “Analog Comparator” on
page 192 for details on how to configure the Analog Comparator.
Brown-out DetectorIf the Brown-out Detector is not needed in the application, this module should be turned
off. If the Brown-out Detector is enabled by the BODEN fuse, it will be enabled in all
sleep modes, and hence, always consume power. In the deeper sleep modes, this will
contribute significantly to the total current consumption. Refer to “Brown-out Detection”
on page 37 for details on how to configure the Brown-out Detector.
Internal Voltage ReferenceThe Internal Voltage Reference will be enabled when needed by the Brown-out detec-
tor, the Analog Comparator or the ADC. If these modules are disabled as described in
the sections above, the internal voltage reference will be disabled and it will not be consuming power. When turned on again, the user must allow the reference to start up
before the output is used. If the reference is kept on in sleep mode, the output can be
used immediately. Refer to “Internal Voltage Reference” on page 39 for details on the
start-up time.
Watchdog TimerIf the Watchdog Timer is not needed in the application, this module should be turned off.
If the Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence,
always consume power. In the deeper sleep modes, this will contribute significantly to
the total current consumption. Refer to “Watchdog Timer” on page 39 for details on how
to configure the Watchdog Timer.
Port PinsWhen entering a sleep mode, all port pins should be configured to use minimum power.
The most important thing is then to ensure that no pins drive resistive loads. In sleep
modes where the both the I/O clock (clk
) and the ADC clock (clk
I/O
) are stopped, the
ADC
input buffers of the device will be disabled. This ensures that no power is consumed by
the input logic when not needed. In some cases, the input logic is needed for detecting
wake-up conditions, and it will then be enabled. Refer to the section “Digital Input
Enable and Sleep Modes” on page 51 for details on which pins are enabled. If the input
buffer is enabled and the input signal is left floating or have an analog signal level close
/2, the input buffer will use excessive power.
to V
CC
2466B–09/01
33
System Control and
Reset
Resetting the AVRDuring reset, all I/O registers are set to their initial values, and the program starts execu-
tion from the Reset Vector. The instruction placed at the Reset Vector must be a JMP –
absolute jump – instruction to the reset handling routine. If the program never enables
an interrupt source, the interrupt vectors are not used, and regular program code can be
placed at these locations. This is also the case if the Reset Vector is in the Application
section while the interrupt vectors are in the boot section or vice versa. The circuit diagram in Figure 15 shows the reset logic. Table 15 defines the electrical parameters of
the reset circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source
goes active. This does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the
internal reset. This allows the power to reach a stable level before normal operation
starts. The time-out period of the delay counter is defined by the user through the
CKSEL fuses. The different selections for the delay period are presented in “Clock
Sources” on page 23.
Reset SourcesThe ATmega16 has five sources of reset:
•Power-on Reset. The MCU is reset when the supply voltage is below the power-on
reset threshold (V
•External Reset. The MCU is reset when a low level is present on the RESET pin for
longer than the minimum pulse length.
•Watchdog Reset. The MCU is reset when the Watchdog timer period expires and
the Watchdog is enabled.
•Brown-out Reset. The MCU is reset when the supply voltage V
Brown-out Reset threshold (V
•JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset
Register, one of the scan chains of the JTAG system. Refer to the section “IEEE
1149.1 (JTAG) Boundary-scan” on page 220 for details.
POT
).
is below the
) and the Brown-out Detector is enabled.
BOT
CC
34
ATmega16(L)
2466B–09/01
Figure 15. Reset Logic
DATA BUS
MCU Control and Status
Register (MCUCSR)
JTRF
BORF
PORF
WDRF
EXTRF
ATmega16(L)
BODEN
BODLEVEL
Pull-up Resistor
SPIKE
FILTER
JTAG Reset
Register
Table 15. Reset Characteristics
Brown-Out
Reset Circuit
Watchdog
Oscillator
Clock
Generator
CKSEL[3:0]
SUT[1:0]
(1)
CK
Delay Counters
TIMEOUT
SymbolParameterConditionMinTypMaxUnits
V
POT
Power-on Reset Threshold
Voltage (rising)
Power-on Reset Threshold
Voltage (falling)
(2)
TBDTBD2.3V
TBDTBD2.3V
2466B–09/01
V
V
t
V
RST
t
RST
BOT
BOD
HYST
RESET Pin Threshold VoltageTBDTBDTBDV
Minimum pulse width on
RESET
Pin
Brown-out Reset Threshold
Vol ta ge
Minimum low voltage period for
Brown-out Detection
BODLEVEL = 1TBD2.7TBD
BODLEVEL = 0TBD4.0TBD
BODLEVEL = 1TBDTBDTBDµs
BODLEVEL = 0TBDTBDTBDµs
Brown-out Detector hysteresisTBD130TBDmV
Notes:1. Values are guidelines only. Actual values are TBD.
2. The Power-on Reset will not work unless the supply voltage has been below V
(falling)
TBDTBDTBDns
V
POT
35
Power-on ResetA Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detec-
tion level is defined in Table 15. The POR is activated whenever V
is below the
CC
detection level. The POR circuit can be used to trigger the start-up reset, as well as to
detect a failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from power-on. Reaching the power-on reset threshold voltage invokes the delay counter, which determines
how long the device is kept in RESET after V
again, without any delay, when V
decreases below the detection level.
CC
rise. The RESET signal is activated
CC
Figure 16. MCU Start-up, RESET
V
V
CC
RESET
TIME-OUT
INTERNAL
RESET
POT
V
RST
t
TOUT
Figure 17. MCU Start-up, RESET
V
V
CC
RESET
TIME-OUT
POT
Tied to VCC.
Extended Externally
V
RST
t
TOUT
INTERNAL
RESET
External ResetAn external reset is generated by a low level on the RESET
than the minimum pulse width (see Table 15) will generate a reset, even if the clock is
not running. Shorter pulses are not guaranteed to generate a reset. When the applied
on its positive edge, the delay
RST
has expired.
TOUT
36
signal reaches the Reset Threshold Voltage – V
counter starts the MCU after the Time-out period t
ATmega16(L)
pin. Reset pulses longer
2466B–09/01
Figure 18. External Reset During Operation
CC
ATmega16(L)
Brown-out DetectionATmega16 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V
level during operation by comparing it to a fixed trigger level. The trigger level for the
BOD can be selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed),
or 4.0V (BODLEVEL programmed). The trigger level has a hysteresis to ensure spike
free Brown-out Detection. The hysteresis on the detection level should be interpreted as
V
BOT+
= V
BOT
+ V
HYST
/2 and V
BOT-
= V
BOT
- V
HYST
/2.
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is
enabled (BODEN programmed), and V
in Figure 19), the Brown-out Reset is immediately activated. When VCC increases
(V
BOT-
above the trigger level (V
time-out period t
has expired.
TOUT
in Figure 19), the delay counter starts the MCU after the
BOT+
The BOD circuit will only detect a drop in V
for longer than t
given in Table 15.
BOD
decreases to a value below the trigger level
CC
if the voltage stays below the trigger level
CC
Figure 19. Brown-out Reset During Operation
V
CC
RESET
TIME-OUT
V
BOT-
V
BOT+
t
TOUT
CC
2466B–09/01
INTERNAL
RESET
37
Watchdog ResetWhen the Watchdog times out, it will generate a short reset pulse of 1 CK cycle dura-
tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period
. Refer to page 39 for details on operation of the Watchdog Timer.
t
TOUT
Figure 20. Watchdog Reset During Operation
CC
CK
MCU Control and Status
Register – MCUCSR
The MCU Control and Status Register provides information on which reset source
caused an MCU reset.
Bit76543210
JTDISC2–JTRFWDRFBORFEXTRFPORFMCUCSR
Read/WriteR/WR/WRR/WR/WR/WR/WR/W
Initial Value000See Bit Description
• Bit 4 - JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register
selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on reset, or
by writing a logic zero to the flag.
• Bit 3 - WDRF: Watchdog Reset Flag
This bit is set if a watchdog reset occurs. The bit is reset by a power-on reset, or by writing a logic zero to the flag.
• Bit 2 - BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a power-on reset, or by
writing a logic zero to the flag.
• Bit 1 - EXTRF: External Reset Flag
This bit is set if an external reset occurs. The bit is reset by a power-on reset, or by writing a logic zero to the flag.
• Bit 0 - PORF: Power-on Reset Flag
38
This bit is set if a power-on reset occurs. The bit is reset only by writing a logic zero to
the flag.
To make use of the reset flags to identify a reset condition, the user should read and
then reset the MCUCSR as early as possible in the program. If the register is cleared
before another reset occurs, the source of the reset can be found by examining the reset
flags.
ATmega16(L)
2466B–09/01
ATmega16(L)
Internal Voltage
Reference
Voltage Reference Enable
Signals and Start-up Time
ATmega16 features an internal bandgap reference. This reference is used for Brownout Detection, and it can be used as an input to the Analog Comparator or the ADC. The
2.56V reference to the ADC is generated from the internal bandgap reference.
The voltage reference has a start-up time that may influence the way it should be used.
The start-up time is given in Table 16. To save power, the reference is not always turned
on. The reference is on during the following situations:
1. When the BOD is enabled (by programming the BODEN fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting
the ACBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the
user must always allow the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power consumption in Power-down mode, the user
can avoid the three conditions above to ensure that the reference is turned off before
entering Power-down mode.
Table 16. Internal Voltage Reference Characteristics
SymbolParameterConditionMinTypMaxUnits
V
BG
t
BG
I
BG
Bandgap reference voltageTBDTBD1.23TBDV
Bandgap reference start-up timeTBD4070µs
Bandgap reference current
consumption
(1)
TBD10TBDµA
Note:1. Values are guidelines only. Actual values are TBD.
Watchdog TimerThe Watchdog Timer is clocked from a separate On-chip oscillator which runs at 1 MHz.
This is the typical value at V
levels. By controlling the Watchdog Timer prescaler, the Watchdog reset interval
V
CC
can be adjusted as shown in Table 17 on page 41. The WDR – Watchdog Reset –
instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is disabled and when a chip reset occurs. Eight different clock cycle periods can be selected
to determine the reset period. If the reset period expires without another Watchdog
reset, the ATmega16 resets and executes from the Reset vector. For timing details on
the Watchdog reset, refer to page 38.
To prevent unintentional disabling of the watchdog, a special turn-off sequence must be
followed when the watchdog is disabled. Refer to the description of the Watchdog Timer
Control Register for details.
= 5V. See characterization data for typical values at other
CC
2466B–09/01
39
Watchdog Timer Control
Register – WDTCR
Figure 21. Watchdog Timer
WATCHDOG
OSCILLATOR
Bit7654 3210
–––WDTOEWDEWDP2WDP1WDP0WDTCR
Read/WriteRRRR/WR/WR/WR/WR/W
Initial Value00000000
• Bits 7..5 - Res: Reserved Bits
These bits are reserved bits in the ATmega16 and will always read as zero.
• Bit 4 - WDTOE: Watchdog Turn-off Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the watchdog
will not be disabled. Once written to one, hardware will clear this bit after four clock
cycles. Refer to the description of the WDE bit for a watchdog disable procedure.
• Bit 3 - WDE: Watchdog Enable
When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is
written to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared
if the WDTOE bit has logic level one. To disable an enabled watchdog timer, the following procedure must be followed:
1. In the same operation, write a logic one to WDTOE and WDE. A logic one must
be written to WDE even though it is set to one before the disable operation starts.
2. Within the next four clock cycles, write a logic 0 to WDE. This disables the
watchdog.
The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the
Watchdog Timer is enabled. The different prescaling values and their corresponding
Timeout Periods are shown in Table 17.
40
ATmega16(L)
2466B–09/01
Table 17. Watchdog Timer Prescale Select
(1)
ATmega16(L)
WDP2WDP1WDP0
Number of WDT
Oscillator Cycles
Typical Time-out
at VCC = 3.0V
Typical Time-out
at VCC = 5.0V
00016KTBD16 ms
00132KTBD32 ms
01064KTBD64 ms
011128KTBD0.13 s
100256KTBD0.26 s
101512KTBD0.5 s
1101,024KTBD1.0 s
1112,048KTBD2.0 s
Note:1. Values are guidelines only. Actual values are TBD.
The following code example shows one assembly and one C function for turning off the
WDT. The example assumes that interrupts are controlled (for example by disabling
interrupts globally) so that no interrupts will occur during execution of these functions.
Assembly Code Example
WDT_off:
; Write logical one to WDTOE and WDE
ldi r16, (1<<WDTOE)|(1<<WDE)
out WDTCR, r16
; Turn off WDT
ldi r16, (0<<WDE)
out WDTCR, r16
ret
2466B–09/01
C Code Example
void WDT_off(void)
{
/* Write logical one to WDTOE and WDE */
WDTCR = (1<<WDTOE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
41
InterruptsThis section describes the specifics of the interrupt handling as performed in
ATmega16. For a general explanation of the AVR interrupt handling, refer to “Reset and
Interrupt Handling” on page 11.
Interrupt Vectors in
ATm eg a1 6
Table 18. Reset and Interrupt Vectors
Program
Vector No.
1$000
2$002INT0External Interrupt Request 0
3$004INT1External Interrupt Request 1
4$006TIMER2 COMPTimer/Counter2 Compare Match
5$008TIMER2 OVFTimer/Counter2 Overflow
6$00ATIMER1 CAPTTimer/Counter1 Capture Event
7$00CTIMER1 COMPATimer/Counter1 Compare Match A
8$00ETIMER1 COMPBTimer/Counter1 Compare Match B
9$010TIMER1 OVFTimer/Counter1 Overflow
10$012TIMER0 OVFTimer/Counter0 Overflow
11$014SPI, STCSerial Transfer Complete
12$016USART, RXCUSART, Rx Complete
13$018USART, UDREUSART Data Register Empty
14$01AUSART, TXCUSART, Tx Complete
15$01CADCADC Conversion Complete
Address
(2)
(1)
SourceInterrupt Definition
RESETExternal Pin, Power-on Reset, Brown-out
Reset, Watchdog Reset, and JTAG AVR
Reset
42
16$01EEE_RDYEEPROM Ready
17$020ANA_COMPAnalog Comparator
18$022TWI2-wire Serial Interface
19$024INT2External Interrupt Request 2
20$026TIMER0 COMPTimer/Counter0 Compare Match
21$028SPM_RDYStore Program Memory Ready
Notes: 1. When the BOOTRST fuse is programmed, the device will jump to the Boot Loader
address at reset, see “Boot Loader Support – Read-While-Write Self-Programming”
on page 240.
2. When the IVSEL bit in GICR is set, interrupt vectors will be moved to the start of the
boot Flash section. The address of each interrupt vector will then be the address in
this table added to the start address of the boot Flash section.
Table 19 shows reset and interrupt vectors placement for the various combinations of
BOOTRST and IVSEL settings. If the program never enables an interrupt source, the
interrupt vectors are not used, and regular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application section while the
interrupt vectors are in the boot section or vice versa.
Note:1. The Boot Reset Address is shown in Table 100 on page 252. For the BOOTRST fuse
“1” means unprogrammed while “0” means programmed.
The most typical and general program setup for the Reset and Interrupt Vector
Addresses in ATmega16 is:
AddressLabelsCodeComments
$000jmpRESET; Reset Handler
$002jmpEXT_INT0; IRQ0 Handler
$004jmpEXT_INT1; IRQ1 Handler
$006jmpTIM2_COMP; Timer2 Compare Handler
$008jmpTIM2_OVF; Timer2 Overflow Handler
$00AjmpTIM1_CAPT; Timer1 Capture Handler
$00CjmpTIM1_COMPA; Timer1 CompareA Handler
$00EjmpTIM1_COMPB; Timer1 CompareB Handler
$010jmpTIM1_OVF; Timer1 Overflow Handler
$012jmpTIM0_OVF; Timer0 Overflow Handler
$014jmpSPI_STC; SPI Transfer Complete Handler
$016jmpUSART_RXC; USART RX Complete Handler
$018jmpUSART_UDRE; UDR Empty Handler
$01AjmpUSART_TXC; USART TX Complete Handler
$01CjmpADC; ADC Conversion Complete Handler
$01EjmpEE_RDY; EEPROM Ready Handler
$020jmpANA_COMP; Analog Comparator Handler
$022jmpTWSI; 2-wire Serial Interface Handler
$024jmpEXT_INT2; IRQ2 Handler
$026jmpTIM0_COMP; Timer0 Compare Handler
$028jmpSPM_RDY; Store Program Memory Ready Handler
;
$02ARESET:ldir16,high(RAMEND) ; Main program start
$02BoutSPH,r16; Set stack pointer to top of RAM
$02Cldir16,low(RAMEND)
$02DoutSPL,r16
$02Esei; Enable interrupts
$02F<instr> xxx
.........
2466B–09/01
43
When the BOOTRST fuse is unprogrammed, the boot section size set to 2K bytes and
the IVSEL bit in the GICR register is set before any interrupts are enabled, the most typical and general program setup for the Reset and Interrupt Vector Addresses is:
AddressLabelsCodeComments
$000RESET:ldir16,high(RAMEND) ; Main program start
$001outSPH,r16; Set stack pointer to top of RAM
$002ldir16,low(RAMEND)
$003outSPL,r16
$004sei; Enable interrupts
$005<instr> xxx
;
.org $1C02
$1C02jmpEXT_INT0; IRQ0 Handler
$1C04jmpEXT_INT1; IRQ1 Handler
.........;
$1C28jmpSPM_RDY; Store Program Memory Ready Handler
When the BOOTRST fuse is programmed and the boot section size set to 2K bytes, the
most typical and general program setup for the Reset and Interrupt Vector Addresses is:
AddressLabelsCodeComments
.org $002
$002jmpEXT_INT0; IRQ0 Handler
$004jmpEXT_INT1; IRQ1 Handler
.........;
$028jmpSPM_RDY; Store Program Memory Ready Handler
;
.org $1C00
$1C00RESET:ldir16,high(RAMEND) ; Main program start
$1C01outSPH,r16; Set stack pointer to top of RAM
$1C02ldir16,low(RAMEND)
$1C03outSPL,r16
$1C04sei; Enable interrupts
$1C05<instr> xxx
44
When the BOOTRST fuse is programmed, the boot section size set to 2K bytes and the
IVSEL bit in the GICR register is set before any interrupts are enabled, the most typical
and general program setup for the Reset and Interrupt Vector Addresses is:
$1C28jmpSPM_RDY; Store Program Memory Ready Handler
;
$1C2ARESET:ldir16,high(RAMEND) ; Main program start
$1C2BoutSPH,r16; Set stack pointer to top of RAM
$1C2Cldir16,low(RAMEND)
$1C2DoutSPL,r16
$1C2Esei; Enable interrupts
$1C2F<instr> xxx
ATmega16(L)
2466B–09/01
ATmega16(L)
Moving Interrupts Between
Application and Boot Space
General Interrupt Control
Register – GICR
The General Interrupt Control Register controls the placement of the interrupt vector
table.
Bit76543210
INT1INT0INT2–––IVSELIVCEGICR
Read/WriteR/WR/WR/WRRRR/WR/W
Initial Value00000000
• Bit 1 - IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the interrupt vectors are placed at the start of the
Flash memory. When this bit is set (one), the interrupt vectors are moved to the beginning of the Boot Loader section of the Flash. The actual address of the start of the boot
Flash section is determined by the BOOTSZ fuses. Refer to the section “Boot Loader
Support – Read-While-Write Self-Programming” on page 240 for details. To avoid unintentional changes of interrupt vector tables, a special write procedure must be followed
to change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are
disabled in the cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not written, interrupts remain disabled for four
cycles. The I-bit in the Status Register is unaffected by the automatic disabling.
Note: If interrupt vectors are placed in the Boot Loader section and Boot Lock bit BLB02
is programmed, interrupts are disabled while executing from the Application section. If
interrupt vectors are placed in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer to
the section “Boot Loader Support – Read-While-Write Self-Programming” on page 240
for details on Boot Lock bits.
• Bit 0 - IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is
cleared by hardware four cycles after it is written or when IVSEL is written. Setting the
IVCE bit will disable interrupts, as explained in the IVSEL description above. See Code
Example below.
2466B–09/01
45
Assembly Code Example
Move_interrupts:
; Enable change of interrupt vectors
ldi r16, (1<<IVCE)
out GICR, r16
; Move interrupts to boot Flash section
ldi r16, (1<<IVSEL)
out GICR, r16
ret
C Code Example
void Move_interrupts(void)
{
/* Enable change of interrupt vectors */
GICR = (1<<IVCE);
/* Move interrupts to boot Flash section */
GICR = (1<<IVSEL);
}
46
ATmega16(L)
2466B–09/01
ATmega16(L)
I/O Ports
IntroductionAll AVR ports have true Read-Modify-Write functionality when used as general digital
I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The
same applies when changing drive value (if configured as output) or enabling/disabling
of pull-up resistors (if configured as input). Each output buffer has symmetrical drive
characteristics with both high sink and source capability. The pin driver is strong enough
to drive LED displays directly. All port pins have individually selectable pull-up resistors
with a supply-voltage invariant resistance. All I/O pins have protection diodes to both
and Ground as indicated in Figure 22. Refer to “Electrical Characteristics” on page
V
CC
282 for a complete list of parameters.
Figure 22. I/O Pin Equivalent Schematic
R
pu
Pxn
C
pin
"General Digital I/O" for
All registers and bit references in this section are written in general form. A lower case
“x” represents the numbering letter for the port, and a lower case “n” represents the bit
number. However, when using the register or bit defines in a program, the precise form
must be used. i.e., PORTB3 for bit no. 3 in Port B, here documented generally as
PORTxn. The physical I/O registers and bit locations are listed in “Register Description
for I/O Ports” on page 62.
Three I/O memory address locations are allocated for each port, one each for the Data
Register – PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The
Port Input Pins I/O location is read only, while the Data Register and the Data Direction
Register are read/write. In addition, the Pull-up Disable – PUD bit in SFIOR disables the
pull-up function for all pins in all ports when set.
Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on
page 48. Most port pins are multiplexed with alternate functions for the peripheral features on the device. How each alternate function interferes with the port pin is described
in “Alternate Port Functions” on page 52. Refer to the individual module sections for a
full description of the alternate functions.
Logic
See Figure 23
Details
2466B–09/01
Note that enabling the alternate function of some of the port pins does not affect the use
of the other pins in the port as general digital I/O.
47
Ports as General Digital
I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 23 shows a
functional description of one I/O-port pin, here generically called Pxn.
Figure 23. General Digital I/O
Pxn
(1)
SLEEP
SYNCHRONIZER
DLQ
D
PINxn
Q
PUD
Q
D
DDxn
Q
CLR
RESET
Q
D
PORTxn
Q
CLR
RESET
Q
Q
WDx
RDx
WPx
RRx
RPx
clk
DATA B US
I/O
PUD:PULLUP DISABLE
SLEEP:SLEEP CONTROL
clk
:I/O CLOCK
I/O
Note:1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk
Configuring the PinEach port pin consists of 3 Register bits: DDxn, PORTxn, and PINxn. As shown in “Reg-
ister Description for I/O Ports” on page 62, the DDxn bits are accessed at the DDRx I/O
address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O
address.
The DDxn bit in the DDRx register selects the direction of this pin. If DDxn is written
logic one, Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up
resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic
zero or the pin has to be configured as an output pin. The port pins are tri-stated when a
reset condition becomes active, even if no clocks are running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is
driven high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero).
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn,
PORTxn} = 0b11), an intermediate state with either pull-up enabled ({DDxn, PORTxn} =
0b01) or output low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up
,
48
ATmega16(L)
2466B–09/01
ATmega16(L)
enabled state is fully acceptable, as a high-impedant environment will not notice the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in
the SFIOR register can be set to disable all pull-ups in all ports.
Switching between input with pull-up and output low generates the same problem. The
user must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state
({DDxn, PORTxn} = 0b10) as an intermediate step.
Table 20 summarizes the control signals for the pin value.
Table 20. Port Pin Configurations
PUD
DDxnPORTxn
00XInputNoTri-state (Hi-Z)
010InputYes
011InputNoTri-state (Hi-Z)
10XOutputNoOutput Low (Sink)
11XOutputNoOutput High (Source)
(in SFIOR)I/OPull-upComment
Pxn will source current if ext. pulled
low.
Reading the Pin ValueIndependent of the setting of Data Direction bit DDxn, the port pin can be read through
the PINxn Register Bit. As shown in Figure 23, the PINxn Register bit and the preceding
latch constitute a synchronizer. This is needed to avoid metastability if the physical pin
changes value near the edge of the internal clock, but it also introduces a delay. Figure
24 shows a timing diagram of the synchronization when reading an externally applied
pin value. The maximum and minimum propagation delays are denoted t
pd,max
and t
pd,min
respectively.
Figure 24. Synchronization when Reading an Externally Applied Pin Value
SYSTEM CLK
INSTRUCTIONS
SYNC LATCH
PINxn
r17
t
pd, max
0x00
XXXXXX
t
pd, min
in r17, PINx
0xFF
2466B–09/01
Consider the clock period starting shortly after the first falling edge of the system clock.
The latch is closed when the clock is low, and goes transparent when the clock is high,
as indicated by the shaded region of the “SYNC LATCH” signal. The signal value is
latched when the system clock goes low. It is clocked into the PINxn register at the suc-
49
ceeding positive clock edge. As indicated by the two arrows t
pd,max
and t
pd,min
, a single
signal transition on the pin will be delayed between ½ and 1-½ system clock period
depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as
indicated in Figure 25. The out instruction sets the “SYNC LATCH” signal at the positive
edge of the clock. In this case, the delay t
through the synchronizer is 1 system clock
pd
period.
Figure 25. Synchronization when Reading a Software Assigned Pin Value
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
out PORTx, r16
0xFF
nopin r17, PINx
0x00
t
pd
0xFF
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and
define the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The
resulting pin values are read back again, but as previously discussed, a nop instruction
is included to be able to read back the value recently assigned to some of the pins.
50
ATmega16(L)
2466B–09/01
ATmega16(L)
Assembly Code Example
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldir16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)
ldir17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)
outPORTB,r16
outDDRB,r17
; Insert nop for synchronization
nop
; Read port pins
inr16,PINB
...
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
_NOP();
/* Read port pins */
i = PINB;
...
(1)
(1)
Digital Input Enable and Sleep
Modes
Note:1. For the assembly program, two temporary registers are used to minimize the time
from pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set,
defining bit 2 and 3 as low and redefining bits 0 and 1 as strong high drivers.
As shown in Figure 23, the digital input signal can be clamped to ground at the input of
the schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep
Controller in Power-down mode, Power-save mode, Standby mode, and Extended
Standby mode to avoid high power consumption if some input signals are left floating, or
have an analog signal level close to V
CC
/2.
SLEEP is overridden for port pins enabled as External Interrupt pins. If the External
Interrupt Request is not enabled, SLEEP is active also for these pins. SLEEP is also
overridden by various other alternate functions as described in “Alternate Port Functions” on page 52.
If a logic high level (“one”) is present on an Asynchronous External Interrupt pin configured as “Interrupt on Any Logic Change on Pin” while the external interrupt is not
enabled, the corresponding External Interrupt Flag will be set when resuming from the
above mentioned Sleep Modes, as the clamping in these Sleep Modes produces the
requested logic change.
2466B–09/01
51
Alternate Port FunctionsMost port pins have alternate functions in addition to being general digital I/Os. Figure
26 shows how the port pin control signals from the simplified Figure 23 can be overridden by alternate functions. The overriding signals may not be present in all port pins, but
the figure serves as a generic description applicable to all port pins in the AVR microcontroller family.
Figure 26. Alternate Port Functions
1
0
1
0
Pxn
1
0
1
0
(1)
PUOExn
PUOVxn
DDOExn
DDOVxn
PVOExn
PVOVxn
DIEOExn
DIEOVxn
SLEEP
SYNCHRONIZER
SET
DLQ
CLR
D
PINxn
Q
PUD
D
Q
DDxn
Q
CLR
WDx
RESET
RDx
D
Q
PORTxn
Q
CLR
WPx
DATA B US
RESET
RRx
RPx
Q
Q
CLR
52
ATmega16(L)
clk
I/O
DIxn
AIOxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP:SLEEP CONTROL
Note:1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
I/O
SLEEP, and PUD are common to all ports. All other signals are unique for each pin.
Table 21 summarizes the function of the overriding signals. The pin and port indexes
from Figure 26 are not shown in the succeeding tables. The overriding signals are generated internally in the modules having the alternate function.
2466B–09/01
,
ATmega16(L)
Table 21. Generic Description of Overriding Signals for Alternate Functions
Signal NameFull NameDescription
PUOEPull-up Override
Enable
PUOVPull-up Override
Val ue
DDOEData Direction
Override Enable
DDOVData Direction
Override Value
PVOEPort Value Override
Enable
If this signal is set, the pull-up enable is controlled by
the PUOV signal. If this signal is cleared, the pull-up is
enabled when {DDxn, PORTxn, PUD} = 0b010.
If PUOE is set, the pull-up is enabled/disabled when
PUOV is set/cleared, regardless of the setting of the
DDxn, PORTxn, and PUD register bits.
If this signal is set, the Output Driver Enable is
controlled by the DDOV signal. If this signal is cleared,
the Output driver is enabled by the DDxn register bit.
If DDOE is set, the Output Driver is enabled/disabled
when DDOV is set/cleared, regardless of the setting of
the DDxn register bit.
If this signal is set and the Output Driver is enabled,
the port value is controlled by the PVOV signal. If
PVOE is cleared, and the Output Driver is enabled, the
port Value is controlled by the PORTxn register bit.
Special Function I/O Register
– SFIOR
PVOVPort Value Override
Val ue
DIEOEDigital Input Enable
Override Enable
DIEOVDigital Input Enable
Override Value
DIDigital InputThis is the Digital Input to alternate functions. In the
AIOAnalog Input/ outputThis is the Analog Input/output to/from alternate
If PVOE is set, the port value is set to PVOV,
regardless of the setting of the PORTxn register bit.
If this bit is set, the Digital Input Enable is controlled by
the DIEOV signal. If this signal is cleared, the Digital
Input Enable is determined by MCU-state (Normal
Mode, Sleep Modes).
If DIEOE is set, the Digital Input is enabled/disabled
when DIEOV is set/cleared, regardless of the MCU
state (Normal Mode, Sleep Modes).
figure, the signal is connected to the output of the
schmitt trigger but before the synchronizer. Unless the
Digital Input is used as a clock source, the module with
the alternate function will use its own synchronizer.
functions. The signal is connected directly to the pad,
and can be used bidirectionally.
The following subsections shortly describe the alternate functions for each port, and
relate the overriding signals to the alternate function. Refer to the alternate function
description for further details.
Bit76543210
ADTS2ADTS1ADTS0ADHSMACMEPUDPSR2PSR10SFIOR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
2466B–09/01
• Bit 2 - PUD: Pull-up disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn
and PORTxn registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01).
See “Configuring the Pin” on page 48 for more details about this feature.
53
Alternate Functions of Port APort A has an alternate function as analog input for the ADC as shown in Table 22. If
some Port A pins are configured as outputs, it is essential that these do not switch when
a conversion is in progress. This might corrupt the result of the conversion.
Table 22. Port A Pins Alternate Functions
Port PinAlternate Function
PA7ADC7 (ADC input channel 7)
PA6ADC6 (ADC input channel 6)
PA5ADC5 (ADC input channel 5)
PA4ADC4 (ADC input channel 4)
PA3ADC3 (ADC input channel 3)
PA2ADC2 (ADC input channel 2)
PA1ADC1 (ADC input channel 1)
PA0ADC0 (ADC input channel 0)
Table 23 and Table 24 relate the alternate functions of Port A to the overriding signals
shown in Figure 26 on page 52.
Table 23. Overriding Signals for Alternate Functions in PA7..PA4
Signal NamePA7/ADC7PA6/ADC6PA5/ADC5PA4/ADC4
PUOE 0000
PUOV 0000
DDOE 0000
DDOV 0000
PVOE 0000
PVOV0000
DIEOE 0000
DIEOV 0000
DI––––
AIOADC7 INPUTADC6 INPUTADC5 INPUTADC4 INPUT
54
ATmega16(L)
2466B–09/01
Table 24. Overriding Signals for Alternate Functions in PA3..PA0
Signal NamePA3/ADC3PA2/ADC2PA1/ADC1PA0/ADC0
PUOE 0000
PUOV0000
DDOE 0000
DDOV 0000
PVOE0000
PVOV0000
DIEOE 0000
DIEOV 0000
DI––––
AIOADC3 INPUTADC2 INPUTADC1 INPUTADC0 INPUT
Alternate Functions of Port BThe Port B pins with alternate functions are shown in Table 25.
SCK: Master clock output, slave clock input pin for SPI channel. When the SPI is
enabled as a slave, this pin is configured as an input regardless of the setting of DDB7.
When the SPI is enabled as a master, the data direction of this pin is controlled by
DDB7. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB7 bit.
• MISO - Port B, Bit 6
2466B–09/01
MISO: Master data input, slave data output pin for SPI channel. When the SPI is
enabled as a master, this pin is configured as an input regardless of the setting of
DDB6. When the SPI is enabled as a slave, the data direction of this pin is controlled by
DDB6. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB6 bit.
55
• MOSI - Port B, Bit 5
MOSI: SPI Master data output, slave data input for SPI channel. When the SPI is
enabled as a slave, this pin is configured as an input regardless of the setting of DDB5.
When the SPI is enabled as a master, the data direction of this pin is controlled by
DDB5. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.
- Port B, Bit 4
• SS
SS
: Slave Select input. When the SPI is enabled as a slave, this pin is configured as an
input regardless of the setting of DDB4. As a slave, the SPI is activated when this pin is
driven low. When the SPI is enabled as a master, the data direction of this pin is controlled by DDB4. When the pin is forced by the SPI to be an input, the pull-up can still be
controlled by the PORTB4 bit.
• AIN1/OC0 - Port B, Bit 3
AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up switched off to avoid the digital port function from interfering with the function
of the analog comparator.
OC0, Output compare match output: The PB3 pin can serve as an external output for
the Timer/Counter0 compare match. The PB3 pin has to be configured as an output
(DDB3 set (one)) to serve this function. The OC0 pin is also the output pin for the PWM
mode timer function.
• AIN0/INT2 - Port B, Bit 2
AIN0, Analog Comparator Positive Input. Configure the port pin as input with the internal
pull-up switched off to avoid the digital port function from interfering with the function of
the analog comparator.
INT2, External Interrupt source 2: The PB2 pin can serve as an external interrupt source
to the MCU.
• T1 - Port B, Bit 1
T1, Timer/Counter1 counter source.
• T0/XCK - Port B, Bit 0
T0, Timer/Counter0 counter source.
XCK, USART external clock. The Data Direction Register (DDB0) controls whether the
clock is output (DDB0 set) or input (DDB0 cleared). The XCK pin is active only when the
USART operates in synchronous mode.
Table 26 and Table 27 relate the alternate functions of Port B to the overriding signals
shown in Figure 26 on page 52. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute
the MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE
INPUT.
56
ATmega16(L)
2466B–09/01
ATmega16(L)
Table 26. Overriding Signals for Alternate Functions in PB7..PB4
Table 27. Overriding Signals for Alternate Functions in PB3..PB0
Signal
NamePB3/OC0/AIN1PB2/INT2/AIN0PB1/T1PB0/T0/XCK
PUOE0000
PUOV0000
DDOE0000
DDOV0000
PVOEOC0 ENABLE00UMSEL
PVOVOC000XCK OUTPUT
DIEOE0INT2 ENABLE00
DIEOV0100
2466B–09/01
DI–INT2 INPUTT1 INPUTXCK INPUT/T0 INPUT
AIOAIN1 INPUTAIN0 INPUT––
57
Alternate Functions of Port CThe Port C pins with alternate functions are shown in Table 28. If the JTAG interface is
enabled, the pull-up resistors on pins PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.
Table 28. Port C Pins Alternate Functions
Port PinAlternate Function
PC7TOSC2 (Timer Oscillator Pin 2)
PC6TOSC1 (Timer Oscillator Pin 1)
PC5TDI (JTAG Test Data In)
PC4TDO (JTAG Test Data Out)
PC 3TM S ( JTAG Te st M ode Se le ct )
PC2TCK (JTAG Test Clock)
PC1SDA (2-wire Serial Bus Data Input/Output Line)
PC0SCL (2-wire Serial Bus Clock Line)
The alternate pin configuration is as follows:
• TOSC2 - Port C, Bit 7
TOSC2, Timer Oscillator pin 2: When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PC7 is disconnected from the port, and
becomes the inverting output of the oscillator amplifier. In this mode, a crystal oscillator
is connected to this pin, and the pin can not be used as an I/O pin.
• TOSC1 - Port C, Bit 6
TOSC1, Timer Oscillator pin 1: When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PC6 is disconnected from the port, and
becomes the input of the inverting oscillator amplifier. In this mode, a crystal oscillator is
connected to this pin, and the pin can not be used as an I/O pin.
• TDI - Port C, Bit 5
TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or
Data Register (scan chains). When the JTAG interface is enabled, this pin can not be
used as an I/O pin.
• TDO - Port C, Bit 4
TDO, JTAG Test Data Out: Serial output data from Instruction register or Data Register.
When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TMS - Port C, Bit 3
TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller
state machine. When the JTAG interface is enabled, this pin can not be used as an I/O
pin.
• TCK - Port C, Bit 2
TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• SDA - Port C, Bit 1
SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable
the 2-wire Serial Interface, pin PC1 is disconnected from the port and becomes the
Serial Data I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on
the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by
58
ATmega16(L)
2466B–09/01
ATmega16(L)
an open drain driver with slew-rate limitation. When this pin is used by the 2-wire Serial
Interface, the pull-up can still be controlled by the PORTC1 bit.
• SCL - Port C, Bit 0
SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable
the 2-wire Serial Interface, pin PC0 is disconnected from the port and becomes the
Serial Clock I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on
the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by
an open drain driver with slew-rate limitation. When this pin is used by the 2-wire Serial
Interface, the pull-up can still be controlled by the PORTC0 bit.
Table 29 and Table 30 relate the alternate functions of Port C to the overriding signals
shown in Figure 26 on page 52.
Table 29. Overriding Signals for Alternate Functions in PC7..PC4
Signal
NamePC7/TOSC2PC6/TOSC1PC5/TDIPC4/TDO
PUOEAS2AS2JTAGENJTAGEN
PUOV0010
DDOEAS2AS2JTAGENJTAGEN
DDOV000SHIFT_IR + SHIFT_DR
PVOE000JTAGEN
PVOV000TDO
DIEOEAS2AS2JTAGENJTAGEN
DIEOV0000
DI––––
AIOT/C2 OSC OUTPUTT/C2 OSC INPUTTDI–
Table 30. Overriding Signals for Alternate Functions in PC3..PC0
Signal
NamePC3/TMSPC2/TCKPC1/SDAPC0/SCL
PUOEJTAGENJTAGENTWENTWEN
PUOV11PORTC1 • PUD
DDOEJTAGENJTAGENTWENTWEN
DDOV00SDA_OUTSCL_OUT
PVOE00TWENTWEN
PVOV0000
DIEOEJTAGENJTAGEN00
(1)
PORTC0 • PUD
2466B–09/01
DIEOV0000
DI––––
AIOTMSTCKSDA INPUTSCL INPUT
Note:1. When enabled, the Two-Wire Serial Interface enables slew-rate controls on the output
pins PC0 and PC1. This is not shown in the figure. In addition, spike filters are connected between the AIO outputs shown in the port figure and the digital logic of the
TWI module.
59
Alternate Functions of Port DThe Port D pins with alternate functions are shown in Table 31.
Table 31. Port D Pins Alternate Functions
Port PinAlternate Function
PD7OC2 (Timer/Counter2 Output Compare Match Output)
PD6ICP (Timer/Counter1 Input Capture Pin)
PD5OC1A (Timer/Counter1 Output CompareA Match Output)
PD4OC1B (Timer/Counter1 Output CompareB Match Output)
PD3INT1 (External Interrupt 1 Input)
PD2INT0 (External Interrupt 0 Input)
PD1TXD (USART Output Pin)
PD0RXD (USART Input Pin)
The alternate pin configuration is as follows:
• OC2 - Port D, Bit 7
OC2, Timer/Counter2 Output Compare match output: The PD7 pin can serve as an
external output for the Timer/Counter2 output compare. The pin has to be configured as
an output (DDD7 set (one)) to serve this function. The OC2 pin is also the output pin for
the PWM mode timer function.
• ICP - Port D, Bit 6
ICP - Input Capture Pin: The PD6 pin can act as an input capture pin for
Timer/Counter1.
• OC1A - Port D, Bit 5
OC1A, Output Compare matchA output: The PD5 pin can serve as an external output
for the Timer/Counter1 output compareA. The pin has to be configured as an output
(DDD5 set (one)) to serve this function. The OC1A pin is also the output pin for the
PWM mode timer function.
• OC1B - Port D, Bit 4
OC1B, Output Compare matchB output: The PD4 pin can serve as an external output
for the Timer/Counter1 output compareB. The pin has to be configured as an output
(DDD4 set (one)) to serve this function. The OC1B pin is also the output pin for the
PWM mode timer function.
• INT1 - Port D, Bit 3
INT1, External Interrupt source 1: The PD3 pin can serve as an external interrupt
source.
• INT0 - Port D, Bit 2
INT0, External Interrupt source 0: The PD2 pin can serve as an external interrupt
source.
• TXD - Port D, Bit 1
TXD, Transmit Data (Data output pin for the USART). When the USART transmitter is
enabled, this pin is configured as an output regardless of the value of DDD1.
• RXD - Port D, Bit 0
60
RXD, Receive Data (Data input pin for the USART). When the USART receiver is
enabled this pin is configured as an input regardless of the value of DDD0. When the
ATmega16(L)
2466B–09/01
ATmega16(L)
USART forces this pin to be an input, the pull-up can still be controlled by the PORTD0
bit.
Table 32 and Table 33 relate the alternate functions of Port D to the overriding signals
shown in Figure 26 on page 52.
Table 32. Overriding Signals for Alternate Functions PD7..PD4
Signal NamePD7/OC2PD6/ICPPD5/OC1APD4/OC1B
PUOE0000
PUOV0000
DDOE0000
DDOV0000
PVOEOC2 ENABLE0OC1A ENABLEOC1B ENABLE
PVOVOC20OC1AOC1B
DIEOE0000
DIEOV0000
DI–ICP INPUT––
AIO––– –
Table 33. Overriding Signals for Alternate Functions in PD3..PD0
External InterruptsThe external interrupts are triggered by the INT0, INT1, and INT2 pins. Observe that, if
enabled, the interrupts will trigger even if the INT0..2 pins are configured as outputs.
This feature provides a way of generating a software interrupt. The external interrupts
can be triggered by a falling or rising edge or a low level (INT2 is only an edge triggered
interrupt). This is set up as indicated in the specification for the MCU Control Register –
MCUCR and MCU Control and Status Register – MCUCSR. When the external interrupt
is enabled and is configured as level triggered (only INT0/INT1), the interrupt will trigger
as long as the pin is held low. Note that recognition of falling or rising edge interrupts on
INT0 and INT1 requires the presence of an I/O clock, described in “Clock Systems and
their Distribution” on page 22. Low level interrupts on INT0/INT1 and the edge interrupt
on INT2 are detected asynchronously. This implies that these interrupts can be used for
waking the part also from sleep modes other than Idle mode. The I/O clock is halted in
all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down Mode, the
changed level must be held for some time to wake up the MCU. This makes the MCU
less sensitive to noise. The changed level is sampled twice by the watchdog oscillator
clock. The period of the watchdog oscillator is 1 µs (nominal) at 5.0V and 25
quency of the watchdog oscillator is voltage dependent as shown in “Electrical
Characteristics” on page 282. The MCU will wake up if the input has the required level
during this sampling or if it is held until the end of the start-up time. The start-up time is
defined by the SUT fuses as described in “System Clock and Clock Options” on page
22. If the level is sampled twice by the watchdog oscillator clock but disappears before
the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The required level must be held long enough for the MCU to complete the wake up
to trigger the level interrupt.
°C. The fre-
MCU Control Register –
MCUCR
The MCU Control Register contains control bits for interrupt sense control and general
MCU functions.
Bit76543210
SM2SESM1SM0ISC11ISC10ISC01ISC00MCUCR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 3, 2 - ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0
The External Interrupt 1 is activated by the external pin INT1 if the SREG I-bit and the
corresponding interrupt mask in the GICR are set. The level and edges on the external
INT1 pin that activate the interrupt are defined in Table 34. The value on the INT1 pin is
sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last
longer than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held
until the completion of the currently executing instruction to generate an interrupt.
Table 34. Interrupt 1 Sense Control
ISC11ISC10Description
00The low level of INT1 generates an interrupt request.
01Any logical change on INT1 generates an interrupt request.
10The falling edge of INT1 generates an interrupt request.
11The rising edge of INT1 generates an interrupt request.
64
ATmega16(L)
2466B–09/01
MCU Control and Status
Register – MCUCSR
ATmega16(L)
• Bit 1, 0 - ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0
The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the
corresponding interrupt mask are set. The level and edges on the external INT0 pin that
activate the interrupt are defined in Table 35. The value on the INT0 pin is sampled
before detecting edges. If edge or toggle interrupt is selected, pulses that last longer
than one clock period will generate an interrupt. Shorter pulses are not guaranteed to
generate an interrupt. If low level interrupt is selected, the low level must be held until
the completion of the currently executing instruction to generate an interrupt.
Table 35. Interrupt 0 Sense Control
ISC01ISC00Description
00The low level of INT0 generates an interrupt request.
01Any logical change on INT0 generates an interrupt request.
10The falling edge of INT0 generates an interrupt request.
11The rising edge of INT0 generates an interrupt request.
Bit76543210
JTDISC2–JTRFWDRFBORFEXTRFPORFMCUCSR
Read/WriteR/WR/WRR/WR/WR/WR/WR/W
Initial Value000See Bit Description
General Interrupt Control
Register – GICR
• Bit 6 - ISC2: Interrupt Sense Control 2
The asynchronous external interrupt 2 is activated by the external pin INT2 if the SREG
I-bit and the corresponding interrupt mask in GICR are set. If ISC2 is written to zero, a
falling edge on INT2 activates the interrupt. If ISC2 is written to one, a rising edge on
INT2 activates the interrupt. Edges on INT2 are registered asynchronously. Pulses on
INT2 wider than the minimum pulse width given in Table 36 will generate an interrupt.
Shorter pulses are not guaranteed to generate an interrupt. When changing the ISC2
bit, an interrupt can occur. Therefore, it is recommended to first disable INT2 by clearing
its Interrupt Enable bit in the GICR register. Then, the ISC2 bit can be changed. Finally,
the INT2 interrupt flag should be cleared by writing a logical one to its Interrupt Flag bit
(INTF2) in the GIFR register before the interrupt is re-enabled.
Minimum pulse width for
asynchronous external interrupt
INT1INT0INT2
TBD50TBDns
–––IVSELIVCEGICR
• Bit 7 - INT1: External Interrupt Request 1 Enable
2466B–09/01
When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one),
the external pin interrupt is enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and
ISC10) in the MCU general Control Register (MCUCR) define whether the external
65
General Interrupt Flag
Register – GIFR
interrupt is activated on rising and/or falling edge of the INT1 pin or level sensed. Activity
on the pin will cause an interrupt request even if INT1 is configured as an output. The
corresponding interrupt of External Interrupt Request 1 is executed from the INT1 interrupt vector.
• Bit 6 - INT0: External Interrupt Request 0 Enable
When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one),
the external pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and
ISC00) in the MCU general Control Register (MCUCR) define whether the external
interrupt is activated on rising and/or falling edge of the INT0 pin or level sensed. Activity
on the pin will cause an interrupt request even if INT0 is configured as an output. The
corresponding interrupt of External Interrupt Request 0 is executed from the INT0 interrupt vector.
• Bit 5 - INT2: External Interrupt Request 2 Enable
When the INT2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one),
the external pin interrupt is enabled. The Interrupt Sense Control2 bit (ISC2) in the MCU
Control and Status Register (MCUCSR) defines whether the external interrupt is activated on rising or falling edge of the INT2 pin. Activity on the pin will cause an interrupt
request even if INT2 is configured as an output. The corresponding interrupt of External
Interrupt Request 2 is executed from the INT2 interrupt vector.
Bit76543210
INTF1INTF0INTF2
Read/WriteR/WR/WR/WRRRRR
Initial Value00000000
–––––GIFR
• Bit 7 - INTF1: External Interrupt Flag 1
When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1
becomes set (one). If the I-bit in SREG and the INT1 bit in GICR are set (one), the MCU
will jump to the corresponding interrupt vector. The flag is cleared when the interrupt
routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
This flag is always cleared when INT1 is configured as a level interrupt.
• Bit 6 - INTF0: External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0
becomes set (one). If the I-bit in SREG and the INT0 bit in GICR are set (one), the MCU
will jump to the corresponding interrupt vector. The flag is cleared when the interrupt
routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
This flag is always cleared when INT0 is configured as a level interrupt.
• Bit 5 - INTF2: External Interrupt Flag 2
When an event on the INT2 pin triggers an interrupt request, INTF2 becomes set (one).
If the I-bit in SREG and the INT2 bit in GICR are set (one), the MCU will jump to the corresponding interrupt vector. The flag is cleared when the interrupt routine is executed.
Alternatively, the flag can be cleared by writing a logical one to it. Note that when entering some sleep modes with the INT2 interrupt disabled, the input buffer on this pin will
be disabled. This may cause a logic change in internal signals which will set the INTF2
flag. See “Digital Input Enable and Sleep Modes” on page 51 for more information.
66
ATmega16(L)
2466B–09/01
ATmega16(L)
8-bit Timer/Counter0
with PWM
Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The
main features are:
• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)
OverviewA simplified block diagram of the 8-bit Timer/Counter is shown in Figure 27. For the
actual placement of I/O pins, refer to “Pinouts ATmega16” on page 2. CPU accessible
I/O registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O
register and bit locations are listed in the “8-bit Timer/Counter Register Description” on
page 77.
Figure 27. 8-bit Timer/Counter Block Diagram
TCCRn
count
clear
direction
BOTTOM
Control Logic
TOP
clk
Tn
Clock Select
Edge
Detector
TOVn
(Int.Req.)
Tn
Wavefo rm
Generation
( From Prescaler )
OCn
(Int.Req.)
OCn
Timer/Counter
TCNTn
= 0
=
0xFF
DATA BU S
=
OCRn
RegistersThe Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers.
Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer
Interrupt Flag Register (TIFR). All interrupts are individually masked with the Timer
Interrupt Mask register (TIMSK). TIFR and TIMSK are not shown in the figure since
these registers are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock
source on the T0 pin. The Clock Select logic block controls which clock source and edge
the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is
inactive when no clock source is selected. The output from the clock select logic is
referred to as the timer clock (clk
T0
).
2466B–09/01
67
The double buffered Output Compare Register (OCR0) is compared with the
Timer/Counter value at all times. The result of the compare can be used by the waveform generator to generate a PWM or variable frequency output on the Output Compare
Pin (OC0). See “Output Compare Unit” on page 69. for details. The compare match
event will also set the compare flag (OCF0) which can be used to generate an output
compare interrupt request.
DefinitionsMany register and bit references in this document are written in general form. A lower
case “n” replaces the Timer/Counter number, in this case 0. However, when using the
register or bit defines in a program, the precise form must be used i.e., TCNT0 for
accessing Timer/Counter0 counter value and so on.
The definitions in Table 37 are also used extensively throughout the document.
Table 37. Definitions
BOTTOMThe counter reaches the BOTTOM when it becomes 0x00
MAXThe counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOPThe counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR0 register. The
assignment is dependent on the mode of operation.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the clock select logic which is controlled by the clock select
(CS02:0) bits located in the Timer/Counter control register (TCCR0). For details on
clock sources and prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on
page 81.
Counter UnitThe main part of the 8-bit Timer/Counter is the programmable bidirectional counter unit.
Figure 28 shows a block diagram of the counter and its surroundings.
Figure 28. Counter Unit Block Diagram
TOVn
TOP
(Int. Req.)
clk
Tn
Clock Select
Edge
Detector
( From Prescaler )
Tn
DATA BU S
count
TCNTnControl Logic
clear
direction
BOTTOM
Signal description (internal signals):
countIncrement or decrement TCNT0 by 1.
68
ATmega16(L)
directionSelect between increment and decrement.
clearClear TCNT0 (set all bits to zero).
clk
Tn
Timer/counter clock, referred to as clkT0 in the following.
TOPSignalize that TCNT0 has reached maximum value.
BOTTOMSignalize that TCNT0 has reached minimum value (zero).
2466B–09/01
ATmega16(L)
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk
clock source, selected by the clock select bits (CS02:0). When no clock source is
selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed
by the CPU, regardless of whether clk
priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits
located in the Timer/Counter control register (TCCR0). There are close connections
between how the counter behaves (counts) and how waveforms are generated on the
output compare output OC0. For more details about advanced counting sequences and
waveform generation, see “Modes of Operation” on page 71.
The Timer/Counter overflow (TOV0) flag is set according to the mode of operation
selected by the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
Output Compare UnitThe 8-bit comparator continuously compares TCNT0 with the output compare register
(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will
set the output compare flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1
and global interrupt flag in SREG is set), the output compare flag generates an output
compare interrupt. The OCF0 flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0 flag can be cleared by software by writing a logical one to
its I/O bit location. The waveform generator uses the match signal to generate an output
according to operating mode set by the WGM01:0 bits and compare output mode
(COM01:0) bits. The max and bottom signals are used by the waveform generator for
handling the special cases of the extreme values in some modes of operation (See
“Modes of Operation” on page 71.).
). clkT0 can be generated from an external or internal
T0
is present or not. A CPU write overrides (has
T0
Figure 29 shows a block diagram of the output compare unit.
Figure 29. Output Compare Unit, Block Diagram
DATA B U S
top
bottom
FOCn
OCRn
=
(8-bit Comparator )
Waveform Generator
WGMn1:0
COMn1:0
TCNTn
OCFn (Int.Req.)
OCn
The OCR0 register is double buffered when using any of the pulse width modulation
(PWM) modes. For the normal and clear timer on compare (CTC) modes of operation,
the double buffering is disabled. The double buffering synchronizes the update of the
2466B–09/01
69
OCR0 compare register to either top or bottom of the counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby
making the output glitch-free.
The OCR0 register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR0 buffer register, and if double
buffering is disabled the CPU will access the OCR0 directly.
Force Output CompareIn non-PWM waveform generation modes, the match output of the comparator can be
forced by writing a one to the force output compare (FOC0) bit. Forcing compare match
will not set the OCF0 flag or reload/clear the timer, but the OC0 pin will be updated as if
a real compare match had occurred (the COM01:0 bits settings define whether the OC0
pin is set, cleared or toggled).
Compare Match Blocking by
TCNT0 Write
Using the Output Compare
Unit
Compare Match Output
Unit
All CPU write operations to the TCNT0 register will block any compare match that occur
in the next timer clock cycle, even when the timer is stopped. This feature allows OCR0
to be initialized to the same value as TCNT0 without triggering an interrupt when the
Timer/Counter clock is enabled.
Since writing TCNT0 in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNT0 when using the output
compare channel, independently of whether the Timer/Counter is running or not. If the
value written to TCNT0 equals the OCR0 value, the compare match will be missed,
resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value
equal to BOTTOM when the counter is downcounting.
The setup of the OC0 should be performed before setting the data direction register for
the port pin to output. The easiest way of setting the OC0 value is to use the force output
compare (FOC0) strobe bits in normal mode. The OC0 register keeps its value even
when changing between waveform generation modes.
Be aware that the COM01:0 bits are not double buffered together with the compare
value. Changing the COM01:0 bits will take effect immediately.
The compare output mode (COM01:0) bits have two functions. The waveform generator
uses the COM01:0 bits for defining the output compare (OC0) state at the next compare
match. Also, the COM01:0 bits control the OC0 pin output source. Figure 30 shows a
simplified schematic of the logic affected by the COM01:0 bit setting. The I/O registers,
I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O
port control registers (DDR and PORT) that are affected by the COM01:0 bits are
shown. When referring to the OC0 state, the reference is for the internal OC0 register,
not the OC0 pin. If a system reset occur, the OC0 register is reset to “0”.
70
ATmega16(L)
2466B–09/01
Figure 30. Compare Match Output Unit, Schematic
ATmega16(L)
COMn1
COMn0
FOCn
clk
I/O
Waveform
Generator
DQ
1
OCn
DQ
PORT
DATA B U S
DQ
DDR
0
OCn
Pin
The general I/O port function is overridden by the output compare (OC0) from the waveform generator if either of the COM01:0 bits are set. However, the OC0 pin direction
(input or output) is still controlled by the data direction register (DDR) for the port pin.
The data direction register bit for the OC0 pin (DDR_OC0) must be set as output before
the OC0 value is visible on the pin. The port override function is independent of the
waveform generation mode.
The design of the output compare pin logic allows initialization of the OC0 state before
the output is enabled. Note that some COM01:0 bit settings are reserved for certain
modes of operation. See “8-bit Timer/Counter Register Description” on page 77.
Compare Output Mode and
Waveform Generation
The waveform generator uses the COM01:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COM01:0 = 0 tells the waveform generator that no
action on the OC0 register is to be performed on the next compare match. For compare
output actions in the non-PWM modes refer to Table 39 on page 78. For fast PWM
mode, refer to Table 40 on page 78, and for phase correct PWM refer to Table 41 on
page 79.
A change of the COM01:0 bits state will have effect at the first compare match after the
bits are written. For non-PWM modes, the action can be forced to have immediate effect
by using the FOC0 strobe bits.
Modes of OperationThe mode of operation, i.e., the behavior of the Timer/Counter and the output compare
pins, is defined by the combination of the waveform generation mode (WGM01:0) and
compare output mode (COM01:0) bits. The compare output mode bits do not affect the
counting sequence, while the waveform generation mode bits do. The COM01:0 bits
control whether the PWM output generated should be inverted or not (inverted or noninverted PWM). For non-PWM modes the COM01:0 bits control whether the output
should be set, cleared, or toggled at a compare match (See “Compare Match Output
Unit” on page 70.).
For detailed timing information refer to Figure 34, Figure 35, Figure 36 and Figure 37 in
“Timer/Counter Timing Diagrams” on page 75.
2466B–09/01
71
Normal ModeThe simplest mode of operation is the normal mode (WGM01:0 = 0). In this mode the
counting direction is always up (incrementing), and no counter clear is performed. The
counter simply overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then
restarts from the bottom (0x00). In normal operation the Timer/Counter overflow flag
TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0
(
flag in this case behaves like a 9th bit, except that it is only set, not cleared. However,
combined with the timer overflow interrupt that automatically clears the
TOV0 flag, the
timer resolution can be increased by software. There are no special cases to consider in
the normal mode, a new counter value can be written anytime.
The output compare unit can be used to generate interrupts at some given time. Using
the output compare to generate waveforms in normal mode is not recommended, since
this will occupy too much of the CPU time.
Clear Timer on Compare
Match (CTC) Mode
In clear timer on compare or CTC mode (WGM01:0 = 2), the OCR0 register is used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the
counter value (TCNT0) matches the OCR0. The OCR0 defines the top value for the
counter, hence also its resolution. This mode allows greater control of the compare
match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 31. The counter value
(TCNT0) increases until a compare match occurs between TCNT0 and OCR0, and then
counter (TCNT0) is cleared.
Figure 31. CTC Mode, Timing Diagram
OCn Interrupt Flag Set
TCNTn
OCn
(Toggle)
Period
14
23
(COMn1:0 = 1)
72
An interrupt can be generated each time the counter value reaches the TOP value by
using the OCF0 flag. If the interrupt is enabled, the interrupt handler routine can be used
for updating the TOP value. However, changing TOP to a value close to BOTTOM when
the counter is running with none or a low prescaler value must be done with care since
the CTC mode does not have the double buffering feature. If the new value written to
OCR0 is lower than the current value of TCNT0, the counter will miss the compare
match. The counter will then have to count to its maximum value (0xFF) and wrap
around starting at 0x00 before the compare match can occur.
For generating a waveform output in CTC mode, the OC0 output can be set to toggle its
logical level on each compare match by setting the compare output mode bits to toggle
mode (COM01:0 = 1). The OC0 value will not be visible on the port pin unless the data
direction for the pin is set to output. The waveform generated will have a maximum fre-
ATmega16(L)
2466B–09/01
ATmega16(L)
quency of f
OC0
= f
/2 when OCR0 is set to zero (0x00). The waveform frequency is
clk_I/O
defined by the following equation:
f
clk_I/O
f
OCn
-----------------------------------------------=
2 N1OCRn+()⋅⋅
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
As for the normal mode of operation, the TOV0 flag is set in the same timer clock cycle
that the counter counts from MAX to 0x00.
Fast PWM ModeThe fast pulse width modulation or fast PWM mode (WGM01:0 = 1) provides a high fre-
quency PWM waveform generation option. The fast PWM differs from the other PWM
option by its single-slope operation. The counter counts from BOTTOM to MAX then
restarts from BOTTOM. In non-inverting compare output mode, the output compare
(OC0) is cleared on the compare match between TCNT0 and OCR0, and set at BOTTOM. In inverting compare output mode, the output is set on compare match and
cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the
fast PWM mode can be twice as high as the phase correct PWM mode that use dualslope operation. This high frequency makes the fast PWM mode well suited for power
regulation, rectification, and DAC applications. High frequency allows physically small
sized external components (coils, capacitors), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the MAX
value. The counter is then cleared at the following timer clock cycle. The timing diagram
for the fast PWM mode is shown in Figure 32. The TCNT0 value is in the timing diagram
shown as a histogram for illustrating the single-slope operation. The diagram includes
non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0
slopes represent compare matches between OCR0 and TCNT0.
Figure 32. Fast PWM Mode, Timing Diagram
OCRn Interrupt Flag Set
OCRn Update
and
TOVn Interrupt Flag Set
TCNTn
OCn
OCn
Period
1
23
4567
(COMn1:0 = 2)
(COMn1:0 = 3)
The Timer/Counter overflow flag (TOV0) is set each time the counter reaches MAX. If
the interrupt is enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0
pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM and an inverted
PWM output can be generated by setting the COM01:0 to 3 (See Table 40 on page 78).
The actual OC0 value will only be visible on the port pin if the data direction for the port
2466B–09/01
73
pin is set as output. The PWM waveform is generated by setting (or clearing) the OC0
register at the compare match between OCR0 and TCNT0, and clearing (or setting) the
OC0 register at the timer clock cycle the counter is cleared (changes from MAX to
BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f
clk_I/O
f
OCnPWM
------------------=
N 256⋅
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0 register represents special cases when generating a
PWM waveform output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the
output will be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0 equal
to MAX will result in a constantly high or low output (depending on the polarity of the output set by the COM01:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved
by setting OC0 to toggle its logical level on each compare match (COM01:0 = 1). The
waveform generated will have a maximum frequency of f
OC0
= f
/2 when OCR0 is
clk_I/O
set to zero. This feature is similar to the OC0 toggle in CTC mode, except the double
buffer feature of the output compare unit is enabled in the fast PWM mode.
Phase Correct PWM ModeThe phase correct PWM mode (WGM01:0 = 3) provides a high resolution phase correct
PWM waveform generation option. The phase correct PWM mode is based on a dualslope operation. The counter counts repeatedly from BOTTOM to MAX and then from
MAX to BOTTOM. In non-inverting compare output mode, the output compare (OC0) is
cleared on the compare match between TCNT0 and OCR0 while upcounting, and set on
the compare match while downcounting. In inverting output compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than
single slope operation. However, due to the symmetric feature of the dual-slope PWM
modes, these modes are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode is fixed to 8 bits. In phase correct
PWM mode the counter is incremented until the counter value matches MAX. When the
counter reaches MAX, it changes the count direction. The TCNT0 value will be equal to
MAX for one timer clock cycle. The timing diagram for the phase correct PWM mode is
shown on Figure 33. The TCNT0 value is in the timing diagram shown as a histogram
for illustrating the dual-slope operation. The diagram includes non-inverted and inverted
PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare
matches between OCR0 and TCNT0.
74
ATmega16(L)
2466B–09/01
Figure 33. Phase Correct PWM Mode, Timing Diagram
TCNTn
ATmega16(L)
OCn Interrupt Flag Set
OCRn Update
TOVn Interrupt Flag Set
OCn
OCn
Period
123
(COMn1:0 = 2)
(COMn1:0 = 3)
The Timer/Counter overflow flag (TOV0) is set each time the counter reaches BOTTOM.
The interrupt flag can be used to generate an interrupt each time the counter reaches
the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on
the OC0 pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM. An
inverted PWM output can be generated by setting the COM01:0 to 3 (see Table 41 on
page 79). The actual OC0 value will only be visible on the port pin if the data direction
for the port pin is set as output. The PWM waveform is generated by clearing (or setting)
the OC0 register at the compare match between OCR0 and TCNT0 when the counter
increments, and setting (or clearing) the OC0 register at compare match between OCR0
and TCNT0 when the counter decrements. The PWM frequency for the output when
using phase correct PWM can be calculated by the following equation:
f
f
OCnPCPWM
clk_I/O
------------------=
N 510⋅
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0 register represent special cases when generating a
PWM waveform output in the phase correct PWM mode. If the OCR0 is set equal to
BOTTOM, the output will be continuously low and if set equal to MAX the output will be
continuously high for non-inverted PWM mode. For inverted PWM the output will have
the opposite logic values.
Timer/Counter Timing
Diagrams
2466B–09/01
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore
shown as a clock enable signal in the following figures. The figures include information
on when interrupt flags are set. Figure 34 contains timing data for basic Timer/Counter
operation. The figure shows the count sequence close to the MAX value in all modes
other than phase correct PWM mode.
75
Figure 34. Timer/Counter Timing Diagram, no Prescaling
clk
I/O
clk
Tn
(clk
/1)
I/O
TCNTn
MAX - 1MAXBOTTOMBOTTOM + 1
TOVn
Figure 35 shows the same timing data, but with the prescaler enabled.
Figure 35. Timer/Counter Timing Diagram, with Prescaler (f
clk
I/O
clk
Tn
(clk
/8)
I/O
TCNTn
MAX - 1MAXBOTTOMBOTTOM + 1
clk_I/O
/8)
TOVn
76
ATmega16(L)
Figure 36 shows the setting of OCF0 in all modes except CTC mode.
Figure 36. Timer/Counter Timing Diagram, Setting of OCF0, with Prescaler (f
clk
I/O
clk
Tn
(clk
/8)
I/O
TCNTn
OCRn
OCRn - 1OCRnOCRn + 1OCRn + 2
OCRn Value
OCFn
/8)
clk_I/O
2466B–09/01
ATmega16(L)
Figure 37 shows the setting of OCF0 and the clearing of TCNT0 in CTC mode.
Figure 37. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with
Prescaler (f
clk
I/O
clk
Tn
(clk
/8)
I/O
clk_I/O
/8)
8-bit Timer/Counter
Register Description
Timer/Counter Control
Register – TCCR0
TCNTn
(CTC)
OCRn
TOP - 1TOPBOTTOMBOTTOM + 1
TOP
OCFn
Bit76543210
FOC0WGM00COM01COM00WGM01CS02CS01CS00TCCR0
Read/WriteWR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 7 - FOC0: Force Output Compare
The FOC0 bit is only active when the WGM00 bit specifies a non-PWM mode. However,
for ensuring compatibility with future devices, this bit must be set to zero when TCCR0 is
written when operating in PWM mode. When writing a logical one to the FOC0 bit, an
immediate compare match is forced on the waveform generation unit. The OC0 output is
changed according to its COM01:0 bits setting. Note that the FOC0 bit is implemented
as a strobe. Therefore it is the value present in the COM01:0 bits that determines the
effect of the forced compare.
2466B–09/01
A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode
using OCR0 as TOP.
The FOC0 bit is always read as zero.
• Bit 6,3 - WGM01:0: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum
(TOP) counter value, and what type of waveform generation to be used. Modes of operation supported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare
match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes. See Table
38 and “Modes of Operation” on page 71.
77
Table 38. Waveform Generation Mode Bit Description
(1)
WGM01
Mode
Note:1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 def-
(CTC0)
000Normal0xFFImmediateMAX
101PWM, Phase Correct0xFFTOPBOTTOM
210CTCOCR0ImmediateMAX
31 1Fast PWM0xFFTOPMAX
initions. However, the functionality and location of these bits are compatible with
previous versions of the timer.
WGM00
(PWM0)
Timer/Counter Mode
of OperationTOP
Update of
OCR0
TOV0 Flag
Set-on
• Bit 5:4 - COM01:0: Compare Match Output Mode
These bits control the output compare pin (OC0) behavior. If one or both of the
COM01:0 bits are set, the OC0 output overrides the normal port functionality of the I/O
pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0 pin must be set in order to enable the output driver.
When OC0 is connected to the pin, the function of the COM01:0 bits depends on the
WGM01:0 bit setting. Table 39 shows the COM01:0 bit functionality when the WGM01:0
bits are set to a normal or CTC mode (non-PWM).
Table 39. Compare Output Mode, non-PWM Mode
COM01COM00Description
00Normal port operation, OC0 disconnected.
01Toggle
10Clear
11Set OC0 on compare match
OC0 on compare match
OC0 on compare match
Table 40 shows the COM01:0 bit functionality when the WGM01:0 bits are set to fast
PWM mode.
Table 40. Compare Output Mode, Fast PWM Mode
COM01COM00Description
00Normal port operation,
01Reserved
10Clear
11Set
Note:1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the
compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode”
on page 73 for more details.
OC0 on compare match, set OC0 at TOP
OC0 on compare match, clear OC0 at TOP
(1)
OC0 disconnected.
Table 41 shows the COM01:0 bit functionality when the WGM01:0 bits are set to phase
correct PWM mode.
Note:1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the
compare match is ignored, but the set or clear is done at TOP. See “Phase Correct
PWM Mode” on page 74 for more details.
OC0 on compare match when up-counting. Set OC0 on compare
match when downcounting.
OC0 on compare match when up-counting. Clear OC0 on compare
match when downcounting.
(1)
• Bit 2:0 - CS02:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter.
Table 42. Clock Select Bit Description
CS02CS01CS00Description
000No clock source (Timer/counter stopped)
001
010clk
011
100
101clk
110External clock source on T0 pin. Clock on falling edge.
clk
/(No prescaling)
I/O
/8 (From prescaler)
I/O
clk
/32 (From prescaler)
I/O
clk
/256 (From prescaler)
I/O
/1024 (From prescaler)
I/O
Timer/Counter Register –
TCNT0
111External clock source on T0 pin. Clock on rising edge.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will
clock the counter even if the pin is configured as an output. This feature allows software
control of the counting.
Bit76543210
TCNT0[7:0]TCNT0
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
The Timer/Counter Register gives direct access, both for read and write operations, to
the Timer/Counter unit 8-bit counter. Writing to the TCNT0 register blocks (removes) the
compare match on the following timer clock. Modifying the counter (TCNT0) while the
counter is running, introduces a risk of missing a compare match between TCNT0 and
the OCR0 register.
2466B–09/01
79
Output Compare Register –
OCR0
Timer/Counter Interrupt Mask
Register – TIMSK
Bit76543210
OCR0[7:0]OCR0
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
The Output Compare Register contains an 8-bit value that is continuously compared
with the counter value (TCNT0). A match can be used to generate an output compare
interrupt, or to generate a waveform output on the OC0 pin.
Bit7654 3 210
OCIE2TOIE 2TICIE1OCIE1AOCIE1BTOIE1OCIE0TOIE0TIMSK
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 1 - OCIE0: Timer/Counter0 Output Compare Match Interrupt Enable
When the OCIE0 bit is written to one, and the I-bit in the Status Register is set (one), the
Timer/Counter0 Compare Match interrupt is enabled. The corresponding interrupt is
executed if a compare match in Timer/Counter0 occurs, i.e., when the OCF0 bit is set in
the Timer/Counter Interrupt Flag Register - TIFR.
• Bit 0 - TOIE0: Timer/Counter0 Overflow Interrupt Enable
When the TOIE0 bit is written to one, and the I-bit in the Status Register is set (one), the
Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if
an overflow in Timer/Counter0 occurs, i.e., when the TOV0 bit is set in the
Timer/Counter Interrupt Flag Register – TIFR.
Timer/Counter Interrupt Flag
Register – TIFR
Bit76543210
OCF2TOV2ICF1OCF1AOCF1BTOV 1OCF0TOV0TIFR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 1 - OCF0: Output Compare Flag 0
The OCF0 bit is set (one) when a compare match occurs between the Timer/Counter0
and the data in OCR0 - Output Compare Register0. OCF0 is cleared by hardware when
executing the corresponding interrupt handling vector. Alternatively, OCF0 is cleared by
writing a logic one to the flag. When the I-bit in SREG, OCIE0 (Timer/Counter0 Compare match Interrupt Enable), and OCF0 are set (one), the Timer/Counter0 Compare
match Interrupt is executed.
• Bit 0 - TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared
by hardware when executing the corresponding interrupt handling vector. Alternatively,
TOV0 is cleared by writing a logic one to the flag. When the SREG I-bit, TOIE0
(Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set (one), the
Timer/Counter0 Overflow interrupt is executed. In phase correct PWM mode, this bit is
set when Timer/Counter0 changes counting direction at $00.
80
ATmega16(L)
2466B–09/01
ATmega16(L)
Timer/Counter0 and
Timer/Counter1
Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the
timer/counters can have different prescaler settings. The description below applies to
both Timer/Counter1 and Timer/Counter0.
Prescalers
Internal Clock SourceThe timer/counter can be clocked directly by the system clock (by setting the CSn2:0 =
1). This provides the fastest operation, with a maximum timer/counter clock frequency
equal to system clock frequency (f
caler can be used as a clock source. The prescaled clock has a frequency of either
f
CLK_I/O
/8, f
CLK_I/O
/64, f
CLK_I/O
/256, or f
Prescaler ResetThe prescaler is free running, i.e., operates independently of the clock select logic of the
timer/counter, and it is shared by Timer/Counter1 and Timer/Counter0. Since the prescaler is not affected by the timer/counter’s clock select, the state of the prescaler will
have implications for situations where a prescaled clock is used. One example of prescaling artifacts occurs when the timer is enabled and clocked by the prescaler (6 >
CSn2:0 > 1). The number of system clock cycles from when the timer is enabled to the
first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the timer/counter to program
execution. However, care must be taken if the other timer/counter that shares the same
prescaler also uses prescaling. A prescaler reset will affect the prescaler period for all
timer/counters it is connected to.
). Alternatively, one of four taps from the pres-
CLK_I/O
/1024.
CLK_I/O
External Clock SourceAn external clock source applied to the T1/T0 pin can be used as timer/counter clock
/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin syn-
(clk
T1
chronization logic. The synchronized (sampled) signal is then passed through the edge
detector. Figure 38 shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector logic. The registers are clocked at the positive edge of the
clk
internal system clock (
). The latch is transparent in the high period of the internal
I/O
system clock.
/clk
The edge detector generates one clk
pulse for each positive (CSn2:0 = 7) or neg-
0
T1
T
ative (CSn2:0 = 6) edge it detects.
Figure 38. T1/T0 Pin Sampling
Tn_sync
(To Clock
Select Logic)
clk
Tn
DQDQ
LE
I/O
DQ
Edge DetectorSynchronization
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system
clock cycles from an edge has been applied to the T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 has been stable for
at least one system clock cycle, otherwise it is a risk that a false timer/counter clock
pulse is generated.
2466B–09/01
Each half period of the external clock applied must be longer than one system clock
cycle to ensure correct sampling. The external clock must be guaranteed to have less
81
than half the system clock frequency (f
ExtClk
< f
/2) given a 50/50% duty cycle. Since
clk_I/O
the edge detector uses sampling, the maximum frequency of an external clock it can
detect is half the sampling frequency (Nyquist sampling theorem). However, due to variation of the system clock frequency and duty cycle caused by oscillator source (crystal,
resonator, and capacitors) tolerances, it is recommended that maximum frequency of an
external clock source is less than f
clk_I/O
/2.5.
An external clock source can not be prescaled.
Figure 39. Prescaler for Timer/Counter0 and Timer/Counter1
clk
PSR10
T0
T1
I/O
Synchronization
Synchronization
Clear
Special Function IO Register –
SFIOR
clk
T1
clk
T0
Note:The synchronization logic on the input pins (T1/T0) is shown in Figure 38.
Bit76543210
ADTS2ADTS1ADTS0ADHSMACMEPUDPSR2PSR10SFIOR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 0 - PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0
When this bit is written to one, the Timer/Counter1 and Timer/Counter0 prescaler will be
reset. The bit will be cleared by hardware after the operation is performed. Writing a
zero to this bit will have no effect. Note that Timer/Counter1 and Timer/Counter0 share
the same prescaler and a reset of this prescaler will affect both timers. This bit will
always be read as zero.
82
ATmega16(L)
2466B–09/01
ATmega16(L)
16-bit
Timer/Counter1
The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation, and signal timing measurement. The main features are:
• Four Independent Interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)
OverviewMost register and bit references in this document are written in general form. A lower
case “n” replaces the Timer/Counter number, and a lower case “x” replaces the output
compare unit channel. However, when using the register or bit defines in a program, the
precise form must be used (i.e., TCNT1 for accessing Timer/Counter1 counter value
and so on). The physical I/O register and bit locations for ATmega16 are listed in the
“16-bit Timer/Counter Register Description” on page 104.
A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 40. CPU
accessible I/O registers, including I/O bits and I/O pins, are shown in bold.
2466B–09/01
83
Figure 40. 16-bit Timer/Counter Block Diagram
(1)
Count
Clear
Direction
Timer/Counter
TCNTn
Control Logic
TOPBOTTOM
=
clk
Tn
=
0
=
OCRnA
Fixed
TOP
Values
=
DATA BU S
OCRnB
ICRn
ICFn (Int.Req.)
Edge
Detector
TOVn
(Int.Req.)
Clock Select
Edge
Detector
( From Prescaler )
OCnA
(Int.Req.)
Wavefo rm
Generation
OCnB
(Int.Req.)
Wavefo rm
Generation
Noise
Canceler
Tn
OCnA
OCnB
( From Analog
Comparator Ouput )
ICPn
TCCRnATCCRnB
Note:1. Refer to Figure 1 on page 2, Table 25 on page 55, and Table 31 on page 60 for
Timer/Counter1 pin placement and description.
RegistersThe Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture
Register (ICR1) are all 16-bit registers. Special procedures must be followed when
accessing the 16-bit registers. These procedures are described in the section “Access-
ing 16-bit Registers” on page 86. The Timer/Counter Control Registers (TCCR1A/B) are
8-bit registers and have no CPU access restrictions. Interrupt requests (abbreviated to
Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR).
All interrupts are individually masked with the Timer Interrupt Mask register (TIMSK).
TIFR and TIMSK are not shown in the figure since these registers are shared by other
timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock
source on the T1 pin. The Clock Select logic block controls which clock source and edge
the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is
inactive when no clock source is selected. The output from the clock select logic is
referred to as the timer clock (clk
).
1
T
The double buffered Output Compare Registers (OCR1A/B) are compared with the
Timer/Counter value at all time. The result of the compare can be used by the waveform
generator to generate a PWM or variable frequency output on the Output Compare Pin
84
ATmega16(L)
2466B–09/01
ATmega16(L)
(OC1A/B). See “Output Compare Units” on page 91. The compare match event will also
set the compare match flag (OCF1A/B) which can be used to generate an output compare interrupt request.
The Input Capture Register can capture the Timer/Counter value at a given external
(edge triggered) event on either the Input Capture Pin (ICP1) or on the analog comparator pins (See “Analog Comparator” on page 192.) The input capture unit includes a
digital filtering unit (Noise Canceler) for reducing the chance of capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be
defined by either the OCR1A register, the ICR1 register, or by a set of fixed values.
When using OCR1A as TOP value in a PWM mode, the OCR1A register can not be
used for generating a PWM output. However, the TOP value will in this case be double
buffered allowing the TOP value to be changed in run time. If a fixed TOP value is
required, the ICR1 register can be used as an alternative, freeing the OCR1A to be used
as PWM output.
DefinitionsThe following definitions are used extensively throughout the document:
Table 43. Definitions
BOTTOMThe counter reaches the BOTTOM when it becomes 0x0000
MAXThe counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
The counter reaches the TOP when it becomes equal to the highest value in the
TOP
count sequence. The TOP value can be assigned to be one of the fixed values:
0x00FF, 0x01FF, or 0x03FF, or to the value stored in the OCR1A or ICR1 register. The assignment is dependent of the mode of operation.
CompatibilityThe 16-bit Timer/Counter has been updated and improved from previous versions of the
16-bit AVR Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier
version regarding:
•All 16-bit Timer/Counter related I/O register address locations, including timer
interrupt registers.
•Bit locations inside all 16-bit Timer/Counter registers, including timer interrupt
registers.
•Interrupt vectors.
The following control bits have changed name, but have same functionality and register
location:
•PWM10 is changed to WGM10.
•PWM11 is changed to WGM11.
•CTC1 is changed to WGM12.
The following bits are added to the 16-bit Timer/Counter control registers:
•FOC1A and FOC1B are added to TCCR1A.
•WGM13 is added to TCCR1B.
2466B–09/01
The 16-bit Timer/Counter has improvements that will affect the compatibility in some
special cases.
85
Accessing 16-bit
Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR
CPU via the 8-bit data bus. The 16-bit register must be byte accessed using two read or
write operations. Each 16-bit timer has a single 8-bit register for temporary storing of the
high byte of the 16-bit access. The same temporary register is shared between all 16-bit
registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or write
operation. When the low byte of a 16-bit register is written by the CPU, the high byte
stored in the temporary register, and the low byte written are both copied into the 16-bit
register in the same clock cycle. When the low byte of a 16-bit register is read by the
CPU, the high byte of the 16-bit register is copied into the temporary register in the
same clock cycle as the low byte is read.
Not all 16-bit accesses uses the temporary register for the high byte. Reading the
OCR1A/B 16-bit registers does not involve using the temporary register.
To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read,
the low byte must be read before the high byte.
The following code examples show how to access the 16-bit timer registers assuming
that no interrupts updates the temporary register. The same principle can be used
directly for accessing the OCR1A/B and ICR1 registers. Note that when using “C”, the
compiler handles the 16-bit access.
Assembly Code Example
...
; Set TCNT1 to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNT1H,r17
out TCNT1L,r16
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
...
C Code Example
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT
/* Read TCNT1 into i */
i = TCNT1;
...
(1)
1 = 0x1FF;
(1)
86
Note:1. The example code assumes that the part specific header file is included.
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs between the two instructions accessing the 16-bit register, and the interrupt
code updates the temporary register by accessing the same or any other of the 16-bit
timer registers, then the result of the access outside the interrupt will be corrupted.
Therefore, when both the main code and the interrupt code update the temporary register, the main code must disable the interrupts during the 16-bit access.
ATmega16(L)
2466B–09/01
ATmega16(L)
The following code examples show how to do an atomic read of the TCNT1 register contents. Reading any of the OCR1A/B or ICR1 registers can be done by using the same
principle.
Assembly Code Example
TIM16_ReadTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Read TCNT1 into r17:r16
in r16,TCNT1L
in r17,TCNT1H
; Restore global interrupt flag
out SREG,r18
ret
C Code Example
unsigned int TIM16_ReadTCNT1( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Read TCNT1 into i */
i = TCNT1;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
(1)
(1)
2466B–09/01
Note:1. The example code assumes that the part specific header file is included.
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
87
The following code examples show how to do an atomic write of the TCNT1 register
contents. Writing any of the OCR1A/B or ICR1 registers can be done by using the same
principle.
Assembly Code Example
TIM16_WriteTCNT1:
; Save global interrupt flag
in r18,SREG
; Disable interrupts
cli
; Set TCNT1 to r17:r16
out TCNT1H,r17
out TCNT1L,r16
; Restore global interrupt flag
out SREG,r18
ret
C Code Example
void TIM16_WriteTCNT1 ( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
_CLI();
/* Set TCNT1 to i */
TCNT1 = i;
/* Restore global interrupt flag */
SREG = sreg;
}
(1)
(1)
Note:1. The example code assumes that the part specific header file is included.
The assembly code example requires that the r17:r16 register pair contains the value to
be written to TCNT1.
Reusing the Temporary High
Byte Register
If writing to more than one 16-bit register where the high byte is the same for all registers
written, then the high byte only needs to be written once. However, note that the same
rule of atomic operation described previously also applies in this case.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the clock select logic which is controlled by the clock select
(CS12:0) bits located in the Timer/Counter control register B (TCCR1B). For details on
clock sources and prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on
page 81.
Counter UnitThe main part of the 16-bit Timer/Counter is the programmable 16-bit bidirectional
counter unit. Figure 41 shows a block diagram of the counter and its surroundings.
88
ATmega16(L)
2466B–09/01
Figure 41. Counter Unit Block Diagram
ATmega16(L)
DATA BU S
TEMP (8-bit)
TCNTnH (8-bit)TCNTnL (8-bit)
TCNTn (16-bit Counter)
(8-bit)
Count
Clear
Direction
Control Logic
TOP BOTTO M
TOVn
(Int.Req.)
clk
Tn
Clock Select
Edge
Detector
( From Prescaler )
Tn
Signal description (internal signals):
CountIncrement or decrement TCNT1 by 1.
DirectionSelect between increment and decrement.
ClearClear TCNT1 (set all bits to zero).
clk
1
T
Timer/counter clock.
TOPSignalize that TCNT1 has reached maximum value.
BOTTOMSignalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: counter high
(TCNT1H) containing the upper 8 bits of the counter, and counter low (TCNT1L) containing the lower 8 bits. The TCNT1H register can only be indirectly accessed by the
CPU. When the CPU does an access to the TCNT1H I/O location, the CPU accesses
the high byte temporary register (TEMP). The temporary register is updated with the
TCNT1H value when the TCNT1L is read, and TCNT1H is updated with the temporary
register value when TCNT1L is written. This allows the CPU to read or write the entire
16-bit counter value within one clock cycle via the 8-bit data bus. It is important to notice
that there are special cases of writing to the TCNT1 register when the counter is counting that will give unpredictable results. The special cases are described in the sections
where they are of importance.
2466B–09/01
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk
). The clk
1
T
can be generated from an external or
1
T
internal clock source, selected by the clock select bits (CS12:0). When no clock source
is selected (CS12:0 = 0) the timer is stopped. However, the TCNT1 value can be
accessed by the CPU, independent of whether clk
is present or not. A CPU write over-
1
T
rides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the waveform generation mode
bits (WGM13:0) located in the Timer/Counter control registers A and B (TCCR1A and
TCCR1B). There are close connections between how the counter behaves (counts) and
how waveforms are generated on the output compare outputs OC1x. For more details
about advanced counting sequences and waveform generation, see “Modes of Operation” on page 94.
The Timer/Counter overflow (TOV1) flag is set according to the mode of operation
selected by the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.
89
Input Capture UnitThe Timer/Counter incorporates an input capture unit that can capture external events
and give them a time-stamp indicating time of occurrence. The external signal indicating
an event, or multiple events, can be applied via the ICP1 pin or alternatively, via the
analog-comparator unit. The time-stamps can then be used to calculate frequency, dutycycle, and other features of the signal applied. Alternatively the time-stamps can be
used for creating a log of the events.
The input capture unit is illustrated by the block diagram shown in Figure 42. The elements of the block diagram that are not directly a part of the input capture unit are gray
shaded. The small “n” in register and bit names indicates the Timer/Counter number.
Figure 42. Input Capture Unit Block Diagram
ICPn
WRITE
TEMP (8-bit)
ICRnH (8-bit)
ICRn (16-bit Register)
ACO*
Analog
Comparator
DATA BU S
ICRnL (8-bit)
ACIC*ICNCICES
Canceler
Noise
(8-bit)
TCNTnH (8-bit)TCNTnL (8-bit)
TCNTn (16-bit Counter)
Edge
Detector
ICFn (Int.Req.)
When a change of the logic level (an event) occurs on the input capture pin (ICP1),
alternatively on the analog comparator output (ACO), and this change confirms to the
setting of the edge detector, a capture will be triggered. When a capture is triggered, the
16-bit value of the counter (TCNT1) is written to the input capture register (ICR1). Theinput capture flag (ICF1) is set at the same system clock as the TCNT1 value is copied
into ICR1 register. If enabled (TICIE1 = 1), the input capture flag generates an input
capture interrupt. The ICF1 flag is automatically cleared when the interrupt is executed.
Alternatively the ICF1 flag can be cleared by software by writing a logical one to its I/O
bit location.
90
Reading the 16-bit value in the input capture register (ICR1) is done by first reading the
low byte (ICR1L) and then the high byte (ICR1H). When the low byte is read the high
byte is copied into the high byte temporary register (TEMP). When the CPU reads the
ICR1H I/O location it will access the TEMP register.
The ICR1 register can only be written when using a waveform generation mode that utilizes the ICR1 register for defining the counter’s TOP value. In these cases the
waveform generation mode (WGM13:0) bits must be set before the TOP value can be
written to the ICR1 register. When writing the ICR1 register the high byte must be written
to the ICR1H I/O location before the low byte is written to ICR1L.
ATmega16(L)
2466B–09/01
ATmega16(L)
For more information on how to access the 16-bit registers refer to “Accessing 16-bit
Registers” on page 86.
Input Capture Trigger SourceThe main trigger source for the input capture unit is the input capture pin (ICP1).
Timer/counter 1 can alternatively use the analog comparator output as trigger source for
the input capture unit. The analog comparator is selected as trigger source by setting
the analog comparator input capture (ACIC) bit in the analog comparator control andstatus register (ACSR). Be aware that changing trigger source can trigger a capture.
The input capture flag must therefore be cleared after the change.
Both the input capture pin (ICP1) and the analog comparator output (ACO) inputs are
sampled using the same technique as for the T1 pin (Figure 38 on page 81). The edge
detector is also identical. However, when the noise canceler is enabled, additional logic
is inserted before the edge detector, which increases the delay by 4 system clock
cycles. Note that the input of the noise canceler and edge detector is always enabled
unless the Timer/Counter is set in a waveform generation mode that uses ICR1 to
define TOP.
An input capture can be triggered by software by controlling the port of the ICP1 pin.
Noise CancelerThe noise canceler improves noise immunity by using a simple digital filtering scheme.
The noise canceler input is monitored over 4 samples, and all 4 must be equal for
changing the output that in turn is used by the edge detector.
The noise canceler is enabled by setting the input capture noise canceler (ICNC1) bit in
Timer/Counter control register B (TCCR1B). When enabled the noise canceler introduces additional 4 system clock cycles of delay from a change applied to the input, to
the update of the ICR1 register. The noise canceler uses the system clock and is therefore not affected by the prescaler.
Using the Input Capture UnitThe main challenge when using the input capture unit is to assign enough processor
capacity for handling the incoming events. The time between two events is critical. If the
processor has not read the captured value in the ICR1 register before the next event
occurs, the ICR1 will be overwritten with a new value. In this case the result of the capture will be incorrect.
When using the input capture interrupt, the ICR1 register should be read as early in the
interrupt handler routine as possible. Even though the input capture interrupt has relatively high priority, the maximum interrupt response time is dependent on the maximum
number of clock cycles it takes to handle any of the other interrupt requests.
Using the input capture unit in any mode of operation when the TOP value (resolution) is
actively changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed
after each capture. Changing the edge sensing must be done as early as possible after
the ICR1 register has been read. After a change of the edge, the input capture flag
(ICF1) must be cleared by software (writing a logical one to the I/O bit location). For
measuring frequency only, the clearing of the ICF1 flag is not required (if an interrupt
handler is used).
Output Compare UnitsThe 16-bit comparator continuously compares TCNT1 with the output compare register
(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set the
output compare flag (OCF1x) at the next timer clock cycle
output compare flag generates an output compare interrupt. The OCF1x flag is automatically cleared when the interrupt is executed. Alternatively the OCF1x flag can be
2466B–09/01
. If enabled (OCIE1x = 1), the
91
cleared by software by writing a logical one to its I/O bit location. The waveform generator uses the match signal to generate an output according to operating mode set by the
waveform generation mode (WGM13:0) bits and compare output mode (COM1x1:0)
bits. The TOP and BOTTOM signals are used by the waveform generator for handling
the special cases of the extreme values in some modes of operation (See “Modes of
Operation” on page 94.)
A special feature of output compare unit A allows it to define the Timer/Counter TOP
value (i.e., counter resolution). In addition to the counter resolution, the TOP value
defines the period time for waveforms generated by the waveform generator.
Figure 43 shows a block diagram of the output compare unit. The small “n” in the register and bit names indicates the device number (n = 1
for Timer/Counter 1), and the “x”
indicates output compare unit (A/B). The elements of the block diagram that are not
directly a part of the output compare unit are gray shaded.
Figure 43. Output Compare Unit, Block Diagram
DATA BU S
TEMP (8-bit)
(8-bit)
OCRnxH Buf. (8-bit)
OCRnx Buffer (16-bit Register)
OCRnxH (8-bit)OCRnxL (8-bit)
OCRnx (16-bit Register)
TOP
BOTTOM
OCRnxL Buf. (8-bit)
=
Waveform Generator
TCNTnH (8-bit)TCNTnL (8-bit)
TCNTn (16-bit Counter)
(16-bit Comparator )
OCFnx (Int.Req.)
OCnx
COMnx1:0WGMn3:0
The OCR1x register is double buffered when using any of the twelve pulse width modulation (PWM) modes. For the normal and clear timer on compare (CTC) modes of
operation, the double buffering is disabled. The double buffering synchronizes the
update of the OCR1x compare register to either TOP or BOTTOM of the counting
sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical
PWM pulses, thereby making the output glitch-free.
The OCR1x register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR1x buffer register, and if double
buffering is disabled the CPU will access the OCR1x directly. The content of the OCR1x
(buffer or compare) register is only changed by a write operation (the Timer/Counter
does not update this register automatically as the TCNT1- and ICR1 register). Therefore
OCR1x is not read via the high byte temporary register (TEMP). However, it is a good
practice to read the low byte first as when accessing other 16-bit registers. Writing the
OCR1x registers must be done via the TEMP register since the compare of all 16 bits is
done continuously. The high byte (OCR1xH) has to be written first. When the high byte
92
ATmega16(L)
2466B–09/01
ATmega16(L)
I/O location is written by the CPU, the TEMP register will be updated by the value written. Then when the low byte (OCR1xL) is written to the lower 8 bits, the high byte will be
copied into the upper 8-bits of either the OCR1x buffer or OCR1x compare register in
the same system clock cycle.
For more information of how to access the 16-bit registers refer to “Accessing 16-bit
Registers” on page 86.
Force Output CompareIn non-PWM waveform generation modes, the match output of the comparator can be
forced by writing a one to the force output compare (FOC1x) bit. Forcing compare match
will not set the OCF1x flag or reload/clear the timer, but the OC1x pin will be updated as
if a real compare match had occurred (the COM11:0 bits settings define whether the
OC1x pin is set, cleared or toggled).
Compare Match Blocking by
TCNT1 Write
Using the Output Compare
Unit
Compare Match Output
Unit
All CPU writes to the TCNT1 register will block any compare match that occurs in the
next timer clock cycle, even when the timer is stopped. This feature allows OCR1x to be
initialized to the same value as TCNT1 without triggering an interrupt when the
Timer/Counter clock is enabled.
Since writing TCNT1 in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNT1 when using any of the
output compare channels, independent of whether the Timer/Counter is running or not.
If the value written to TCNT1 equals the OCR1x value, the compare match will be
missed, resulting in incorrect waveform generation. Do not write the TCNT1 equal to
TOP in PWM modes with variable TOP values. The compare match for the TOP will be
ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNT1 value
equal to BOTTOM when the counter is downcounting.
The setup of the OC1x should be performed before setting the data direction register for
the port pin to output. The easiest way of setting the OC1x value is to use the force output compare (FOC1x) strobe bits in normal mode. The OC1x register keeps its value
even when changing between waveform generation modes.
Be aware that the COM1x1:0 bits are not double buffered together with the compare
value. Changing the COM1x1:0 bits will take effect immediately.
The compare output mode (COM1x1:0) bits have two functions. The waveform generator uses the COM1x1:0 bits for defining the output compare (OC1x) state at the next
compare match. Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 44 shows a simplified schematic of the logic affected by the COM1x1:0 bit setting.
The I/O registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of
the general I/O port control registers (DDR and PORT) that are affected by the
COM1x1:0 bits are shown. When referring to the OC1x state, the reference is for the
internal OC1x register, not the OC1x pin. If a system reset occur, the OC1x register is
reset to “0”.
2466B–09/01
93
Figure 44. Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCnx
clk
I/O
Waveform
Generator
DQ
1
OCnx
DQ
PORT
DATA B U S
DQ
DDR
0
OCnx
Pin
The general I/O port function is overridden by the output compare (OC1x) from the
waveform generator if either of the COM1x1:0 bits are set. However, the OC1x pin
direction (input or output) is still controlled by the data direction register (DDR) for the
port pin. The data direction register bit for the OC1x pin (DDR_OC1x) must be set as
output before the OC1x value is visible on the pin. The port override function is generally
independent of the waveform generation mode, but there are some exceptions. Refer to
Table 44, Table 45 and Table 46 for details.
The design of the output compare pin logic allows initialization of the OC1x state before
the output is enabled. Note that some COM1x1:0 bit settings are reserved for certain
modes of operation. See “16-bit Timer/Counter Register Description” on page 104.
The COM1x1:0 bits have no effect on the input capture unit.
Compare Output Mode and
Waveform Generation
The waveform generator uses the COM1x1:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COM1x1:0 = 0 tells the waveform generator that no
action on the OC1x register is to be performed on the next compare match. For compare
output actions in the non-PWM modes refer to Table 44 on page 104. For fast PWM
mode refer to Table 45 on page 104, and for phase correct and phase and frequency
correct PWM refer to Table 46 on page 105.
A change of the COM1x1:0 bits state will have effect at the first compare match after the
bits are written. For non-PWM modes, the action can be forced to have immediate effect
by using the FOC1x strobe bits.
Modes of OperationThe mode of operation, i.e., the behavior of the Timer/Counter and the output compare
pins, is defined by the combination of the waveform generation mode (WGM13:0) and
compare output mode (COM1x1:0) bits. The compare output mode bits do not affect the
counting sequence, while the waveform generation mode bits do. The COM1x1:0 bits
control whether the PWM output generated should be inverted or not (inverted or noninverted PWM). For non-PWM modes the COM1x1:0 bits control whether the output
should be set, cleared or toggle at a compare match (See “Compare Match Output Unit”
on page 93.)
94
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 102.
ATmega16(L)
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ATmega16(L)
Normal ModeThe simplest mode of operation is the normal mode (WGM13:0 = 0). In this mode the
counting direction is always up (incrementing), and no counter clear is performed. The
counter simply overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and
then restarts from the BOTTOM (0x0000). In normal operation the Timer/Counter over-flow flag (TOV1) will be set in the same timer clock cycle as the TCNT1 becomes zero.
The TOV1 flag in this case behaves like a 17th bit, except that it is only set, not cleared.
However, combined with the timer overflow interrupt that automatically clears the TOV1
flag, the timer resolution can be increased by software. There are no special cases to
consider in the normal mode, a new counter value can be written anytime.
The input capture unit is easy to use in normal mode. However, observe that the maximum interval between the external events must not exceed the resolution of the counter.
If the interval between events are too long, the timer overflow interrupt or the prescaler
must be used to extend the resolution for the capture unit.
The output compare units can be used to generate interrupts at some given time. Using
the output compare to generate waveforms in normal mode is not recommended, since
this will occupy too much of the CPU time.
Clear Timer on Compare
Match (CTC) Mode
In clear timer on compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 register are used to manipulate the counter resolution. In CTC mode the counter is cleared
to zero when the counter value (TCNT1) matches either the OCR1A (WGM13:0 = 4) or
the ICR1 (WGM13:0 = 12). The OCR1A or ICR1 define the top value for the counter,
hence also its resolution. This mode allows greater control of the compare match output
frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 45. The counter value
(TCNT1) increases until a compare match occurs with either OCR1A or ICR1, and then
counter (TCNT1) is cleared.
Figure 45. CTC Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TCNTn
OCnA
(Toggle)
(COMnA1:0 = 1)
2466B–09/01
Period
14
23
An interrupt can be generated at each time the counter value reaches the TOP value by
either using the OCF1A or ICF1 flag according to the register used to define the TOP
value. If the interrupt is enabled, the interrupt handler routine can be used for updating
the TOP value. However, changing the TOP to a value close to BOTTOM when the
counter is running with none or a low prescaler value must be done with care since the
CTC mode does not have the double buffering feature. If the new value written to
OCR1A or ICR1 is lower than the current value of TCNT1, the counter will miss the compare match. The counter will then have to count to its maximum value (0xFFFF) and
wrap around starting at 0x0000 before the compare match can occur. In many cases
95
this feature is not desirable. An alternative will then be to use the fast PWM mode using
f
OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double
buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle
its logical level on each compare match by setting the compare output mode bits to toggle mode (COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the
data direction for the pin is set to output (DDR_OC1A = 1). The waveform generated will
have a maximum frequency of f
OC1A
= f
/2 when OCR1A is set to zero (0x0000). The
clk_I/O
waveform frequency is defined by the following equation:
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the normal mode of operation, the TOV1 flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
Fast PWM ModeThe fast pulse width modulation or fast PWM mode (WGM13:0 = 5,6,7,14, or 15) pro-
vides a high frequency PWM waveform generation option. The fast PWM differs from
the other PWM options by its single-slope operation. The counter counts from BOTTOM
to TOP then restarts from BOTTOM. In non-inverting compare output mode, the output
compare (OC1x) is set on the compare match between TCNT1 and OCR1x, and cleared
at TOP. In inverting compare output mode output is cleared on compare match and set
at TOP. Due to the single-slope operation, the operating frequency of the fast PWM
mode can be twice as high as the phase correct and phase and frequency correct PWM
modes that use dual-slope operation. This high frequency makes the fast PWM mode
well suited for power regulation, rectification, and DAC applications. High frequency
allows physically small sized external components (coils, capacitors), hence reduces
total system cost.
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either
ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to
0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM
resolution in bits can be calculated by using the following equation:
R
FPWM
TOP 1+()log
-----------------------------------=
2
()log
In fast PWM mode the counter is incremented until the counter value matches either
one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 5, 6, or 7), the value in
ICR1 (WGM13:0 = 14), or the value in OCR1A (WGM13:0 = 15). The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in Figure 46. The figure shows fast PWM mode when OCR1A or ICR1 is used to
define TOP. The TCNT1 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM
outputs. The small horizontal line marks on the TCNT1 slopes represent compare
matches between OCR1x and TCNT1. The OC1x interrupt flag will be set when a compare match occurs.
96
ATmega16(L)
2466B–09/01
Figure 46. Fast PWM Mode, Timing Diagram
TCNTn
ATmega16(L)
OCRnx / TOP Update
and
TOVn Interrupt Flag Set
and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCnx
OCnx
Period
17
2345 68
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter overflow flag (TOV1) is set each time the counter reaches TOP. In
addition the OC1A or ICF1 flag is set at the same timer clock cycle as TOV1 is set when
either OCR1A or ICR1 is used for defining the TOP value. If one of the interrupts are
enabled, the interrupt handler routine can be used for updating the TOP and compare
values.
When changing the TOP value the program must ensure that the new TOP value is
higher or equal to the value of all of the compare registers. If the TOP value is lower
than any of the compare registers, a compare match will never occur between the
TCNT1 and the OCR1x. Note that when using fixed TOP values the unused bits are
masked to zero when any of the OCR1x registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining
the TOP value. The ICR1 register is not double buffered. This means that if ICR1 is
changed to a low value when the counter is running with none or a low prescaler value,
there is a risk that the new ICR1 value written is lower than the current value of TCNT1.
The result will then be that the counter will miss the compare match at the TOP value.
The counter will then have to count to the MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. The OCR1A register however, is
double buffered. This feature allows the OCR1A I/O location to be written anytime.
When the OCR1A I/O location is written the value written will be put into the OCR1A
buffer register. The OCR1A compare register will then be updated with the value in the
buffer register at the next timer clock cycle the TCNT1 matches TOP. The update is
done at the same timer clock cycle as the TCNT1 is cleared and the TOV1 flag is set.
2466B–09/01
Using the ICR1 register for defining TOP works well when using fixed TOP values. By
using ICR1, the OCR1A register is free to be used for generating a PWM output on
OC1A. However, if the base PWM frequency is actively changed (by changing the TOP
value), using the OCR1A as TOP is clearly a better choice due to its double buffer
feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the
OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an
inverted PWM output can be generated by setting the COM1x1:0 to 3 (See Table 44 on
page 104). The actual OC1x value will only be visible on the port pin if the data direction
for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by
97
seting (or clearing) the OC1x register at the compare match between OCR1x and
f
TCNT1, and clearing (or setting) the OC1x register at the timer clock cycle the counter is
cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f
clk_I/O
OCnxPWM
-----------------------------------=
N1TOP+()⋅
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x register represents special cases when generating a
PWM waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM
(0x0000) the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the
OCR1x equal to TOP will result in a constant high or low output (depending on the polarity of the output set by the COM1x1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved
by setting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). The
waveform generated will have a maximum frequency of f
OC1A
= f
/2 when OCR1A is
clk_I/O
set to zero (0x0000). This feature is similar to the OC1A toggle in CTC mode, except the
double buffer feature of the output compare unit is enabled in the fast PWM mode.
1,2,3,10, or 11) provides a high resolution phase correct PWM waveform generation
option. The phase correct PWM mode is, like the phase and frequency correct PWM
mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM
(0x0000) to TOP and then from TOP to BOTTOM. In non-inverting compare output
mode, the output compare (OC1x) is cleared on the compare match between TCNT1
and OCR1x while upcounting, and set on the compare match while downcounting. In
inverting output compare mode, the operation is inverted. The dual-slope operation has
lower maximum operation frequency than single slope operation. However, due to the
symmetric feature of the dual-slope PWM modes, these modes are preferred for motor
control applications.
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ATmega16(L)
The PWM resolution for the phase correct PWM mode can be fixed to 8,9, or 10-bit, or
defined by either ICR1 or OCR1A. The minimum resolution allowed is 2 bit (ICR1 or
OCR1A set to 0x0003), and the maximum resolution is 16 bit (ICR1 or OCR1A set to
MAX). The PWM resolution in bits can be calculated by using the following equation:
R
PCPWM
TOP 1+()log
-----------------------------------=
2
()log
In phase correct PWM mode the counter is incremented until the counter value matches
either one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 1, 2, or 3), the
value in ICR1 (WGM13:0 = 10), or the value in OCR1A (WGM13:0 = 11). The counter
has then reached the TOP and changes the count direction. The TCNT1 value will be
equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM
mode is shown on Figure 47. The figure shows phase correct PWM mode when OCR1A
or ICR1 is used to define TOP. The TCNT1 value is in the timing diagram shown as a
histogram for illustrating the dual-slope operation. The diagram includes non-inverted
and inverted PWM outputs. The small horizontal line marks on the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x interrupt flag will be set
when a compare match occurs.
2466B–09/01
Figure 47. Phase Correct PWM Mode, Timing Diagram
TCNTn
ATmega16(L)
OCRnx / TOP Update
and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnx
OCnx
Period
1234
(COMnx1:0 = 2)
(COMnx1:0 = 3)
The Timer/Counter overflow flag (TOV1) is set each time the counter reaches BOTTOM.
When either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 flag
is set accordingly at the same timer clock cycle as the OCR1x registers are updated with
the double buffer value (at TOP). The interrupt flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is
higher or equal to the value of all of the compare registers. If the TOP value is lower
than any of the compare registers, a compare match will never occur between the
TCNT1 and the OCR1x. Note that when using fixed TOP values, the unused bits are
masked to zero when any of the OCR1x registers are written. As the third period shown
in Figure 47 illustrates, changing the TOP actively while the Timer/Counter is running in
the phase correct mode can result in an unsymmetrical output. The reason for this can
be found in the time of update of the OCR1x register. Since the OCR1x update occurs at
TOP, the PWM period starts and ends at TOP. This implies that the length of the falling
slope is determined by the previous TOP value, while the length of the rising slope is
determined by the new TOP value. When these two values differ the two slopes of the
period will differ in length. The difference in length gives the unsymmetrical result on the
output.
2466B–09/01
It is recommended to use the phase and frequency correct mode instead of the phase
correct mode when changing the TOP value while the Timer/Counter is running. When
using a static TOP value there are practically no differences between the two modes of
operation.
In phase correct PWM mode, the compare units allow generation of PWM waveforms on
the OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an
inverted PWM output can be generated by setting the COM1x1:0 to 3 (See Table 44 on
page 104). The actual OC1x value will only be visible on the port pin if the data direction
for the port pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x register at the compare match between OCR1x and TCNT1
when the counter increments, and clearing (or setting) the OC1x register at compare
match between OCR1x and TCNT1 when the counter decrements. The PWM frequency
99
for the output when using phase correct PWM can be calculated by the following
equation:
f
clk_I/O
f
OCnxPCPWM
----------------------------=
2 NTOP⋅⋅
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x register represent special cases when generating a
PWM waveform output in the phase correct PWM mode. If the OCR1x is set equal to
BOTTOM the output will be continuously low and if set equal to TOP the output will be
continuously high for non-inverted PWM mode. For inverted PWM the output will have
the opposite logic values.
Phase and Frequency Correct
PWM Mode
The phase and frequency correct pulse width modulation, or phase and frequency cor-
rect PWM mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency
correct PWM waveform generation option. The phase and frequency correct PWM
mode is, like the phase correct PWM mode, based on a dual-slope operation. The
counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting compare output mode, the output compare (OC1x) is cleared on
the compare match between TCNT1 and OCR1x while upcounting, and set on the compare match while downcounting. In inverting compare output mode, the operation is
inverted. The dual-slope operation gives a lower maximum operation frequency compared to the single-slope operation. However, due to the symmetric feature of the dualslope PWM modes, these modes are preferred for motor control applications.
The main difference between the phase correct, and the phase and frequency correct
PWM mode is the time the OCR1x register is updated by the OCR1x buffer register,
(see Figure 47 and Figure 48).
The PWM resolution for the phase and frequency correct PWM mode can be defined by
either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to
0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM
resolution in bits can be calculated using the following equation:
R
PFCPWM
TOP 1+()log
-----------------------------------=
2
()log
In phase and frequency correct PWM mode the counter is incremented until the counter
value matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A
(WGM13:0 = 9). The counter has then reached the TOP and changes the count direction. The TCNT1 value will be equal to TOP for one timer clock cycle. The timing
diagram for the phase correct and frequency correct PWM mode is shown on Figure 48.
The figure shows phase and frequency correct PWM mode when OCR1A or ICR1 is
used to define TOP. The TCNT1 value is in the timing diagram shown as a histogram for
illustrating the dual-slope operation. The diagram includes non-inverted and inverted
PWM outputs. The small horizontal line marks on the TCNT1 slopes represent compare
matches between OCR1x and TCNT1. The OC1x interrupt flag will be set when a compare match occurs.
100
ATmega16(L)
2466B–09/01
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