• Endurance: 100,000 Write/Erase Cycles
– 128/256/512 Bytes Internal SRAM (ATtiny25/45/85)
– Programming Lock for Self-Programming Flash Program and EEPROM Data
Security
• Peripheral Features
– 8-bit Timer/Counter with Prescaler and Two PWM Channels
– 8-bit High Speed Timer/Counter with Separate Prescaler
• 2 High Frequency PWM Outputs with Separate Output Compare Registers
• Programmable Dead Time Generator
– Universal Serial Interface with Start Condition Detector
– 10-bit ADC
• 4 Single Ended Channels
• 2 Differential ADC Channel Pairs with Programmable Gain (1x, 20x)
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
• Special Microcontroller Features
– debugWIRE On-chip Debug System
– In-System Programmable via SPI Port
– External and Internal Interrupt Sources
– Low Power Idle, ADC Noise Reduction, and Power-down Modes
– Enhanced Power-on Reset Circuit
– Programmable Brown-out Detection Circuit
– Internal Calibrated Oscillator
The ATtiny25/45/85 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 ATtiny25/45/85
achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize
power consumption versus processing speed.
2
ATtiny25/45/85
7598G–AVR–03/08
2.1Block Diagram
PROGRAM
COUNTER
INTERNAL
OSCILLA
TOR
WATCHDOG
TIMER
STACK
POINTER
PROGRAM
FLASH
SRAM
MCU CONTROL
REGISTER
GENERAL
PURPOSE
REGISTERS
INSTRUCTION
REGISTER
TIMER/
COUNTER0
SERIAL
UNIVERSAL
INTERFACE
TIMER/
COUNTER1
INSTRUCTION
DECODER
DATA DIR.
REG.PORT B
DATA REGISTER
PORT B
PROGRAMMING
LOGIC
TIMING AND
CONTROL
MCU STATUS
REGISTER
STATUS
REGISTER
ALU
PORT B DRIVERS
PB0-PB5
VCC
GND
CONTROL
LINES
8-BIT DATABUS
Z
ADC /
ANALOG COMPARATOR
INTERRUPT
UNIT
DATA
EEPROM
CALIBRATED
OSCILLATORS
Y
X
RESET
ATtiny25/45/85
Figure 2-1.Block Diagram
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.
7598G–AVR–03/08
The ATtiny25/45/85 provides the following features: 2/4/8K byte of In-System Programmable
Flash, 128/256/512 bytes EEPROM, 128/256/256 bytes SRAM, 6 general purpose I/O lines, 32
general purpose working registers, one 8-bit Timer/Counter with compare modes, one 8-bit high
3
speed Timer/Counter, Universal Serial Interface, Internal and External Interrupts, a 4-channel,
10-bit ADC, a programmable Watchdog Timer with internal Oscillator, and three software selectable power saving m o d e s . Th e Idl e mode stops the C P U whil e al l o w i n g the SRAM,
Timer/Counter, ADC, Analog Comparator, and Interrupt system to continue functioning. The
Power-down mode saves the register contents, disabling all chip functions until the next Interrupt or Hardware Reset. The ADC Noise Reduction mode stops the CPU and all I/O modules
except ADC, to minimize switching noise during ADC conversions.
The device is manufactured using Atmel’s high density non-volatile memory technology. The
On-chip ISP Flash allows the Program memory to be re-programmed In-System through an SPI
serial interface, by a conventional non-volatile memory programmer or by an On-chip boot code
running on the AVR core.
The ATtiny25/45/85 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.
2.2Automotive Quality Grade
The ATtiny25/45/85 have been developed and manufactured according to the most stringent
requirements of the international standard ISO-TS-16949. This data sheet contains limit values
extracted from the results of extensive characterization (Temperature and Voltage). The quality
and reliability of the ATtiny25/45/85 have been verified during regular product qualification as
per AEC-Q100 grade 1.
As indicated in the ordering information paragraph, the products are available in three different
temperature grades, but with equivalent quality and reliability objectives. Different temperature
identifiers have been defined as listed in Table 2-1.
Table 2-1.Temperature Grade Identification for Automotive Products
Temperature
Temperature
-40 ; +85T
-40 ; +105T1Reduced Automotive Temperature Range
-40 ; +125ZFull AutomotiveTemperature Range
IdentifierComments
Similar to Industrial Temperature Grade but with Automotive
Quality
4
ATtiny25/45/85
7598G–AVR–03/08
2.3Pin Descriptions
2.3.1VCC
Supply voltage.
2.3.2GND
Ground.
2.3.3Port B (PB5..PB0)
Port B is a 6-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 tri-stated 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 ATtiny25/45/85 as listed on
page 54.
2.3.4RESET
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 8-1 on page
36. Shorter pulses are not guaranteed to generate a reset.
ATtiny25/45/85
3.About Code Examples
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.
4.AVR CPU Core
4.1Introduction
This 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.
7598G–AVR–03/08
5
4.2Architectural Overview
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Program
Counter
Control Lines
32 x 8
General
Purpose
Registrers
ALU
Status
and Control
I/O Lines
EEPROM
Data Bus 8-bit
Data
SRAM
Direct Addressing
Indirect Addressing
Interrupt
Unit
Watchdog
Timer
Analog
Comparator
I/O Module 2
I/O Module1
I/O Module n
Figure 4-1.Block Diagram of the AVR Architecture
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.
6
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
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 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 are 16-bits wide. There are
also 32-bit instructions.
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, 0x20 - 0x5F.
4.3ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose
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 “Instruction Set” section for a detailed description.
4.4Status Register
The 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
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
ITHSVNZCSREG
7598G–AVR–03/08
• 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
7
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 I-bit 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 ⊕ V
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
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.
8
ATtiny25/45/85
7598G–AVR–03/08
4.5General Purpose Register File
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 4-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 4-2.AVR CPU General Purpose Working Registers
GeneralR140x0E
PurposeR150x0F
WorkingR160x10
RegistersR170x11
ATtiny25/45/85
70Addr.
R0 0x00
R10x01
R20x02
…
R130x0D
…
R260x1AX-register Low Byte
R270x1BX-register High Byte
R280x1CY-register Low Byte
R290x1DY-register High Byte
R300x1EZ-register Low Byte
R310x1FZ-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.
As shown in Figure 4-2, 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.
4.5.1The X-register, Y-register, and Z-register
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 4-3.
Figure 4-3.The X-, Y-, and Z-registers
15XHXL0
X-register7070
R27 (0x1B)R26 (0x1A)
15YHYL0
7598G–AVR–03/08
9
4.6Stack Pointer
Y-register7070
R29 (0x1D)R28 (0x1C)
15ZHZL0
Z-register7070
R31 (0x1F)R30 (0x1E)
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).
The 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 0x60. 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.
Bit15141312111098
SP15SP14SP13SP12SP11SP10SP9SP8SPH
SP7SP6SP5SP4SP3SP2SP1SP0SPL
76543210
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
R/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
10011111
4.7Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR
CPU is driven by the CPU clock clk
chip. No internal clock division is used.
Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Har-
vard 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.
, directly generated from the selected clock source for the
CPU
10
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
clk
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1T2T3T4
CPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1T2T3T4
clk
CPU
Figure 4-4.The Parallel Instruction Fetches and Instruction Executions
Figure 4-5 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.
Figure 4-5.Single Cycle ALU Operation
4.8Reset and Interrupt Handling
7598G–AVR–03/08
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.
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 45. 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.
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
11
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.
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.
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) */
12
ATtiny25/45/85
7598G–AVR–03/08
4.8.1Interrupt Response Time
0x0000
0x03FF/0x07FF
Program Memory
The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After four clock cycles the Program Vector address for the actual interrupt handling routine
is executed. During this four 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 three 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 four 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 four clock cycles. During these four clock
cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is
incremented by two, and the I-bit in SREG is set.
5.AVR ATtiny25/45/85 Memories
This section describes the different memories in the ATtiny25/45/85. The AVR architecture has
two main memory spaces, the Data memory and the Program memory space. In addition, the
ATtiny25/45/85 features an EEPROM Memory for data storage. All three memory spaces are linear and regular.
ATtiny25/45/85
5.1In-System Re-programmable Flash Program Memory
The ATtiny25/45/85 contains 2/4/8K byte 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
1024/2048/4096 x 16.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATtiny25/45/85
Program Counter (PC) is 10/11/12 bits wide, thus addressing the 1024/2048/4096 Program
memory locations. “Memory Programming” on page 129 contains a detailed description on Flash
data serial downloading using the SPI pins.
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 Tim-
ing” on page 10.
Figure 5-1.Program Memory Map
7598G–AVR–03/08
13
5.2SRAM Data Memory
32 Registers
64 I/O Registers
Internal SRAM
(128/256/512 x 8)
0x0000 - 0x001F
0x0020 - 0x005F
0x0DF/0x015F/0x025F
0x0060
Data Memory
Figure 5-2 shows how the ATtiny25/45/85 SRAM Memory is organized.
The lower 224/352/607 Data memory locations address both the Register File, the I/O memory
and the internal data SRAM. The first 32 locations address the Register File, the next 64 locations the standard I/O memory, and the last 128/256/512 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 post-increment, the address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O Registers, and the 128/256/512 bytes of internal data SRAM in the ATtiny25/45/85 are all accessible through all these addressing modes.
The Register File is described in “General Purpose Register File” on page 9.
Figure 5-2.Data Memory Map
5.2.1Data Memory Access Times
14
ATtiny25/45/85
This section describes the general access timing concepts for internal memory access. The
internal data SRAM access is performed in two clk
cycles as described in Figure 5-3.
CPU
7598G–AVR–03/08
Figure 5-3.On-chip Data SRAM Access Cycles
clk
WR
RD
Data
Data
Address
Address valid
T1T2T3
Compute Address
Read
Write
CPU
Memory Access Instruction
Next Instruction
5.3EEPROM Data Memory
The ATtiny25/45/85 contains 128/256/512 bytes of data EEPROM memory. It is organized as a
separate 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 Serial data
downloading to the EEPROM, see page 133.
ATtiny25/45/85
5.3.1EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access times for the EEPROM are given in Table 5-1. 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 heavily filtered power
supplies, VCC is likely to rise or fall slowly on Power-up/down. This 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 “Atomic Byte Programming” on page 17 and “Split Byte Programming” on page 18 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.
5.3.2EEPROM Address Register High – EEARH
Bit76543210
-------EEAR8EEARH
Read/WriteRRRRRRRR/W
Initial ValueXXXXXXXX
7598G–AVR–03/08
15
• Bit 7..1 – Res6..0: Reserved Bits
These bits are reserved for future use and will always read as 0 in ATtiny25/45/85.
• Bits 0 – EEAR8: EEPROM Address
The EEPROM Address Register – EEARH – specifies the high EEPROM address in the
128/256/512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0
and 127/255/511. The initial value of EEAR is undefined. A proper value must be written before
the EEPROM may be accessed.
5.3.3EEPROM Address Register – EEARL
Bit76543210
EEAR7EEAR6EEAR5EEAR4EEAR3EEAR2EEAR1EEAR0EEARL
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial ValueXXXXXXXX
• Bits 7..0 – EEAR7..0: EEPROM Address
The EEPROM Address Register – EEARL – specifies the low EEPROM address in the
128/256/512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0
and 127/255/511. The initial value of EEAR is undefined. A proper value must be written before
the EEPROM may be accessed.
5.3.4EEPROM Data Register – EEDR
Bit76543210
EEDR7EEDR6EEDR5EEDR4EEDR3EEDR2EEDR1EEDR0EEDR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial ValueXXXXXXXX
• 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 operation, the
EEDR contains the data read out from the EEPROM at the address given by EEAR.
5.3.5EEPROM Control Register – EECR
Bit76543210
––EEPM1EEPM0EERIEEEMPEEEPEEEREEECR
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial Value00XX00X0
• Bit 7 – Res: Reserved Bit
This bit is reserved for future use and will always read as 0 in ATtiny25/45/85. For compatibility
with future AVR devices, always write this bit to zero. After reading, mask out this bit.
• Bit 6 – Res: Reserved Bit
This bit is reserved in the ATtiny25/45/85 and will always read as zero.
The EEPROM Programming mode bits setting defines which programming action that will be
triggered when writing EEPE. It is possible to program data in one atomic operation (erase the
old value and program the new value) or to split the Erase and Write operations in two different
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
operations. The Programming times for the different modes are shown in Table 5-1. While EEPE
is set, any write to EEPMn will be ignored. During reset, the EEPMn bits will be reset to 0b00
unless the EEPROM is busy programming.
Table 5-1.EEPROM Mode Bits
Programming
EEPM1EEPM0
003.4 msErase and Write in one operation (Atomic Operation)
011.8 msErase Only
101.8 msWrite Only
11–Reserved for future use
• 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 Non-volatile memory is ready for programming.
• Bit 2 – EEMPE: EEPROM Master Program Enable
The EEMPE bit determines whether writing EEPE to one will have effect or not.
TimeOperation
When EEMPE is set, setting EEPE within four clock cycles will program the EEPROM at the
selected address. If EEMPE is zero, setting EEPE will have no effect. When EEMPE has been
written to one by software, hardware clears the bit to zero after four clock cycles.
• Bit 1 – EEPE: EEPROM Program Enable
The EEPROM Program Enable Signal EEPE is the programming enable signal to the EEPROM.
When EEPE is written, the EEPROM will be programmed according to the EEPMn bits setting.
The EEMPE bit must be written to one before a logical one is written to EEPE, otherwise no
EEPROM write takes place. When the write access time has elapsed, the EEPE bit is cleared
by hardware. When EEPE 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 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 EEPE 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.
5.3.6Atomic Byte Programming
Using Atomic Byte Programming is the simplest mode. When writing a byte to the EEPROM, the
user must write the address into the EEAR Register and data into EEDR Register. If the EEPMn
bits are zero, writing EEPE (within four cycles after EEMPE is written) will trigger the erase/write
operation. Both the erase and write cycle are done in one operation and the total programming
time is given in Table 20-1. The EEPE bit remains set until the erase and write operations are
completed. While the device is busy with programming, it is not possible to do any other
EEPROM operations.
7598G–AVR–03/08
17
5.3.7Split Byte Programming
It is possible to split the erase and write cycle in two different operations. This may be useful if
the system requires short access time for some limited period of time (typically if the power supply voltage falls). In order to take advantage of this method, it is required that the locations to be
written have been erased before the write operation. But since the erase and write operations
are split, it is possible to do the erase operations when the system allows doing time-critical
operations (typically after Power-up).
5.3.8Erase
To erase a byte, the address must be written to EEAR. If the EEPMn bits are 0b01, writing the
EEPE (within four cycles after EEMPE is written) will trigger the erase operation only (programming time is given in Table 20-1). The EEPE bit remains set until the erase operation completes.
While the device is busy programming, it is not possible to do any other EEPROM operations.
5.3.9Write
To write a location, the user must write the address into EEAR and the data into EEDR. If the
EEPMn bits are 0b10, writing the EEPE (within four cycles after EEMPE is written) will trigger
the write operation only (programming time is given in Table 20-1). The EEPE bit remains set
until the write operation completes. If the location to be written has not been erased before write,
the data that is stored must be considered as lost. While the device is busy with programming, it
is not possible to do any other EEPROM operations.
The calibrated Oscillator is used to time the EEPROM accesses. Make sure the Oscillator frequency is within the requirements described in “Oscillator Calibration Register – OSCCAL” on
page 26.
The following code examples show one assembly and one C function for erase, write, or atomic
write of the EEPROM. The examples assume that interrupts are controlled (e.g., by disabling
interrupts globally) so that no interrupts will occur during execution of these functions
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.
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage 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 VCC reset protection circuit can
be used. If a reset occurs while a write operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.
20
ATtiny25/45/85
7598G–AVR–03/08
5.4I/O Memory
General I/O
Modules
CPU CoreRAM
clk
I/O
AVR Clock
Control Unit
clk
CPU
Flash and
EEPROM
clk
FLASH
Source clock
Watchdog Timer
Watchdog
Oscillator
Reset Logic
Clock
Multiplexer
Watchdog clock
Calibrated RC
Oscillator
Calibrated RC
Oscillator
External Clock
ADC
clk
ADC
Crystal
Oscillator
Low-Frequency
Crystal Oscillator
System Clock
Prescaler
PLL
Oscillator
clk
PCK
clk
PCK
ATtiny25/45/85
The I/O space definition of the ATtiny25/45/85 is shown in “Register Summary” on page 179.
All ATtiny25/45/85 I/Os and peripherals are placed in the I/O space. All I/O locations may be
accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32
general purpose working registers and the I/O space. I/O Registers within the address range
0x00 - 0x1F 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 0x00 - 0x3F must be used. When addressing I/O Registers as data space using
LD and ST instructions, 0x20 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 only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and Peripherals Control Registers are explained in later sections.
6.System Clock and Clock Options
6.1Clock Systems and their Distribution
Figure 6-1 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 Manage-
ment and Sleep Modes” on page 30. The clock systems are detailed below.
Figure 6-1.Clock Distribution
7598G–AVR–03/08
21
6.1.1CPU Clock – clk
CPU
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.
6.1.2I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counter. 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.
6.1.3Flash Clock – clk
FLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock.
6.1.4ADC 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.
6.1.5Internal PLL for Fast Peripheral Clock Generation - clk
The internal PLL in ATtiny25/45/85 generates a clock frequency that is 8x multiplied from a
source input. The source of the PLL input clock is the output of the internal RC oscillator having
a frequency of 8.0 MHz. Thus the output of the PLL, the fast peripheral clock is 64 MHz. The fast
peripheral clock, or a clock prescaled from that, can be selected as the clock source for
Timer/Counter1. See the Figure 6-2 on page 23.
The PLL is locked on the RC oscillator and adjusting the RC oscillator via OSCCAL register will
adjust the fast peripheral clock at the same time. However, even if the RC oscillator is taken to a
higher frequency than 8 MHz, the fast peripheral clock frequency saturates at 85 MHz (worst
case) and remains oscillating at the maximum frequency. It should be noted that the PLL in this
case is not locked any longer with the RC oscillator clock.
PCK
22
Therefore, it is recommended not to take the OSCCAL adjustments to a higher frequency than 8
MHz in order to keep the PLL in the correct operating range. The internal PLL is enabled only
when the PLLE bit in PLLCSR is set or the PLLCK fuse is programmed (‘0’). The bit PLOCK
from PLLCSR is set when PLL is locked.
Both internal RC oscillator and PLL are switched off in power down and stand-by sleep modes.
ATtiny25/45/85
7598G–AVR–03/08
Figure 6-2.PCK Clocking System
8.0 MHz / 6.4 MHz
RC OSCILLATOR
OSCCAL
XTAL1
XTAL2
OSCILLATORS
DIVIDE
BY 4
SYSTEM
CLOCK
PLL
8x / 4x
PLLCK & CKSEL FUSES
PLLE
PCK
Lock
Detector
PLOCK
64 / 25.6 MHz
System
Clock
Prescaler
CLKPS3..0
ATtiny25/45/85
6.2Clock Sources
The 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 6-1.Device Clocking Options Select
Device Clocking Option CKSEL3..0
External Clock0000
PLL Clock0001
Calibrated Internal RC Oscillator 8.0 MHz0010
Watchdog Oscillator 128 kHz0100
External Low-frequency Crystal0110
External Crystal/Ceramic Resonator1000-1111
Reserved0101, 0111, 0011
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 startup, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts
from reset, there is 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 6-
2.
(1)
7598G–AVR–03/08
23
Table 6-2.Number of Watchdog Oscillator Cycles
XTAL2
XTAL1
GND
C2
C1
6.3Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is therefore the Internal RC Oscillator running at 8 MHz with longest start-up
time and an initial system clock prescaling of 8. This default setting ensures that all users can
make their desired clock source setting using an In-System or High-voltage Programmer.
6.4Crystal Oscillator
XTAL1 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 6-3. Either a quartz crystal or a
ceramic resonator may be used.
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 6-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Typ Time-outNumber of Cycles
4 ms512
64 ms8K (8,192)
24
Figure 6-3.Crystal Oscillator Connections
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 6-3.
Table 6-3.Crystal Oscillator Operating Modes
CKSEL3..1 Frequency Range (MHz)
(1)
100
1010.9 - 3.012 - 22
1103.0 - 8.012 - 22
1118.0 -12 - 22
Notes:1. This option should not be used with crystals, only with ceramic resonators.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
6-4.
ATtiny25/45/85
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
0.4 - 0.9–
7598G–AVR–03/08
ATtiny25/45/85
Table 6-4.Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
CKSEL0SUT1..0
000258 CK
001258 CK
0101K CK
0111K CK
1001K CK
Power-save
(1)
(1)
(2)
(2)
(2)
10116K CK14CK
11016K CK14CK + 4 ms
11116K CK14CK + 64 ms
Additional Delay
from Reset
(VCC = 5.0V)Recommended Usage
14CK + 4 ms
14CK + 64 ms
14CK
14CK + 4 ms
14CK + 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
Notes:1. These options should only be used when not operating close to the maximum frequency 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.
6.5Low-frequency Crystal Oscillator
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 CKSEL fuses to ‘0110’. The crystal should be connected
as shown in Figure 6-3. Refer to the 32 kHz Crystal Oscillator Application Note for details on
oscillator operation and how to choose appropriate values for C1 and C2.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in
Table 6-5.
Table 6-5.Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power Down and Power
SUT1..0
001K CK
011K CK
1032K CK64 msStable frequency at start-up
11Reserved
Notes:1. These options should only be used if frequency stability at start-up is not important for the
application.
Save
Additional Delay from
Power On Reset
(VCC = 5.0V)Recommended usage
(1)
(1)
4 ms
64 msSlowly rising power
Fast rising power or BOD
enabled
7598G–AVR–03/08
25
6.6Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator provides an 8.0 MHz clock. The frequency is the nominal
value at 3V and 25°C. If the frequency exceeds the specification of the device (depends on VCC),
the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 during
start-up. “System Clock Prescaler” on page 29. for more details. This clock may be selected as
the system clock by programming the CKSEL Fuses as shown in Table 6-6. If selected, it will
operate with no external components. During reset, hardware loads the calibration byte into the
OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this
calibration gives a frequency within ± 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
Note:1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-7.
Table 6-7.Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
8.0 MHz
Start-up Time
SUT1..0
006 CK14CK + 4 msBOD enabled
016 CK14CK + 4 msFast rising power
(1)
10
11Reserved
Note:1. The device is shipped with this option selected.
from Power-down
6 CK14CK + 64 msSlowly rising power
6.6.1Oscillator 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 non-zero values to this register will increase the frequency of the internal Oscillator. Writing 0xFF 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 8.8 MHz frequency. Otherwise, the EEPROM or Flash write may fail.
Additional Delay from
Reset (VCC = 5.0V)Recommended Usage
26
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the
lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
EXTERNAL
CLOCK
SIGNAL
CLKI
GND
quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher
frequency than OSCCAL = 0x80.
The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00
gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the
range. Incrementing CAL6..0 by 1 will give a frequency increment of less than 2% in the frequency range 7.3 - 8.1 MHz.
Avoid changing the calibration value in large steps when calibrating the calibrated internal RC
Oscillator to ensure stable operation of the MCU. A variation in frequency of more than 2% from
one cycle to the next can lead to unpredicatble behavior. Changes in OSCCAL should not
exceed 0x20 for each calibration. It is required to ensure that the MCU is kept in Reset during
such changes in the clock frequency
Table 6-8.Internal RC Oscillator Frequency Range
6.7External Clock
Min Frequency in Percentage of
OSCCAL Value
0x0050%100%
0x3F75%150%
0x7F100%200%
Nominal Frequency
Max Frequency in Percentage of
Nominal Frequency
To drive the device from an external clock source, CLKI should be driven as shown in Figure 6-
4. To run the device on an external clock, the CKSEL Fuses must be programmed to “00”.
Figure 6-4.External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-9.
7598G–AVR–03/08
Table 6-9.Start-up Times for the External Clock Selection
Start-up Time from Power-
SUT1..0
006 CK14CKBOD enabled
016 CK14CK + 4 msFast rising power
106 CK14CK + 64 msSlowly rising power
11Reserved
down and Power-save
Additional Delay from
ResetRecommended Usage
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page
29 for details.
27
6.7.1High Frequency PLL Clock - PLL
There is an internal PLL that provides nominally 64 MHz clock rate locked to the RC Oscillator
for the use of the Peripheral Timer/Counter1 and for the system clock source. When selected as
a system clock source, by programming the CKSEL fuses to ‘0001’, it is divided by four like
shown in Table 6-10. When this clock source is selected, start-up times are determined by the
SUT fuses as shown in Table 6-11. See also “PCK Clocking System” on page 23.
Table 6-10.PLLCK Operating Modes
CKSEL3..0Nominal Frequency
000116 MHz
Table 6-11.Start-up Times for the PLLCK
CLK
SUT1..0
001K CK14CK + 8msBOD enabled
0116K CK14CK + 8msFast rising power
101K CK14CK + 68 msSlowly rising power
1116K CK14CK + 68 msSlowly rising power
6.8128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The frequ ency is nominal at 3V and 25 °C. This clock may be select as the sy stem clo ck by
programming the CKSEL Fuses to “11”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-12.
Table 6-12.Start-up Times for the 128 kHz Internal Oscillator
SUT1..0
006 CK14CKBOD enabled
016 CK14CK + 4 msFast rising power
106 CK14CK + 64 msSlowly rising power
Start-up Time from Power
Down and Power Save
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)Recommended usage
Additional Delay from
ResetRecommended Usage
11Reserved
6.9Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT
Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. Note that the clock will not be output during reset and the normal operation
of I/O pin will be overridden when the fuse is programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on CLKO. If the System Clock
Prescaler is used, it is the divided system clock that is output.
28
ATtiny25/45/85
7598G–AVR–03/08
6.10System Clock Prescaler
The ATtiny25/45/85 system clock can be divided by setting the Clock Prescale Register –
CLKPR. This feature can be used to decrease power consumption when the requirement for
processing power is low. This can be used with all clock source options, and it will affect the
clock frequency of the CPU and all synchronous peripherals. clk
are divided by a factor as shown in Table 6-13.
6.10.1Clock Prescale Register – CLKPR
Bit76543210
CLKPCE–––CLKPS3CLKPS2CLKPS1CLKPS0CLKPR
Read/WriteR/WRRRR/WR/WR/WR/W
Initial Value0000See Bit Description
• Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE
bit is only updated when the other bits in CLKPR are simultaniosly written to zero. CLKPCE is
cleared by hardware four cycles after it is written or when the CLKPS bits are written. Rewriting
the CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the
CLKPCE bit.
ATtiny25/45/85
I/O
, clk
ADC
, clk
, and clk
CPU
FLASH
• Bits 6..4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
These bits define the division factor between the selected clock source and the internal system
clock. These bits can be written run-time to vary the clock frequency to suit the application
requirements. As the divider divides the master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is used. The division factors are given in
Table 6-13.
To avoid unintentional changes of clock frequency, a special write procedure must be followed
to change the CLKPS bits:
1.Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
CLKPR to zero.
2.Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed,
the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to
“0011”, giving a division factor of eight at start up. This feature should be used if the selected
clock source has a higher frequency than the maximum frequency of the device at the present
operating conditions. Note that any value can be written to the CLKPS bits regardless of the
CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor is
chosen if the selcted clock source has a higher frequency than the maximum frequency of the
7598G–AVR–03/08
29
device at the present operating conditions. The device is shipped with the CKDIV8 Fuse
programmed.
Table 6-13.Clock Prescaler Select
CLKPS3CLKPS2CLKPS1CLKPS0Clock Division Factor
00001
00012
00104
00118
010016
010132
011064
0111128
1000256
1001Reserved
1010Reserved
1011Reserved
1100Reserved
1101Reserved
1110Reserved
1111Reserved
6.10.2Switching Time
When switching between prescaler settings, the System Clock Prescaler ensures that no
glitches occur in the clock system and that no intermediate frequency is higher than neither the
clock frequency corresponding to the previous setting, nor the clock frequency corresponding to
the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock,
which may be faster than the CPU’s clock frequency. Hence, it is not possible to determine the
state of the prescaler – even if it were readable, and the exact time it takes to switch from one
clock division to another cannot be exactly predicted.
From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2*T2 before the
new clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is the
previous clock period, and T2 is the period corresponding to the new prescaler setting.
7.Power Management and Sleep Modes
The high performance and industry leading code efficiency makes the AVR microcontrollers an
ideal choise for low power applications.
30
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.
ATtiny25/45/85
7598G–AVR–03/08
To enter any of the three sleep modes, the SE bit in MCUCR must be written to logic one and a
SLEEP instruction must be executed. The SM1..0 bits in the MCUCR Register select which
sleep mode (Idle, ADC Noise Reduction, or Power-down) will be activated by the SLEEP instruction. See Table 7-1 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,
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 6-1 on page 21 presents the different clock systems in the ATtiny25/45/85, and their dis-
tribution. The figure is helpful in selecting an appropriate sleep mode.
7.0.1MCU Control Register – MCUCR
The MCU Control Register contains control bits for power management.
Bit76543210
BODSPUDSESM1SM0BODSEISC01ISC00MCUCR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 7 – BODS: BOD Sleep
BOD disable functionality is available in some devices, only. See “Limitations” on page 33.
ATtiny25/45/85
In order to disable BOD during sleep (see Table 7-2 on page 33) the BODS bit must be written to
logic one. This is controlled by a timed sequence and the enable bit, BODSE in MCUCR.
First,both BODS and BODSE must be set to one. Second, within four clock cycles, BODS must
be set to one and BODSE must be set to zero. The BODS bit is active three clock cycles after it
is set. A sleep instruction must be executed while BODS is active in order to turn off the BOD for
the actual sleep mode. The BODS bit is automatically cleared after three clock cycles.
In devices where Sleeping BOD has not been implemented this bit is unused and will always
read zero.
• Bit 5 – SE: Sleep Enable
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 programmer’s
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.
• Bits 4, 3 – SM1..0: Sleep Mode Select Bits 2..0
These bits select between the three available sleep modes as shown in Table 7-1.
Table 7-1.Sleep Mode Select
SM1SM0Sleep Mode
00Idle
01ADC Noise Reduction
7598G–AVR–03/08
10Power-down
11Stand-by mode
• Bit 2 – BODSE: BOD Sleep Enable
BOD disable functionality is available in some devices, only. See “Limitations” on page 33.
31
The BODSE bit enables setting of BODS control bit, as explained on BODS bit description. BOD
disable is controlled by a timed sequence.
This bit is unused in devices where software BOD disable has not been implemented and will
read as zero in those devices.
7.1Idle Mode
When the SM1..0 bits are written to 00, the SLEEP instruction makes the MCU enter Idle mode,
stopping the CPU but allowing Analog Comparator, ADC, Timer/Counter, Watchdog, and the
interrupt system to continue operating. This sleep mode basically halts clk
allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal
ones like the Timer Overflow. 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.
7.2ADC Noise Reduction Mode
When the SM1..0 bits are written to 01, the SLEEP instruction makes the MCU enter ADC Noise
Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, and the
Watchdog to continue operating (if enabled). This sleep mode halts clk
while allowing the other clocks to run.
I/O
CPU
, clk
and clk
, and clk
CPU
FLASH
, while
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, an SPM/EEPROM ready interrupt, an external level interrupt on INT0 or a pin change
interrupt can wake up the MCU from ADC Noise Reduction mode.
7.3Power-down Mode
When the SM1..0 bits are written to 10, the SLEEP instruction makes the MCU enter Powerdown mode. In this mode, the Oscillator is stopped, while the external interrupts, and the Watchdog continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out
Reset, an external level interrupt on INT0, or a pin change interrupt can wake up the MCU. This
sleep mode halts all generated clocks, allowing operation of asynchronous modules only.
32
ATtiny25/45/85
7598G–AVR–03/08
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 58
for details..
Table 7-2.Active Clock Domains and Wake-up Sources in the Different Sleep Modes
Active Clock DomainsOscillatorsWake-up Sources
ATtiny25/45/85
CPU
Sleep Mode
IdleXXXXXXXXXX
ADC Noise
Reduction
Power-downX
clk
FLASH
clk
Note:1. For INT0, only level interrupt.
IO
clk
ADC
clk
XXX
PCK
clk
Main Clock
Source Enabled
INT0 and
Pin Change
SPM/
EEPROM
Ready
USI Start Condition
ADC
Other I/O
(1)
(1)
XXXX
XX
7.4Limitations
BOD disable functionality has been implemented in the following devices, only:
•
ATtiny25, revision D, and newer
•
ATtiny45, revision D, and newer
•
ATtiny85, revision C, and newer
7.5Power Reduction Register
The Power Reduction Register, PRR, provides a method to stop the clock to individualperipherals to reduce power consumption. The current state of the peripheral is frozenand the I/O
registers can not be read or written. Resources used by the peripheral when stopping the clock
will remain occupied, hence the peripheral should in most cases be disabled before stopping the
clock. Waking up a module, which is done by clearing the bit in PRR, puts the module in the
same state as before shutdown.
Watchdog
Interrupt
7598G–AVR–03/08
Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall
power consumption. In all other sleep modes, the clock is already stopped.
Bit76543210
–---PRTIM1PRTIM0PRUSIPRADCPRR
Read/WriteRRRRR/WR/WR/WR/W
Initial Value00000000
• Bits 7, 6, 5, 4- Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 3- PRTIM1: Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1
is enabled, operation will continue like before the shutdown.
• Bit 2- PRTIM0: Power Reduction Timer/Counter0
33
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0
is enabled, operation will continue like before the shutdown.
• Bit 1 - PRUSI: Power Reduction USI
Writing a logic one to this bit shuts down the USI by stopping the clock to the module. When
waking up the USI again, the USI should be re initialized to ensure proper operation.
• Bit 0 - PRADC: Power Reduction ADC
Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down.
The analog comparator cannot use the ADC input MUX when the ADC is shut down.
7.6Minimizing 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.
7.6.1Analog to Digital Converter
If 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 107
for details on ADC operation.
7.6.2Analog Comparator
When 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 m odes. Oth erwise, the Internal Vo ltage Reference will be enable d,
independent of sleep mode. Refer to “Analog Comparator” on page 104 for details on how to
configure the Analog Comparator.
7.6.3Brown-out Detector
If 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 BODLEVEL Fuses, 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 38 for details on how to
configure the Brown-out Detector.
7.6.4Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, 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 Volt-
age Reference” on page 40 for details on the start-up time.
34
ATtiny25/45/85
7598G–AVR–03/08
7.6.5Watchdog Timer
7.6.6Port Pins
ATtiny25/45/85
If 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 41 for details on how to configure the Watchdog Timer.
When 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
both the I/O clock (clk
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 has an
analog signal level close to VCC/2, the input buffer will use excessive power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal
level close to VCC/2 on an input pin can cause significant current even in active mode. Digital
input buffers can be disabled by writing to the Digital Input Disable Register (DIDR0). Refer to
“Digital Input Disable Register 0 – DIDR0” on page 107 for details.
) and the ADC clock (clk
I/O
) are stopped, the input buffers of the device
ADC
8.System Control and Reset
8.0.1Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution
from the Reset Vector. The instruction placed at the Reset Vector must be a RJMP – Relative
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. The circuit diagram in Figure 8-1 shows the reset logic. Table 8-1 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 SUT and CKSEL Fuses. The different selections for the delay period are presented in “Clock Sources” on page 23.
8.0.2Reset Sources
The ATtiny25/45/85 has four 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 VCC is below the Brown-out
Reset threshold (V
POT
).
) and the Brown-out Detector is enabled.
BOT
7598G–AVR–03/08
35
Figure 8-1.Reset Logic
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit
BODLEVEL [1..0]
Delay Counters
CKSEL[1:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
Clock
Generator
SPIKE
FILTER
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit
8.0.3Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in Table 8-1. The POR is activated whenever VCC is below the 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 VCC rise. The RESET signal is activated again, without any delay,
when VCC decreases below the detection level.
Power-on Reset Threshold Voltage (rising)1.11.41.7V
V
POT
V
PORMAX
V
PORMIN
V
CCRR
V
RST
Power-on Reset Threshold Voltage (falling)
VCC Max. start voltage to ensure internal Poweron Reset signal
VCC Min. start voltage to ensure internal Poweron Reset signal
VCC Rise Rate to ensure Power-on Reset0.01V/ms
RESET Pin Threshold Voltage 0.1 V
Note:1. Before rising the supply has to be between V
(1)
PORMIN
0.81.31.6V
-0.1V
0.9V
and V
CC
PORMAX
to ensure Reset.
0.4V
CC
V
An External Reset is generated by a low level on the RESET pin if enabled. Reset pulses longer
than the minimum pulse width (see Table 8-1) will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches
the Reset Threshold Voltage – V
after the Time-out period – t
TOUT –
– on its positive edge, the delay counter starts the MCU
RST
has expired.
37
Figure 8-4.External Reset During Operation
CC
8.0.5Brown-out Detection
ATtiny25/45/85 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level
during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be
selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free
Brown-out Detection. The hysteresis on the detection level should be interpreted as V
V
BOT
Table 8-2.BODLEVEL Fuse Coding
+ V
HYST
/2 and V
BOT-
= V
BOT
- V
HYST
/2.
(1)
BOT+
=
BODLEVEL [2..0] FusesMin V
BOT
Typ V
BOT
Max V
BOT
Units
111BOD Disabled
1101.71.82.0
1012.52.72.9
1004.04.34.6
(2)
(2)
(2)
(2)
during the production test. This guar-
BOT
Note:1. V
0112.3
0102.2
0011.9
0002.0
may be below nominal minimum operating voltage for some devices. For devices where
BOT
this is the case, the device is tested down to VCC = V
antees that a Brown-out Reset will occur before VCC drops to a voltage where correct
operation of the microcontroller is no longer guaranteed.
2. Centered value, not tested.
Table 8-3.Brown-out Characteristics
SymbolParameterMinTypMaxUnits
V
V
t
BOD
RAM
HYST
RAM Retention Voltage
Brown-out Detector Hysteresis50mV
Min Pulse Width on Brown-out Reset2µs
(1)
50mV
V
38
Notes:1. This is the limit to which VDD can be lowered without losing RAM data
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT
CK
CC
8.0.6Watchdog Reset
When the BOD is enabled, and VCC decreases to a value below the trigger level (V
in Figure
BOT-
8-5), the Brown-out Reset is immediately activated. When VCC increases above the trigger level
(V
in Figure 8-5), the delay counter starts the MCU after the Time-out period t
BOT+
TOUT
has
expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for
longer than t
given in Table 8-1.
BOD
Figure 8-5.Brown-out Reset During Operation
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On
the falling edge of this pulse, the delay timer starts counting the Time-out period t
TOUT
. Refer to
page 41 for details on operation of the Watchdog Timer.
Figure 8-6.Watchdog Reset During Operation
8.0.7MCU Status Register – MCUSR
The MCU Status Register provides information on which reset source caused an MCU Reset.
Bit76543210
Read/WriteRRRRR/WR/WR/WR/W
Initial Value0000See Bit Description
• Bits 7..4 – Res: Reserved Bits
––––WDRFBORFEXTRFPORFMCUSR
7598G–AVR–03/08
39
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• 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
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 MCUSR 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.
8.1Internal Voltage Reference
ATtiny25/45/85 features an internal bandgap reference. This reference is used for Brown-out
Detection, and it can be used as an input to the Analog Comparator or the ADC.
8.1.1Voltage Reference Enable Signals and Start-up Time
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 8-4. 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 BODLEVEL [2..0] Fuse bits).
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 8-4.Internal Voltage Reference Characteristics
SymbolParameterConditionMinTypMaxUnits
V
BG
t
BG
I
BG
Bandgap reference voltage
Bandgap reference start-up time
Bandgap reference current
consumption
VCC = 1.1V / 2.7V,
TA = 25°C
VCC = 2.7V,
TA = 25°C
VCC = 2.7V,
TA = 25°C
1.01.11.2V
4070µs
15µA
40
ATtiny25/45/85
7598G–AVR–03/08
8.2Watchdog Timer
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
MCU RESET
WATCHDOG
PRESCALER
128 kHz
OSCILLATOR
WATCHDOG
RESET
WDP0
WDP1
WDP2
WDP3
WDE
The Watchdog Timer is clocked from an On-chip Oscillator which runs at 128 kHz. By controlling
the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as shown in Table
8-7 on page 43. 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. Ten different
clock cycle periods can be selected to determine the reset period. If the reset period expires
without another Watchdog Reset, the ATtiny25/45/85 resets and executes from the Reset Vector. For timing details on the Watchdog Reset, refer to Table 8-7 on page 43.
The Wathdog Timer can also be configured to generate an interrupt instead of a reset. This can
be very helpful when using the Watchdog to wake-up from Power-down.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out period,
two different safety levels are selected by the fuse WDTON as shown in Table 8-5. Refer to
“Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 44 for
details.
Table 8-5.WDT Configuration as a Function of the Fuse Settings of WDTON
ATtiny25/45/85
Safety
WDTON
Level
Unprogrammed1DisabledTimed sequenceNo limitations
Programmed2EnabledAlways enabledTimed sequence
Figure 8-7.Watchdog Timer
8.2.1Watchdog Timer Control Register – WDTCR
Bit76543210
WDIFWDIEWDP3WDCEWDEWDP2WDP1WDP0WDTCR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value0000X000
WDT Initial
State
How to Disable the
WDT
How to Change Timeout
7598G–AVR–03/08
• Bit 7 – WDIF: Watchdog Timeout Interrupt Flag
This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt. WDIF is cleared by hardware when executing the corresponding interrupt
handling vector. Alternatively, WDIF is cleared by writing a logic one to the flag. When the I-bit in
SREG and WDIE are set, the Watchdog Time-out Interrupt is executed.
41
• Bit 6 – WDIE: Watchdog Timeout Interrupt Enable
When this bit is written to one, WDE is cleared, and the I-bit in the Status Register is set, the
Watchdog Time-out Interrupt is enabled. In this mode the corresponding interrupt is executed
instead of a reset if a timeout in the Watchdog Timer occurs.
If WDE is set, WDIE is automatically cleared by hardware when a time-out occurs. This is useful
for keeping the Watchdog Reset security while using the interrupt. After the WDIE bit is cleared,
the next time-out will generate a reset. To avoid the Watchdog Reset, WDIE must be set after
each interrupt.
Table 8-6.Watchdog Timer Configuration
WDEWDIEWatchdog Timer StateAction on Time-out
00StoppedNone
01RunningInterrupt
10RunningReset
11RunningInterrupt
• Bit 4 – WDCE: Watchdog Change 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. This bit must also be set when
changing the prescaler bits. “Timed Sequences for Changing the Configuration of the Watchdog
Timer” on page 44.
• 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 WDCE 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 WDCE 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.
In safety level 2, it is not possible to disable the Watchdog Timer, even with the algorithm
described above. “Timed Sequences for Changing the Configuration of the Watchdog Timer” on
page 44.
In safety level 1, WDE is overridden by WDRF in MCUSR. See “MCU Status Register –
MCUSR” on page 39 for description of WDRF. This means that WDE is always set when WDRF
is set. To clear WDE, WDRF must be cleared before disabling the Watchdog with the procedure
described above. This feature ensures multiple resets during conditions causing failure, and a
safe start-up after the failure.
Note:If the watchdog timer is not going to be used in the application, it is important to go through a
watchdog disable procedure in the initialization of the device. If the Watchdog is accidentally
enabled, for example by a runaway pointer or brown-out condition, the device will be reset, which
in turn will lead to a new watchdog reset. To avoid this situation, the application software should
always clear the WDRF flag and the WDE control bit in the initialization routine.
The WDP3..0 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 8-7.
Table 8-7.Watchdog Timer Prescale Select
Number of WDT Oscillator
WDP3WDP2WDP1WDP0
00002K cycles16 ms
00014K cycles32 ms
00108K cycles64 ms
001116K cycles0.125 s
010032K cycles0.25 s
010164K cycles0.5 s
0110128K cycles1.0 s
0111256K cycles2.0 s
1000512K cycles4.0 s
10011024K cycles8.0 s
1010
1011
1100
1101
1110
1111
Cycles
Reserved
Typical Time-out at
VCC = 5.0V
(1)
Note:1. If selected, one of the valid settings below 0b1010 will be used.
7598G–AVR–03/08
43
The following code example shows one assembly and one C function for turning off the WDT.
The example assumes that interrupts are controlled (e.g., by disabling interrupts globally) so that
no interrupts will occur during execution of these functions.
Assembly Code Example
WDT_off:
WDR
; Clear WDRF in MCUSR
ldir16, (0<<WDRF)
outMCUSR, r16
; Write logical one to WDCE and WDE
; Keep old prescaler setting to prevent unintentional Watchdog
Reset
in r16, WDTCR
ori r16, (1<<WDCE)|(1<<WDE)
out WDTCR, r16
; Turn off WDT
ldi r16, (0<<WDE)
out WDTCR, r16
ret
C Code Example
(1)
(1)
void WDT_off(void)
{
_WDR();
/* Clear WDRF in MCUSR */
MCUSR = 0x00
/* Write logical one to WDCE and WDE */
WDTCR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
Note:1. The example code assumes that the part specific header file is included.
8.3Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the two safety levels. Separate
procedures are described for each level.
8.3.1Safety Level 1
In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit
to one without any restriction. A timed sequence is needed when disabling an enabled Watchdog Timer. To disable an enabled Watchdog Timer, the following procedure must be followed:
1.In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE regardless of the previous value of the WDE bit.
2.Within the next four clock cycles, in the same operation, write the WDE and WDP bits
as desired, but with the WDCE bit cleared.
44
ATtiny25/45/85
7598G–AVR–03/08
8.3.2Safety Level 2
In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A
timed sequence is needed when changing the Watchdog Time-out period. To change the
Watchdog Time-out, the following procedure must be followed:
1.In the same operation, write a logical one to WDCE and WDE. Even though the WDE
always is set, the WDE must be written to one to start the timed sequence.
2.Within the next four clock cycles, in the same operation, write the WDP bits as desired,
but with the WDCE bit cleared. The value written to the WDE bit is irrelevant.
9.Interrupts
This section describes the specifics of the interrupt handling as performed in ATtiny25/45/85.
For a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling”
If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular
program code can be placed at these locations. The most typical and general program setup for
the Reset and Interrupt Vector Addresses in ATtiny25/45/85 is:
7598G–AVR–03/08
Address Labels CodeComments
0x0000rjmpRESET; Reset Handler
0x0001rjmpEXT_INT0; IRQ0 Handler
0x0002rjmpPCINT0; PCINT0 Handler
45
10. I/O Ports
0x0003rjmpTIM1_COMPA; Timer1 CompareA Handler
0x0004rjmpTIM1_OVF; Timer1 Overflow Handler
0x0005rjmpTIM0_OVF; Timer0 Overflow Handler
0x0006rjmpEE_RDY; EEPROM Ready Handler
0x0007rjmpANA_COMP; Analog Comparator Handler
0x0008rjmpADC; ADC Conversion Handler
0x0009rjmpTIM1_COMPB; Timer1 CompareB Handler
0x000ArjmpTIM0_COMPA;
0x000BrjmpTIM0_COMPB;
0x000CrjmpWDT;
0x000DrjmpUSI_START;
0x000ErjmpUSI_OVF;
0x000FRESET: ldir16, low(RAMEND); Main program start
0x0010ldir17, high(RAMEND); Tiny85 has also SPH
0x0011outSPL, r16; Set Stack Pointer to top of RAM
0x0012outSPH, r17; Tiny85 has also SPH
0x0013sei; Enable interrupts
0x0014<instr> xxx
... ... ... ...
10.1Introduction
All 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 VCC and Ground as indicated in Figure 10-1. Refer to “Electrical Char-
acteristics” on page 147 for a complete list of parameters.
46
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
C
pin
Logic
R
pu
See Figure
"General Digital I/O" for
Details
Pxn
Figure 10-1. I/O Pin Equivalent Schematic
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. For example,
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 57.
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.
However, writing a logic one to a bit in the PINx Register, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable – PUD bit in MCUCR 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
47. 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 alter-
nate functions.
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.
10.2Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a functional description of one I/O-port pin, here generically called Pxn.
Note:1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk
10.2.1Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register
Description for I/O-Ports” on page 57, 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 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).
10.2.2Toggling the Pin
Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn.
Note that the SBI instruction can be used to toggle one single bit in a port.
SLEEP, and PUD are common to all ports.
I/O
,
48
ATtiny25/45/85
7598G–AVR–03/08
10.2.3Switching Between Input and Output
XXXin r17, PINx
0x000xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
t
pd, max
t
pd, min
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 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 MCUCR 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 10-1 summarizes the control signals for the pin value.
Table 10-1.Port Pin Configurations
DDxnPORTxn
00XInputNoTri-state (Hi-Z)
010InputYesPxn will source current if ext. pulled low.
011InputNoTri-state (Hi-Z)
10XOutputNoOutput Low (Sink)
ATtiny25/45/85
PUD
(in MCUCR)I/OPull-upComment
10.2.4Reading the Pin Value
11XOutputNoOutput High (Source)
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the
PINxn Register bit. As shown in Figure 10-2, 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 10-3 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
respectively.
pd,min
Figure 10-3. Synchronization when Reading an Externally Applied Pin value
7598G–AVR–03/08
49
Consider the clock period starting shortly after the first falling edge of the system clock. The latch
out PORTx, r16nopin r17, PINx
0xFF
0x000xFF
SYSTEM CLK
r16
INSTRUCTIONS
SYNC LATCH
PINxn
r17
t
pd
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 succeeding positive clock edge. As indicated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed
between ½ and 1½ 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 10-4. The out instruction sets the “SYNC LATCH” signal at the positive edge of
the clock. In this case, the delay tpd through the synchronizer is one system clock period.
Figure 10-4. Synchronization when Reading a Software Assigned Pin Value
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 5 as input with a pull-up assigned to port pin 4. 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
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
Assembly Code Example
...
; Define pull-ups and set outputs high
; Define directions for port pins
ldir16,(1<<PB4)|(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
...
(1)
C Code Example
unsigned char i;
...
/* Define pull-ups and set outputs high */
/* Define directions for port pins */
PORTB = (1<<PB4)|(1<<PB1)|(1<<PB0);
DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);
/* Insert nop for synchronization*/
_NOP();
/* Read port pins */
i = PINB;
...
Note:1. For the assembly program, two temporary registers are used to minimize the time from pull-
ups are set on pins 0, 1 and 4, until the direction bits are correctly set, defining bit 2 and 3 as
low and redefining bits 0 and 1 as strong high drivers.
10.2.5Digital Input Enable and Sleep Modes
As shown in Figure 10-2, 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, and Standby mode to avoid high power consumption if
some input signals are left floating, or have an analog signal level close to VCC/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 Rising Edge, Falling Edge, or 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 mode, as the clamping in these sleep mode produces the requested
logic change.
7598G–AVR–03/08
51
10.2.6Unconnected Pins
clk
RPx
RRx
WRx
RDx
WDx
PUD
SYNCHRONIZER
WDx: WRITE DDRx
WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN
PUD: PULLUP DISABLE
clk
I/O
: I/O CLOCK
RDx: READ DDRx
DLQ
Q
SET
CLR
0
1
0
1
0
1
DIxn
AIOxn
DIEOExn
PVOVxn
PVOExn
DDOVxn
DDOExn
PUOExn
PUOVxn
PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VAL UE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VAL UE
DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx
RESET
RESET
Q
Q
D
CLR
Q
Q
D
CLR
Q
Q
D
CLR
PINxn
PORTxn
DDxn
DATA BUS
0
1
DIEOVxn
SLEEP
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP:SLEEP CONTROL
Pxn
I/O
0
1
PTOExn
PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE
WPx:WRITE PINx
WPx
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even
though most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to reduce current consumption in all other modes where the digital
inputs are enabled (Reset, Active mode and Idle mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.
In this case, the pull-up will be disabled during reset. If low power consumption during reset is
important, it is recommended to use an external pull-up or pulldown. Connecting unused pins
directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is
accidentally configured as an output.
10.3Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5
shows how the port pin control signals from the simplified Figure 10-2 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.
(1)
Figure 10-5. Alternate Port Functions
52
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
Note:1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk
SLEEP, and PUD are common to all ports. All other signals are unique for each pin.
I/O
,
Table 10-2 summarizes the function of the overriding signals. The pin and port indexes from Figure 10-5 are not shown in the succeeding tables. The overriding signals are generated internally
in the modules having the alternate function.
Table 10-2.Generic Description of Overriding Signals for Alternate Functions
Signal NameFull NameDescription
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.
PUOE
PUOV
DDOE
DDOV
PVOE
Pull-up Override
Enable
Pull-up Override
Value
Data Direction
Override Enable
Data Direction
Override Value
Port Value
Override Enable
PVOV
PTOE
DIEOE
DIEOV
DIDigital Input
AIO
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.
10.3.1MCU Control Register – MCUCR
Bit76543210
BODSPUDSESM1SM0BODSEISC01ISC00MCUCR
Port Value
Override Value
Port Toggle
Override Enable
Digital Input
Enable Override
Enable
Digital Input
Enable Override
Value
Analog
Input/Output
If PVOE is set, the port value is set to PVOV, regardless of the
setting of the PORTxn Register bit.
If PTOE is set, the PORTxn Register bit is inverted.
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 mode).
If DIEOE is set, the Digital Input is enabled/disabled when
DIEOV is set/cleared, regardless of the MCU state (Normal
mode, sleep mode).
This is the Digital Input to alternate functions. In the 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.
This is the Analog Input/Output to/from alternate functions. The
signal is connected directly to the pad, and can be used bidirectionally.
7598G–AVR–03/08
53
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bit 6 – 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 “Con-
figuring the Pin” on page 48 for more details about this feature.
10.3.2Alternate Functions of Port B
The Port B pins with alternate function are shown in Table 10-3.
5. Serial Data Input, Analog Comparator Negative Input, Timer/Counter0 Output Compare and
PWM Output B, Timer/Counter1 Output Compare and PWM Output A, USI Data Output (threewire mode), or Pin Change Interrupt.
6. Serial Data Output, Analog Comparator Positive Input, Timer/Counter0 Output Compare and
PWM Output A, Timer/Counter1 Inverted Output Compare and PWM Output A, USI Data Input
(three-wire mode), USI Data (two-wire mode), Voltage Ref., or Pin Change Interrupt.
(1)
(2)
(3)
(4)
(5)
(6)
54
• Port B, Bit 5 - RESET/dW/ADC0/PCINT5
RESET: External Reset input is active low and enabled by unprogramming (“1”) the RSTDISBL
Fuse. Pullup is activated and output driver and digital input are deactivated when the pin is used
as the RESET pin.
dW: When the debugWIRE Enable (DWEN) Fuse is programmed and Lock bits are unprogrammed, the debugWIRE system within the target device is activated. The RESET port pin is
configured as a wire-AND (open-drain) bi-directional I/O pin with pull-up enabled and becomes
the communication gateway between target and emulator.
ADC0: Analog to Digital Converter, Channel 0
PCINT5: Pin Change Interrupt source 5.
• Port B, Bit 4- XTAL2/CLKO/ADC2/OC1B/PCINT4
ATtiny25/45/85
.
7598G–AVR–03/08
ATtiny25/45/85
XTAL2: Chip Clock Oscillator pin 2. Used as clock pin for all chip clock sources except internal
calibrateble RC Oscillator and external clock. When used as a clock pin, the pin can not be used
as an I/O pin. When using internal calibratable RC Oscillator or External clock as a Chip clock
sources, PB4 serves as an ordinary I/O pin.
CLKO: The devided system clock can be output on the pin PB4. The divided system clock will be
output if the CKOUT Fuse is programmed, regardless of the PORTB4 and DDB4 settings. It will
also be output during reset.
ADC2: Analog to Digital Converter, Channel 2
OC1B: Output Compare Match output: The PB4 pin can serve as an external output for the
Timer/Counter1 Compare Match B when configured as an output (DDB4 set). The OC1B pin is
also the output pin for the PWM mode timer function.
PCINT4: Pin Change Interrupt source 4.
• Port B, Bit 3 - XTAL1/ADC3/OC1B/PCINT3
XTAL1: Chip Clock Oscillator pin 1. Used for all chip clock sources except internal calibrateble
RC oscillator. When used as a clock pin, the pin can not be used as an I/O pin.
ADC3: Analog to Digital Converter, Channel 3
OC1B: Inverted Output Compare Match output: The PB3 pin can serve as an external output for
the Timer/Counter1 Compare Match B when configured as an output (DDB3 set). The OC1B pin
is also the inverted output pin for the PWM mode timer function.
PCINT3: Pin Change Interrupt source 3.
• Port B, Bit 2 - SCK/ADC1/T0/USCK/SCL/INT0/PCINT2
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 DDB2. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDPB2. When the pin is
forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit.
.
.
7598G–AVR–03/08
ADC1: Analog to Digital Converter, Channel 1
T0: Timer/Counter0 counter source.
USCK: Three-wire mode Universal Serial Interface Clock.
SCL: Two-wire mode Serial Clock for USI Two-wire mode.
INT0: External Interrupt source 0.
PCINT2: Pin Change Interrupt source 2.
• Port B, Bit 1 - MISO/AIN1/OC0B/OC1A/DO/PCINT1
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 DDB1. When the SPI is
enabled as a Slave, the data direction of this pin is controlled by DDB1. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB1 bit.
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.
.
55
OC0B: Output Compare Match output. The PB1 pin can serve as an external output for the
Timer/Counter0 Compare Match B. The PB1 pin has to be configured as an output (DDB1 set
(one)) to serve this function. The OC0B pin is also the output pin for the PWM mode timer
function.
OC1A: Output Compare Match output: The PB1 pin can serve as an external output for the
Timer/Counter1 Compare Match B when configured as an output (DDB1 set). The OC1A pin is
also the output pin for the PWM mode timer function.
DO: Three-wire mode Universal Serial Interface Data output. Three-wire mode Data output overrides PORTB1 value and it is driven to the port when data direction bit DDB1 is set (one).
PORTB1 still enables the pull-up, if the direction is input and PORTB1 is set (one).
PCINT1: Pin Change Interrupt source 1.
• Port B, Bit 0 - MOSI/AIN0/OC0A/OC1A/DI/SDA/AREF/PCINT0
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 DDB0. When the SPI is
enabled as a Master, the data direction of this pin is controlled by DDB0. When the pin is forced
by the SPI to be an input, the pull-up can still be controlled by the PORTB0 bit.
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.
OC0A: Output Compare Match output. The PB0 pin can serve as an external output for the
Timer/Counter0 Compare Match A when configured as an output (DDB0 set (one)). The OC0A
pin is also the output pin for the PWM mode timer function.
OC1A: Inverted Output Compare Match output: The PB0 pin can serve as an external output for
the Timer/Counter1 Compare Match B when configured as an output (DDB0 set). The OC1A pin
is also the inverted output pin for the PWM mode timer function.
SDA: Two-wire mode Serial Interface Data.
AREF: External Analog Reference for ADC. Pullup and output driver are disabled on PB0 when
the pin is used as an external reference or Internal Voltage Reference with external capacitor at
the AREF pin.
DI: Data Input in USI Three-wire mode. USI Three-wire mode does not override normal port
functions, so pin must be configure as an input for DI function.
PCINT0: Pin Change Interrupt source 0.
56
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
Table 10-4 and Table 10-5 relate the alternate functions of Port B to the overriding signals
shown in Figure 10-5 on page 52.
Table 10-4.Overriding Signals for Alternate Functions in PB5..PB3
Signal
Name
PUOERSTDISBL
PUOV100
DDOERSTDISBL
DDOVdebugWire Transmit00
PVOE0OC1B Enable_OC1B Enable
PVOV0OC1B_OC1B
PTOE000
DIEOE
DIEOVADC0DADC2DADC3D
DIPCINT5 InputPCINT4 InputPCINT3 Input
AIORESET Input, ADC0 InputADC2 InputADC3 Input
Note:1. 1 when the Fuse is “0” (Programmed).
PB5/RESET/
ADC0/PCINT5
(1)
• DWEN
(1)
• DWEN
RSTDISBL
PCIE + ADC0D)
(1)
+ (PCINT5 •
PB4/ADC2/XTAL2/
OC1B/PCINT4
(1)
(1)
00
00
PCINT4 • PCIE + ADC2DPCINT3 • PCIE + ADC3D
PB3/ADC3/XTAL1/
_OC1B/PCINT3
Table 10-5.Overriding Signals for Alternate Functions in PB3..PB0
PB0/MOSI/DI/SDA/AIN0/AR
Signal
Name
PUOE000
PB2/SCK/ADC1/T0/
USCK/SCL/INT0/PCINT2
PB1/MISO/DO/AIN1/
OC1A/OC0B/PCINT1
EF/_OC1A/OC0A/
PCINT0
PUOV000
DDOEUSI_TWO_WIRE0USI_TWO_WIRE
DDOV
PVOEUSI_TWO_WIRE • DDB2
PVOV0OC0B + OC1A + DOOC0A + _OC1A
PTOEUSITC00
DIEOE
DIEOVADC1DAIN1DAIN0D
DI
AIOADC1 Input
(USI_SCL_HOLD +
PORTB2) • DDB2
PCINT2 • PCIE + ADC1D +
USISIE
T0/USCK/SCL/INT0/
PCINT2 Input
10.4Register Description for I/O-Ports
7598G–AVR–03/08
0(SDA + PORTBO) • DDB0
OC0B Enable + OC1A
Enable +
USI_THREE_WIRE
PCINT1 • PCIE + AIN1D
PCINT1 InputDI/SDA/PCINT0 Input
Analog Comparator
Negative Input
OC0A Enable + _OC1A
Enable + (USI_TWO_WIRE
• DDB0)
PCINT0 • PCIE + AIN0D +
USISIE
Analog Comparator Positive
Input
57
10.4.1Port B Data Register – PORTB
Bit76543210
––PORTB5PORTB4PORTB3PORTB2PORTB1PORTB0PORTB
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial Value00000000
10.4.2Port B Data Direction Register – DDRB
Bit76543210
––
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial Value00000000
10.4.3Port B Input Pins Address – PINB
Bit76543210
––
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial Value00N/AN/AN/AN/AN/AN/A
11. External Interrupts
The External Interrupts are triggered by the INT0 pin or any of the PCINT5..0 pins. Observe that,
if enabled, the interrupts will trigger even if the INT0 or PCINT5..0 pins are configured as outputs. This feature provides a way of generating a software interrupt. Pin change interrupts PCI
will trigger if any enabled PCINT5..0 pin toggles. The PCMSK Register control which pins contribute to the pin change inter rupts. Pin change interrupts on PCINT5..0 are detected
asynchronously. This implies that these interrupts can be used for waking the part also from
sleep modes other than Idle mode.
DDB5DDB4DDB3DDB2DDB1DDB0DDRB
PINB5PINB4PINB3PINB2PINB1PINB0PINB
The INT0 interrupts can be triggered by a falling or rising edge or a low level. This is set up as
indicated in the specification for the MCU Control Register – MCUCR. When the INT0 interrupt is
enabled and is configured as level triggered, the interrupt will trigger as long as the pin is held
low. Note that recognition of falling or rising edge interrupts on INT0 requires the presence of an
I/O clock, described in “Clock Systems and their Distribution” on page 21. Low level interrupt on
INT0 is detected asynchronously. This implies that this interrupt 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, the required level
must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If
the level disappears before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described
in “System Clock and Clock Options” on page 21.
11.0.1MCU Control Register – MCUCR
The External Interrupt Control Register A contains control bits for interrupt sense control.
Bit76543210
BODSPUDSESM1SM0BODSEISC01ISC00MCUCR
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
• Bits 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0
58
ATtiny25/45/85
7598G–AVR–03/08
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 11-1. 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 11-1.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.
11.0.2General Interrupt Mask Register – GIMSK
Bit76543210
–INT0PCIE–––––GIMSK
Read/WriteRR/WR/WRRRRR
Initial Value00000000
ATtiny25/45/85
• Bits 7, 4..0 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• 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
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 – PCIE: Pin Change Interrupt Enable
When the PCIE bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin
change interrupt is enabled. Any change on any enabled PCINT5..0 pin will cause an
interrupt. The corresponding interrupt of Pin Change Interrupt Request is executed from
the PCI Interrupt Vector. PCINT5..0 pins are enabled individually by the PCMSK0 Register.
11.0.3General Interrupt Flag Register – GIFR
Bit76543210
–INTF0PCIF–––––GIFR
Read/WriteRR/WR/WRRRRR
Initial Value00000000
7598G–AVR–03/08
• Bits 7, 4..0 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 6 – INTF0: External Interrupt Flag 0
59
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 GIMSK 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 – PCIF: Pin Change Interrupt Flag
When a logic change on any PCINT5..0 pin triggers an interrupt request, PCIF becomes set
(one). If the I-bit in SREG and the PCIE bit in GIMSK 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.
11.0.4Pin Change Mask Register – PCMSK
Bit76543210
––PCINT5PCINT4PCINT3PCINT2PCINT1PCINT0PCMSK
Read/WriteRRR/WR/WR/WR/WR/WR/W
Initial Value00111111
• Bits 7, 6 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
Each PCINT5..0 bit selects whether pin change interrupt is enabled on the corresponding I/O
pin. If PCINT5..0 is set and the PCIE bit in GIMSK is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT5..0 is cleared, pin change interrupt on the corresponding I/O pin
is disabled.
12. 8-bit Timer/Counter0 with PWM
Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output
Compare Units, and with PWM support. It allows accurate program execution timing (event management) and wave generation. The main features are:
Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
12.1Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 12-1. For the actual
placement of I/O pins, refer to “Pinout ATtiny25/45/85” 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 71.
60
ATtiny25/45/85
7598G–AVR–03/08
Figure 12-1. 8-bit Timer/Counter Block Diagram
Clock Select
Timer/Counter
DATA BUS
OCRnA
OCRnB
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
=
Fixed
TOP
Val ue
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int.Req.)
OCnA
(Int.Req.)
OCnB
(Int.Req.)
TCCRnATCCRnB
Tn
Edge
Detector
( From Prescaler )
clk
Tn
ATtiny25/45/85
12.1.1Registers
12.1.2Definitions
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) 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.
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 (clkT0).
The double buffered Output Compare Registers (OCR0A and OCR0B) 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 pins (OC0A and
OC0B). “Output Compare Unit” on page 63. for details. The Compare Match event will also set
the Compare Flag (OCF0A or OCF0B) which can be used to generate an Output Compare interrupt request.
Many register and bit references in this section are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare Unit A or Compare Unit B. However, when using the register or
bit def i n es i n a prog r am, t h e pr ec ise f or m mu st b e us e d, i . e . , TCNT0 f o r ac c e ssing
Timer/Counter0 counter value and so on.
7598G–AVR–03/08
61
The definitions in Table 34 are also used extensively throughout the document.
DATA BUS
TCNTnControl Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear
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 OCR0A Register. The
assignment is dependent on the mode of operation.
12.2Timer/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 (TCCR0B). For details on clock sources and prescaler, see “Timer/Counter Prescaler” on page 77.
12.3Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
12-2 shows a block diagram of the counter and its surroundings.
Figure 12-2. Counter Unit Block Diagram
Signal description (internal signals):
Depending of the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT0). clkT0 can be generated from an external or internal 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 clkT0 is present or not. A CPU write overrides (has 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 (TCCR0A) and the WGM02 bit located in the Timer/Counter
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ATtiny25/45/85
countIncrement or decrement TCNT0 by 1.
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).
7598G–AVR–03/08
Control Register B (TCCR0B). There are close connections between how the counter behaves
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMnX1:0
bottom
(counts) and how waveforms are generated on the Output Compare output OC0A. For more
details about advanced counting sequences and waveform generation, see “Modes of Opera-
tion” on page 65.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by
the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
12.4Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers
(OCR0A and OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a
match. A match will set the Output Compare Flag (OCF0A or OCF0B) at the next timer clock
cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output
Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is executed. Alternatively, the 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 WGM02:0 bits and Compare Output mode (COM0x1:0) 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 (“Modes of Operation” on page 65.).
Figure 12-3 shows a block diagram of the Output Compare unit.
ATtiny25/45/85
Figure 12-3. Output Compare Unit, Block Diagram
The OCR0x Registers are 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 OCR0x Compare
Registers 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.
7598G–AVR–03/08
63
The OCR0x Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x directly.
12.4.1Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be forced by
writing a one to the Force Output Compare (FOC0x) bit. Forcing Compare Match will not set the
OCF0x Flag or reload/clear the timer, but the OC0x pin will be updated as if a real Compare
Match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set, cleared or
toggled).
12.4.2Compare Match Blocking by TCNT0 Write
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 OCR0x to be initialized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is
enabled.
12.4.3Using the Output Compare Unit
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
Unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT0
equals the OCR0x 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
down-counting.
The setup of the OC0x should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal mode. The OC0x Registers keep their values even when
changing between Waveform Generation modes.
Be aware that the COM0x1:0 bits are not double buffered together with the compare value.
Changing the COM0x1:0 bits will take effect immediately.
12.5Compare Match Output Unit
The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses
the COM0x1:0 bits for defining the Output Compare (OC0x) state at the next Compare Match.
Also, the COM0x1:0 bits control the OC0x pin output source. Figure 12-4 shows a simplified
schematic of the logic affected by the COM0x1:0 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 COM0x1:0 bits are shown. When referring to the
OC0x state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset
occur, the OC0x Register is reset to “0”.
64
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
PORT
DDR
DQ
DQ
OCn
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BUS
FOCn
clk
I/O
Figure 12-4. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform
Generator if either of the COM0x1:0 bits are set. However, the OC0x 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 OC0x pin (DDR_OC0x) must be set as output before the OC0x 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 OC0x state before the output is enabled. Note that some COM0x1:0 bit settings are reserved for certain modes of
operation. “8-bit Timer/Counter Register Description” on page 71.
12.5.1Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes.
For all modes, setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the
OC0x Register is to be performed on the next Compare Match. For compare output actions in
the non-PWM modes refer to Table 12-1 on page 72. For fast PWM mode, refer to Table 12-2 on
page 72, and for phase correct PWM refer to Table 12-3 on page 72.
A change of the COM0x1:0 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
FOC0x strobe bits.
12.6Modes of Operation
The 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 (WGM02:0) and Compare Output
mode (COM0x1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a Compare
Match (“Compare Match Output Unit” on page 64.).
For detailed timing information refer to Figure 12-8, Figure 12-9, Figure 12-10 and Figure 12-11
in “Timer/Counter Timing Diagrams” on page 70.
7598G–AVR–03/08
65
12.6.1Normal Mode
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
14
Period
23
(COMnx1:0 = 1)
The simplest mode of operation is the Normal mode (WGM02:0 = 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 ninth
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.
12.6.2Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter
value (TCNT0) matches the OCR0A. The OCR0A 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 12-5. The counter value (TCNT0)
increases until a Compare Match occurs between TCNT0 and OCR0A, and then counter
(TCNT0) is cleared.
Figure 12-5. CTC Mode, Timing Diagram
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF0A 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 OCR0A 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 OC0A output can be set to toggle its logical
level on each Compare Match by setting the Compare Output mode bits to toggle mode
(COM0A1:0 = 1). The OC0A 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 frequency of f
f
/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by the following
clk_I/O
OC0
=
equation:
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.
The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM waveform generation option. The fast PWM differs from the other PWM option by
its single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when WGM2:0 = 7. In noninverting Compare Output mode, the Output Compare (OC0x) is cleared on the Compare Match
between TCNT0 and OCR0x, 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 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), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 12-6. 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 OCR0x and TCNT0.
Figure 12-6. Fast PWM Mode, Timing Diagram
7598G–AVR–03/08
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled, the interrupt handler routine can be used for updating the compare value.
67
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins.
f
OC nxPW M
f
clk_I/O
N 256⋅
------------------=
Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allowes
the AC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available
for the OC0B pin (See Table 12-2 on page 72). The actual OC0x 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
setting (or clearing) the OC0x Register at the Compare Match between OCR0x and TCNT0, and
clearing (or setting) the OC0x 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:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result
in a constantly high or low output (depending on the polarity of the output set by the COM0A1:0
bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to toggle its logical level on each Compare Match (COM0x1:0 = 1). The waveform
generated will have a maximum frequency of f
OC0
= f
/2 when OCR0A is set to zero. This
clk_I/O
feature is similar to the OC0A toggle in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
12.6.4Phase Correct PWM Mode
The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct
PWM waveform generation option. The phase correct PWM mode is based on a dual-slope
operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1, and OCR0A when WGM2:0 = 5. In noninverting Compare Output mode, the Output Compare (OC0x) is cleared on the Compare Match
between TCNT0 and OCR0x 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.
In phase correct PWM mode the counter is incremented until the counter value matches TOP.
When the counter reaches TOP, it changes the count direction. The TCNT0 value will be equal
to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown
on Figure 12-7. 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 OCR0x
and TCNT0.
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
OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted
PWM output can be generated by setting the COM0x1:0 to three: Setting the COM0A0 bits to
one allows the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is
not available for the OC0B pin (See Table 12-3 on page 72). The actual OC0x 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 OC0x Register at the Compare Match between OCR0x
and TCNT0 when the counter increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0 when the counter decrements. The PWM frequency for
the output when using phase correct PWM can be calculated by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR0A 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.
7598G–AVR–03/08
At the very start of period 2 in Figure 12-7 OCn has a transition from high to low even though
there is no Compare Match. The point of this transition is to guaratee symmetry around BOTTOM. There are two cases that give a transition without Compare Match.
• OCR0A changes its value from MAX, like in Figure 12-7. When the OCR0A value is MAX the
OCn pin value is the same as the result of a down-counting Compare Match. To ensure
69
symmetry around BOTTOM the OCn value at MAX must correspond to the result of an up-
clk
Tn
(clk
I/O
/1)
TOVn
clk
I/O
TCNTnMAX - 1MAXBOTTOMBOTTOM + 1
TOVn
TCNTn
MAX - 1MAXBOTTOMBOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
counting Compare Match.
• The timer starts counting from a value higher than the one in OCR0A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the
way up.
12.7Timer/Counter Timing Diagrams
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 12-8 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.
Figure 12-8. Timer/Counter Timing Diagram, no Prescaling
Figure 12-9 shows the same timing data, but with the prescaler enabled.
Figure 12-9. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O
/8)
Figure 12-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC
mode and PWM mode, where OCR0A is TOP.
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ATtiny25/45/85
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ATtiny25/45/85
OCFnx
OCRnx
TCNTn
OCRnx Value
OCRnx - 1OCRnxOCRnx + 1OCRnx + 2
clk
I/O
clk
Tn
(clk
I/O
/8)
OCFnx
OCRnx
TCNTn
(CTC)
TOP
TOP - 1TOPBOTTOMBOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Figure 12-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (f
clk_I/O
/8)
Figure 12-11 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast
PWM mode where OCR0A is TOP.
Figure 12-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Pres-
caler (f
clk_I/O
/8)
12.88-bit Timer/Counter Register Description
12.8.1Timer/Counter Control Register A – TCCR0A
Bit76543210
COM0A1COM0A0COM0B1COM0B0––WGM01WGM00TCCR0A
Read/WriteR/WR/WR/WR/WRRR/WR/W
Initial Value00000000
• Bits 7:6 – COM01A:0: Compare Match Output A Mode
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0
bits are set, the OC0A 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 OC0A pin
must be set in order to enable the output driver.
7598G–AVR–03/08
71
When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the
WGM02:0 bit setting. Table 12-1 shows the COM0A1:0 bit functionality when the WGM02:0 bits
are set to a normal or CTC mode (non-PWM).
Table 12-1.Compare Output Mode, non-PWM Mode
COM01COM00Description
00Normal port operation, OC0A disconnected.
01Toggle OC0A on Compare Match
10Clear OC0A on Compare Match
11Set OC0A on Compare Match
Table 12-2 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM
mode.
Table 12-2.Compare Output Mode, Fast PWM Mode
COM01COM00Description
00Normal port operation, OC0A disconnected.
01
10Clear OC0A on Compare Match, set OC0A at TOP
11Set OC0A on Compare Match, clear OC0A at TOP
Note:1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 67
for more details.
WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
(1)
Table 12-3 shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to phase cor-
WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
Clear OC0A on Compare Match when up-counting. Set OC0A on
Compare Match when down-counting.
(1)
72
11
Note:1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Com-
• Bits 5:4 – COM0B1:0: Compare Match Output B Mode
These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B1:0
bits are set, the OC0B 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 OC0B pin
must be set in order to enable the output driver.
ATtiny25/45/85
Set OC0A on Compare Match when up-counting. Clear OC0A on
Compare Match when down-counting.
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on
page 68 for more details.
7598G–AVR–03/08
ATtiny25/45/85
When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the
WGM02:0 bit setting. Table 12-1 shows the COM0A1:0 bit functionality when the WGM02:0 bits
are set to a normal or CTC mode (non-PWM).
Table 12-4.Compare Output Mode, non-PWM Mode
COM01COM00Description
00Normal port operation, OC0B disconnected.
01Toggle OC0B on Compare Match
10Clear OC0B on Compare Match
11Set OC0B on Compare Match
Table 12-2 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast PWM
mode.
Table 12-5.Compare Output Mode, Fast PWM Mode
COM01COM00Description
00Normal port operation, OC0B disconnected.
01Reserved
10Clear OC0B on Compare Match, set OC0B at TOP
11Set OC0B on Compare Match, clear OC0B at TOP
Note:1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 67
for more details.
(1)
Table 12-3 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to phase cor-
Clear OC0B on Compare Match when up-counting. Set OC0B on
Compare Match when down-counting.
Set OC0B on Compare Match when up-counting. Clear OC0B on
Compare Match when down-counting.
(1)
7598G–AVR–03/08
Note:1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on
page 68 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bits 1:0 – WGM01:0: Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used, see Table 12-7. Modes of operation supported by the Timer/Counter
73
unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and two types of
Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page 65).
Table 12-7.Waveform Generation Mode Bit Description
Timer/Counter
Mode of
ModeWGM2WGM1WGM0
0000Normal0xFFImmediateMAX
OperationTOP
Update of
OCRx at
TOV Flag
(1)(2)
Set on
1001
2010CTCOCRAImmediateMAX
3011Fast PWM0xFFTOPMAX
4100Reserved–––
5101
6110Reserved–––
7111Fast PWMOCRATOPTOP
Notes:1. MAX = 0xFF
2. BOTTOM = 0x00
12.8.2Timer/Counter Control Register B – TCCR0B
Bit76543210
FOC0AFOC0B––WGM02CS02CS01CS00TCCR0B
Read/WriteWWRRR/WR/WR/WR/W
Initial Value00000000
• Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
PWM, Phase
Correct
PWM, Phase
Correct
0xFFTOPBOTTOM
OCRATOPBOTTOM
74
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0A bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC0A output is
changed according to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a
strobe. Therefore it is the value present in the COM0A1:0 bits that determines the effect of the
forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0A as TOP.
The FOC0A bit is always read as zero.
• Bit 6 – FOC0B: Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0B bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC0B output is
changed according to its COM0B1:0 bits setting. Note that the FOC0B bit is implemented as a
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
strobe. Therefore it is the value present in the COM0B1:0 bits that determines the effect of the
forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0B as TOP.
The FOC0B bit is always read as zero.
• Bits 5:4 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 3 – WGM02: Waveform Generation Mode
See the description in the “Timer/Counter Control Register A – TCCR0A” on page 71.
• Bits 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 12-8.Clock Select Bit Description
CS02CS01CS00Description
000No clock source (Timer/Counter stopped)
001clk
010clk
011clk
100clk
101clk
110External clock source on T0 pin. Clock on falling edge.
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.
12.8.3Timer/Counter Register – TCNT0
Bit76543210
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 OCR0x Registers.
/(No prescaling)
I/O
/8 (From prescaler)
I/O
/64 (From prescaler)
I/O
/256 (From prescaler)
I/O
/1024 (From prescaler)
I/O
TCNT0[7:0]TCNT0
12.8.4Output Compare Register A – OCR0A
Bit76543210
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
7598G–AVR–03/08
OCR0A[7:0]OCR0A
75
The Output Compare Register A 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 OC0A pin.
12.8.5Output Compare Register B – OCR0B
Bit76543210
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
The Output Compare Register B 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 OC0B pin.
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 3 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable
When the OCIE0B bit is written to one, an d the I-bit in the S tatus Register is set, the
Timer/Counter Compare Match B interrupt is enabled. The corresponding interrupt is executed if
a Compare Match in Timer/Counter occurs, i.e., when the OCF0B bit is set in the Timer/Counter
Interrupt Flag Register – TIFR0.
OCR0B[7:0]OCR0B
• Bit 2 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable
When the OCIE 0A bit is written to one , and the I-bit in the St atus Register is set, the
Timer/Counter0 Compare Match A interrupt is enabled. The corresponding interrupt is executed
if a Compare Matc h in Timer/Counter0 occurs, i.e., when the OCF 0A bit is set in the
Timer/Counter 0 Interrupt Flag Register – TIFR0.
• Bit 1 – TOIE0: Timer/Counter0 Overflow Interrupt Enable
Whe n t h e TOIE0 bit is writt en t o o ne, and the I-bit in the S tatus Regis t er is set, 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 0 Interrupt Flag Register – TIFR0.
12.8.7Timer/Counter 0 Interrupt Flag Register – TIFR
Bit76543210
–OCF1AOCF1BOCF0AOCF0BTOV1TOV0–TIFR
Read/WriteRR/WR/WR/WR/WR/WR/WR
Initial Value00000000
• Bits 7, 0 – Res: Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and will always read as zero.
• Bit 4– OCF0A: Output Compare Flag 0 A
76
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data
in OCR0A – Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF0A is cleared by writing a logic one to
the flag. When the I-bit in SREG, OCIE0A (Timer/Counter0 Compare Match Interrupt Enable),
and OCF0A are set, the Timer/Counter0 Compare Match Interrupt is executed.
• Bit 3 – OCF0B: Output Compare Flag 0 B
The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in
OCR0B – Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF0B is cleared by writing a logic one to
the flag. When the I-bit in SREG, OCIE0B (Timer/Counter Compare B Match Interrupt Enable),
and OCF0B are set, the Timer/Counter Compare Match Interrupt is executed.
• Bit 1 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set 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, the Timer/Counter0 Overflow interrupt is executed.
The setting of this flag is dependent of the WGM02:0 bit setting. Refer to Table 12-7, “Waveform
Generation Mode Bit Description” on page 74.
13. Timer/Counter Prescaler
The 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
clock source. The prescaled clock has a frequency of either f
f
/1024.
CLK_I/O
13.0.1Prescaler Reset
The prescaler is free running, i.e., operates independently of the Clock Select logic of the
Timer/Counter. 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.
13.0.2External Clock Source
An external clock source applied to the T0 pin can be used as Timer/Counter clock (clkT0). The
T0 pin is sampled once every system clock cycle by the pin synchronization logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 13-1 shows a functional
equivalent block diagram of the T0 synchronization and edge detector logic. The registers are
clocked at the positive edge of the internal system clock (
high period of the internal system clock.
). Alternatively, one of four taps from the prescaler can be used as a
CLK_I/O
/8, f
CLK_I/O
clk
). The latch is transparent in the
I/O
CLK_I/O
/64, f
CLK_I/O
/256, or
7598G–AVR–03/08
77
The edge detector generates one clk
Tn_sync
(To Clock
Select Logic)
Edge DetectorSynchronization
DQDQ
LE
DQ
Tn
clk
I/O
PSR10
Clear
clk
T0
T0
clk
I/O
Synchronization
pulse for each positive (CSn2:0 = 7) or negative (CSn2:0
0
T
= 6) edge it detects.
Figure 13-1. T0 Pin Sampling
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 T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when 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.
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 than half the system clock frequency (f
ExtClk
< f
/2) given a 50/50% duty cycle. Since the edge detector uses
clk_I/O
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 13-2. Prescaler for Timer/Counter0
13.0.3General Timer/Counter Control Register – GTCCR
Note:1. The synchronization logic on the input pins (
Bit76543210
TSMPWM1BCOM1B1COM1B0FOC1BFOC1APSR1PSR0GTCCR
T0)
is shown in Figure 13-1.
78
ATtiny25/45/85
7598G–AVR–03/08
Read/WriteR/WRRRRRRR/W
TIMER/COUNTER1 COUNT ENABLE
PSR1
CS10
CS11
CS12
PCK
64/32 MHz
0
CS13
14-BIT
T/C PRESCALER
T1CK/2
T1CK
T1CK/4
T1CK/8
T1CK/16
T1CK/32
T1CK/64
T1CK/128
T1CK/256
T1CK/512
T1CK/1024
T1CK/2048
T1CK/4096
T1CK/8192
T1CK/16384
S
A
CK
PCKE
T1CK
Initial Value00000000
• Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the
value that is written to the PSR0 bit is kept, hence keeping the Prescaler Reset signal asserted.
This ensures that the Timer/Counter is halted and can be configured without the risk of advancing during configuration. When the TSM bit is written to zero, the PSR0 bit is cleared by
hardware, and the Timer/Counter start counting.
• Bit 0 – PSR0: Prescaler Reset Timer/Counter0
When this bit is one, the Timer/Counter0 prescaler will be Reset. This bit is normally cleared
immediately by hardware, except if the TSM bit is set.
14. Counter and Compare Units
Figure 14-1 shows the Timer/Counter1 prescaler that supports two clocking modes, a synchro-
nous clocking mode and an asynchronous clocking mode. The synchronous clocking mode uses
the system clock (CK) as the clock timebase and asynchronous mode uses the fast peripheral
clock (PCK) as the clock time base. The PCKE bit from the PLLCSR register enables the asynchronous mode when it is set (‘1’).
ATtiny25/45/85
14.1Timer/Counter1
7598G–AVR–03/08
Figure 14-1. Timer/Counter1 Prescaler
In the asynchronous clocking mode the clock selections are from PCK to PCK/16384 and stop,
and in the synchronous clocking mode the clock selections are from CK to CK/16384 and stop.
The clock options are described in Table 14-2 on page 82 and the Timer/Counter1 Control Register, TCCR1. Setting the PSR1 bit in GTCCR register resets the prescaler. The PCKE bit in the
PLLCSR register enables the asynchronous mode. The frequency of the fast peripheral clock is
64 MHz (or 32 MHz in Low Speed Mode).
The Timer/Counter1 general operation is described in the asynchronous mode and the operation in the synchronous mode is mentioned only if there are differences between these two
modes. Figure 14-2 shows Timer/Counter 1 synchronization register block diagram and synchronization delays in between registers. Note that all clock gating details are not shown in the
79
figure. The Timer/Counter1 register values go through the internal synchronization registers,
8-BIT DATABUS
OCR1A
OCR1A_SI
TCNT_SO
OCR1B
OCR1B_SI
OCR1C
OCR1C_SI
TCCR1
TCCR1_SI
GTCCR
GTCCR_SI
TCNT1
TCNT1_SI
OCF1A
OCF1A_SI
OCF1B
OCF1B_SI
TOV1
TOV1_SI
TOV1_SO
OCF1B_SO
OCF1A_SO
TCNT1
S
A
S
A
PCKE
CK
PCK
IO-registersInput synchronization
registers
Timer/Counter1Output synchronization
registers
SYNC
MODE
ASYNC
MODE
1 CK Delay1/2 CK Delay
1 - 2 PCK Delay1 PCK Delay ~1 CK DelayNo Delay
TCNT1
OCF1A
OCF1B
TOV1
1/2 CK Delay1 CK Delay
which cause the input synchronization delay, before affecting the counter operation. The registers TCCR1, GTCCR, OCR1A, OCR1B, and OCR1C can be read back right after writing the
register. The read back values are delayed for the Timer/Counter1 (TCNT1) register and flags
(OCF1A, OCF1B, and TOV1), because of the input and output synchronization.
The Timer/Counter1 features a high resolution and a high accuracy usage with the lower prescaling opportunities. It can also support two accurate, high speed, 8-bit Pulse Width Modulators
using cl o ck spe e d s up t o 64 M Hz ( or 32 MHz i n Low Sp ee d Mode) . In th i s mode,
Timer/Counter1 and the output compare registers serve as dual stand-alone PWMs with nonoverlapping non-inverted and inverted outputs. Refer to page 87 for a detailed description on
this function. Similarly, the high prescaling opportunities make this unit useful for lower speed
functions or exact timing functions with infrequent actions.
Timer/Counter1 and the prescaler allow running the CPU from any clock source while the prescaler is operating on the fast 64 MHz (or 32 MHz in Low Speed Mode) PCK clock in the
asynchronous mode.
Note that the system clock frequency must be lower than one third of the PCK frequency. The
synchronization mechanism of the asynchronous Timer/Counter1 needs at least two edges of
the PCK when the system clock is high. If the frequency of the system clock is too high, it is a
risk that data or control values are lost.
7598G–AVR–03/08
ATtiny25/45/85
8-BIT DATABUS
TIMER INT. FLAG
REGISTER (TIFR)
TIMER/COUNTER1
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TIMER INT. MASK
REGISTER (TIMSK)
TIMER/COUNTER1
(TCNT1)
T/C CLEAR
T/C1 CONTROL
LOGIC
TOV1
OCF1B
OCF1B
TOV1
TOIE0
TOIE1
OCIE1B
OCIE1A
OCF1A
OCF1A
CK
PCK
T/C1 OVER-
FLOW IRQ
T/C1 COMPARE
MATCH B IRQ
OC1A
(PB1)
T/C1 COMPARE
MATCH A IRQ
T/C CONTROL
REGISTER 1 (TCCR1)
COM1B1
PWM1A
PWM1B
COM1B0
FOC1A
FOC1B
(OCR1A)(OCR1B)(OCR1C)
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TOV0
COM1A1
COM1A0
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
GLOBAL T/C CONTROL
REGISTER (GTCCR)
CS12
PSR1
CS11
CS10
CS13
CTC1
OC1A
(PB0)
OC1B
(PB4)
OC1B
(PB3)
DEAD TIME GENERATOR
DEAD TIME GENERATOR
The following Figure 14-3 shows the block diagram for Timer/Counter1.
Figure 14-3. Timer/Counter1 Block Diagram
Three status flags (overflow and compare matches) are found in the Timer/Counter Interrupt
Flag Register - TIFR. Control signals are found in the Timer/Counter Control Registers TCCR1
and GTCCR. The interrupt enable/disable settings are found in the Timer/Counter Interrupt
Mask Register - TIMSK.
The Timer/Counter1 contains three Output Compare Registers, OCR1A, OCR1B, and OCR1C
as the data source to be compared with the Timer/Counter1 contents. In normal mode the Output Compare functions are operational with all three output compare registers. OCR1A
determines action on the OC1A pin (PB1), and it can generate Timer1 OC1A interrupt in normal
mode and in PWM mode. Likewise, OCR1B determines action on the OC1B pin (PB3) and it can
generate Timer1 OC1B interrupt in normal mode and in PWM mode. OCR1C holds the
Timer/Counter maximum value, i.e. the clear on compare match value. In the normal mode an
overflow interrupt (TOV1) is generated when Timer/Counter1 counts from $FF to $00, while in
the PWM mode the overflow interrupt is generated when Timer/Counter1 counts either from $FF
to $00 or from OCR1C to $00. The inverted PWM outputs OC1A and OC1B are not connected in
normal mode.
In PWM mode, OCR1A and OCR1B provide the data values against which the Timer Counter
value is compared. Upon compare match the PWM outputs (OC1A, OC1A, OC1B, OC1B) are
generated. In PWM mode, the Timer Counter counts up to the value specified in the output compare register OCR1C and starts again from $00. This feature allows limiting the counter “full”
value to a specified value, lower than $FF. Together with the many prescaler options, flexible
PWM frequency selection is provided. Table 14-6 lists clock selection and OCR1C values to
7598G–AVR–03/08
81
obtain PWM frequencies from 20 kHz to 250 kHz in 10 kHz steps and from 250 kHz to 500 kHz
in 50 kHz steps. Higher PWM frequencies can be obtained at the expense of resolution.
14.1.1Timer/Counter1 Control Register - TCCR1
Bit76543210
$30 ($50)
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial value00000000
CTC1PWM1ACOM1A1COM1A0CS13CS12CS11CS10TCCR1
• Bit 7- CTC1 : Clear Timer/Counter on Compare Match
When the CTC1 control bit is set (one), Timer/Counter1 is reset to $00 in the CPU clock cycle
after a compare match with OCR1C register value. If the control bit is cleared, Timer/Counter1
continues counting and is unaffected by a compare match.
• Bit 6- PWM1A: Pulse Width Modulator A Enable
When set (one) this bit enables PWM mode based on comparator OCR1A in Timer/Counter1
and the counter value is reset to $00 in the CPU clock cycle after a compare match with OCR1C
register value.
• Bits 5,4 - COM1A1, COM1A0: Comparator A Output Mode, Bits 1 and 0
The COM1A1 and COM1A0 control bits determine any output pin action following a compare
match with compare register A in Timer/Counter1. Output pin actions affect pin PB1 (OC1A).
Since this is an alternative function to an I/O port, the corresponding direction control bit must be
set (one) in order to control an output pin. Note that OC1A is not connected in normal mode.
Table 14-1.Comparator A Mode Select
COM1A1COM1A0Description
00Timer/Counter Comparator A disconnected from output pin OC1A.
01Toggle the OC1A output line.
10Clear the OC1A output line.
11Set the OC1A output line
In PWM mode, these bits have different functions. Refer to Table 14-4 on page 88 for a detailed
description.
When set (one) this bit enables PWM mode based on comparator OCR1B in Timer/Counter1
and the counter value is reset to $00 in the CPU clock cycle after a compare match with OCR1C
register value.
• Bits 5,4 - COM1B1, COM1B0: Comparator B Output Mode, Bits 1 and 0
The COM1B1 and COM1B0 control bits determine any output pin action following a compare
match with compare register B in Timer/Counter1. Output pin actions affect pin PB3 (OC1B).
Since this is an alternative function to an I/O port, the corresponding direction control bit must be
set (one) in order to control an output pin. Note that OC1B is not connected in normal mode.
Table 14-3.Comparator B Mode Select
COM1B1COM1B0Description
00Timer/Counter Comparator B disconnected from output pin OC1B.
01Toggle the OC1B output line.
10Clear the OC1B output line.
11Set the OC1B output line
In PWM mode, these bits have different functions. Refer to Table 14-4 on page 88 for a detailed
description.
• Bit 3- FOC1B: Force Output Compare Match 1B
7598G–AVR–03/08
83
Writing a logical one to this bit forces a change in the compare match output pin PB3 (OC1B)
according to the values already set in COM1B1 and COM1B0. If COM1B1 and COM1B0 written
in the same cycle as FOC1B, the new settings will be used. The Force Output Compare bit can
be used to change the output pin value regardless of the timer value. The automatic action programmed in COM1B1 and COM1B0 takes place as if a compare match had occurred, but no
interrupt is generated. The FOC1B bit always reads as zero. FOC1B is not in use if PWM1B bit
is set.
• Bit 2- FOC1A: Force Output Compare Match 1A
Writing a logical one to this bit forces a change in the compare match output pin PB1 (OC1A)
according to the values already set in COM1A1 and COM1A0. If COM1A1 and COM1A0 written
in the same cycle as FOC1A, the new settings will be used. The Force Output Compare bit can
be used to change the output pin value regardless of the timer value. The automatic action programmed in COM1A1 and COM1A0 takes place as if a compare match had occurred, but no
interrupt is generated. The FOC1A bit always reads as zero. FOC1A is not in use if PWM1A bit
is set.
• Bit 1- PSR1 : Prescaler Reset Timer/Counter1
When this bit is set (one), the Timer/Counter prescaler (TCNT1 is unaffected) 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. This bit will always read as zero.
14.1.3Timer/Counter1 - TCNT1
Bit76543210
$2F ($4F)MSBLSBTCNT1
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial value00000000
This 8-bit register contains the value of Timer/Counter1.
Timer/Counter1 is realized as an up counter with read and write access. Due to synchronization
of the CPU, Timer/Counter1 data written into Timer/Counter1 is delayed by one and half CPU
clock cycles in synchronous mode and at most one CPU clock cycles for asynchronous mode.
The output compare register A is an 8-bit read/write register.
The Timer/Counter Output Compare Register A contains data to be continuously compared with
Timer/Counter1. Actions on compare matches are specified in TCCR1. A compare match does
only occur if Timer/Counter1 counts to the OCR1A value. A software write that sets TCNT1 and
OCR1A to the same value does not generate a compare match.
A compare match will set the compare interrupt flag OCF1A after a synchronization delay following the compare event.
The output compare register B is an 8-bit read/write register.
The Timer/Counter Output Compare Register B contains data to be continuously compared with
Timer/Counter1. Actions on compare matches are specified in TCCR1. A compare match does
only occur if Timer/Counter1 counts to the OCR1B value. A software write that sets TCNT1 and
OCR1B to the same value does not generate a compare match.
A compare match will set the compare interrupt flag OCF1B after a synchronization delay following the compare event.
The output compare register C is an 8-bit read/write register.
The Timer/Counter Output Compare Register C contains data to be continuously compared with
Timer/Counter1. A compare match does only occur if Timer/Counter1 counts to the OCR1C
value. A software write that sets TCNT1 and OCR1C to the same value does not generate a
compare match. If the CTC1 bit in TCCR1 is set, a compare match will clear TCNT1.
This register has the same function in normal mode and PWM mode.
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
• Bit 6 - OCIE1A: Timer/Counter1 Output Compare Interrupt Enable
When the OCI E1A bit is set (one) and the I-bit in the Status Register is set (one), the
Timer/Counter1 Compare MatchA, interrupt is enabled. The corresponding interrupt at vector
$003 is executed if a compare matchA occurs. The Compare Flag in Timer/Counter1 is set (one)
in the Timer/Counter Interrupt Flag Register.
7598G–AVR–03/08
• Bit 5 - OCIE1B: Timer/Counter1 Output Compare Interrupt Enable
When the OCI E1B bit is set (one) and the I-bit in the Status Register is set (one), the
Timer/Counter1 Compare MatchB, interrupt is enabled. The corresponding interrupt at vector
$009 is executed if a compare matchB occurs. The Compare Flag in Timer/Counter1 is set (one)
in the Timer/Counter Interrupt Flag Register.
85
• Bit 4– OCIE0A: Timer/Counter Output Compare Match A Interrupt Enable
When the OCIE0A bit is written to one, an d the I-bit in the S tatus Register is set, the
Timer/Counter Compare Match A interrupt is enabled. The corresponding interrupt is executed if
a Compare Match in Timer/Counter occurs, i.e., when the OCF0A bit is set in the Timer/Counter
Interrupt Flag Register – TIFR0.
• Bit 3 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable
When the OCIE0B bit is written to one, an d the I-bit in the S tatus Register is set, the
Timer/Counter Compare Match B interrupt is enabled. The corresponding interrupt is executed if
a Compare Match in Timer/Counter occurs, i.e., when the OCF0B bit is set in the Timer/Counte
Interrupt Flag Register – TIFR0.
• Bit 2 - TOIE1: Timer/Counter1 Overflow Interrupt Enable
Whe n the TOIE1 bi t i s set (one) and t he I-bit in the Status Regi ster is set (on e), the
Timer/Counter1 Overflow interrupt is enabled. The corresponding interrupt (at vector $004) is
executed if an overflow in Timer/Counter1 occurs. The Overflow Flag (Timer1) is set (one) in the
Timer/Counter Interrupt Flag Register - TIFR.
• Bit 0 - Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
14.1.8Timer/Counter Interrupt Flag Register - TIFR
Bit76543210
$38 ($58)-OCF1AOCF1BOCF0AOCF0BTOV1TOV0-TIFR
Read/WriteRR/WR/WRRR/WR/WR
Initial value00000000
• Bit 7 - Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
• Bit 6 - OCF1A: Output Compare Flag 1A
The OCF1A bit is set (one) when compare match occurs between Timer/Counter1 and the data
value in OCR1A - Output Compare Register 1A. OCF1A is cleared by hardware when executing
the corresponding interrupt handling vector. Alternatively, OCF1A is cleared, after synchronization clock cycle, by writing a logic one to the flag. When the I-bit in SREG, OCIE1A, and OCF1A
are set (one), the Timer/Counter1 A compare match interrupt is executed.
• Bit 5 - OCF1B: Output Compare Flag 1B
The OCF1B bit is set (one) when compare match occurs between Timer/Counter1 and the data
value in OCR1B - Output Compare Register 1A. OCF1B is cleared by hardware when executing
the corresponding interrupt handling vector. Alternatively, OCF1B is cleared, after synchronization clock cycle, by writing a logic one to the flag. When the I-bit in SREG, OCIE1B, and OCF1B
are set (one), the Timer/Counter1 B compare match interrupt is executed.
• Bit 2 - TOV1: Timer/Counter1 Overflow Flag
In normal mode (PWM1A=0 and PWM1B=0) the bit TOV1 is set (one) when an overflow occurs
in Timer/Counter1. The bit TOV1 is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, TOV1 is cleared, after synchronization clock cycle, by
writing a logical one to the flag.
86
ATtiny25/45/85
7598G–AVR–03/08
In PWM mode (either PWM1A=1 or PWM1B=1) the bit TOV1 is set (one) when compare match
occurs between Timer/Counter1 and data value in OCR1C - Output Compare Register 1C.
Clearing the Timer/Counter1 with the bit CTC1 does not generate an overflow.
When the SREG I-bit, and TOIE1 (Timer/Counter1 Overflow Interrupt Enable), and TOV1 are set
(one), the Timer/Counter1 Overflow interrupt is executed.
• Bit 0 - Res: Reserved Bit
This bit is a reserved bit in the ATtiny25/45/85 and always reads as zero.
14.1.9PLL Control and Status Register - PLLCSR
Bit76543210
$27 ($27)LSM----PCKEPLLEPLOCKPLLCSR
Read/WriteR/WRRRRR/WR/WR
Initial value0000000/10
• Bit 7- LSM: Low Speed Mode
The high speed mode is enabled as default and the fast peripheral clock is 64 MHz, but the low
speed mode can be set by writing the LSM bit to one. Then the fast peripheral clock is scaled
down to 32 MHz. The low speed mode must be set, if the supply voltage is below 2.7 volts,
because the Timer/Counter1 is not running fast enough on low voltage levels. It is highly recommended that Timer/Counter1 is stopped whenever the LSM bit is changed.
ATtiny25/45/85
• Bit 6.. 3- Res : Reserved Bits
These bits are reserved bits in the ATtiny25/45/85 and always read as zero.
• Bit 2- PCKE: PCK Enable
The PCKE bit change the Timer/Counter1 clock source. When it is set, the asynchronous clock
mode is enabled and fast 64 MHz (or 32 MHz in Low Speed Mode) PCK clock is used as
Timer/Counter1 clock source. If this bit is cleared, the synchronous clock mode is enabled, and
system clock CK is used as Timer/Counter1 clock source. This bit can be set only if PLLE bit is
set. It is safe to set this bit only when the PLL is locked i.e the PLOCK bit is 1. The bit PCKE can
only be set, if the PLL has been enabled earlier.
• Bit 1- PLLE: PLL Enable
When the PLLE is set, the PLL is started and if needed internal RC-oscillator is started as a PLL
reference clock. If PLL is selected as a system clock source the value for this bit is always 1.
• Bit 0- PLOCK: PLL Lock Detector
When the PLOCK bit is set, the PLL is locked to the reference clock, and it is safe to enable PCK
for Timer/Counter1. After the PLL is enabled, it takes about 100 micro seconds for the PLL to
lock.
14.1.10Timer/Counter1 Initialization for Asynchronous Mode
To change Timer/Counter1 to the asynchronous mode, first enable PLL, wait 100 µs before polling the PLOCK bit until it is set, and then set the PCKE bit.
14.1.11Timer/Counter1 in PWM Mode
When the PWM mode is selected, Timer/Counter1 and the Output Compare Register C OCR1C form a dual 8-bit, free-running and glitch-free PWM generator with outputs on the
7598G–AVR–03/08
87
PB1(OC1A) and PB3(OC1B) pins and inverted outputs on pins PB0(OC1A) and PB2(OC1B). As
PWM1x
PWM1x
x = A or B
t
non-overlap
=0
t
non-overlap=0
default non-overlapping times for complementary output pairs are zero, but they can be inserted
using a Dead Time Generator (see description on page 100).
Figure 14-4. The PWM Output Pair
When the counter value match the contents of OCR1A or OCR1B, the OC1A and OC1B outputs
are set or cleared according to the COM1A1/COM1A0 or COM1B1/COM1B0 bits in the
Timer/Counter1 Control Register A - TCCR1, as shown in Table 14-4.
Timer/Counter1 acts as an up-counter, counting from $00 up to the value specified in the output
compare register OCR1C, and starting from $00 up again. A compare match with OC1C will set
an overflow interrupt flag (TOV1) after a synchronization delay following the compare event.
Table 14-4.Compare Mode Select in PWM Mode
COM11COM10Effect on Output Compare Pins
00
01
10
11
OC1x not connected.
OC1x not connected.
OC1x cleared on compare match. Set whenTCNT1 = $01.
OC1x set on compare match. Cleared when TCNT1 = $00.
OC1x cleared on compare match. Set when TCNT1 = $01.
OC1x not connected.
OC1x Set on compare match. Cleared when TCNT1= $01.
OC1x not connected.
Note that in PWM mode, writing to the Output Compare Registers OCR1A or OCR1B, the data
value is first transferred to a temporary location. The value is latched into OCR1A or OCR1B
when the Timer/Counter reaches OCR1C. This prevents the occurrence of odd-length PWM
pulses (glitches) in the event of an unsynchronized OCR1A or OCR1B. See Figure 14-5 for an
example.
88
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
PWM Output OC1x
PWM Output OC1x
Unsynchronized OC1x Latch
Synchronized OC1x Latch
Counter Value
Compare Value
Counter Value
Compare Value
Compare Value changes
Glitch
Compare Value changes
f
PWM
f
TCK1
OCR1C + 1()
------------------------------------=
Figure 14-5. Effects of Unsynchronized OCR Latching
During the time between the write and the latch operation, a read from OCR1A or OCR1B will
read the contents of the temporary location. This means that the most recently written value
always will read out of OCR1A or OCR1B.
When OCR1A or OCR1B contain $00 or the top value, as specified in OCR1C register, the outpu t PB 1 (O C1 A ) or P B3 ( OC 1B ) is h e ld l ow o r hi gh a c co rdi ng t o th e s e t ti n gs o f
COM1A1/COM1A0. This is shown in Table 14-5.
Table 14-5.PWM Outputs OCR1x = $00 or OCR1C, x = A or B
COM1x1COM1x0OCR1xOutput OC1xOutput OC1x
01$00LH
01OCR1CHL
10$00LNot connected.
10OCR1CHNot connected.
11$00HNot connected.
11OCR1CLNot connected.
In PWM mode, the Timer Overflow Flag - TOV1 is set when the TCNT1 counts to the OCR1C
value and the TCNT1 is reset to $00. The Timer Overflow Interrupt1 is executed when TOV1 is
set provided that Timer Overflow Interrupt and global interrupts are enabled. This also applies to
the Timer Output Compare flags and interrupts.
The frequency of the PWM will be Timer Clock 1 Frequency divided by (OCR1C value + 1). See
the following equation:
7598G–AVR–03/08
89
Resolution shows how many bit is required to express the value in the OCR1C register. It is calculated by following equation
Resolution
= log2(OCR1C + 1).
PWM
Table 14-6.Timer/Counter1 Clock Prescale Select in the Asynchronous Mode
The Dead Time Generator is provided for the Timer/Counter1 PWM output pairs to allow driving
external power control switches safely. The Dead Time Generator is a separate block that can
be connected to Timer/Counter1 and it is used to insert dead times (non-overlapping times) for
the Timer/Counter1 complementary output pairs (OC1A-OC1A and OC1B-OC1B). The sharing
of tasks is as follows: the timer/counter generates the PWM output and the Dead Time Generator generates the non-overlapping PWM output pair from the timer/counter PWM signal. Two
Dead Time Generators are provided, one for each PWM output. The non-overlap time is adjust-
90
ATtiny25/45/85
7598G–AVR–03/08
ATtiny25/45/85
TIMER/COUNTER1
OC1A
OC1A
OC1B
OC1B
DEAD TIME GENERATOR
PWM GENERATOR
PCKE
T15M
PCK
CK
DT1AH
DT1BH
DEAD TIME GENERATOR
PWM1BPWM1A
DT1AL
DT1BL
CLOCK CONTROL
OC1x
OC1x
T/C1 CLOCK
PWM1x
4-BIT COUNTER
COMPARATOR
DT1xL
DT1xH
DT1x
I/O REGISTER
DEAD TIME
PRESCALER
DTPS11..10
able and the PWM output and it’s com plementary output are adjusted separately, and
independently for both PWM outputs.
Figure 15-1. Timer/Counter1 & Dead Time Generators
The dead time generation is based on the 4-bit down counters that count the dead time, as
shown in Figure 46. There is a dedicated prescaler in front of the Dead Time Generator that can
divide the Timer/Counter1 clock (PCK or CK) by 1, 2, 4 or 8. This provides for large range of
dead times that can be generated. The prescaler is controlled by two control bits DTPS11..10
from the I/O register at address 0x23. The block has also a rising and falling edge detector that
is used to start the dead time counting period. Depending on the edge, one of the transitions on
the rising edges, OC1x or OC1x is delayed until the counter has counted to zero. The comparator is used to compare the counter with zero and stop the dead time insertion when zero has
been reached. The counter is loaded with a 4-bit DT1xH or DT1xL value from DT1x I/O register,
depending on the edge of the PWM generator output when the dead time insertion is started.
7598G–AVR–03/08
Figure 15-2. Dead Time Generator
The length of the counting period is user adjustable by selecting the dead time prescaler setting
in 0x23 register, and selecting then the dead time value in I/O register DT1x. The DT1x register
consists of two 4-bit fields, DT1xH and DT1xL that control the dead time periods of the PWM
output and its’ complementary output separately. Thus the rising edge of OC1x and OC1x can
have different dead time periods. The dead time is adjusted as the number of prescaled dead
time generator clock cycles.
91
Figure 15-3. The Complementary Output Pair
OC1x
x = A or B
t
non-overlap / rising edge
t
non-overlap / falling edge
OC1x
PWM1x
15.0.1Timer/Counter1 Dead Time Prescaler register 1 - DTPS1
Bit76543210
$23 ($43)DTPS11 DTPS10DTPS1
Read/WriteRRRRRRR/WR/W
Initial value00000000
The dead time prescaler register, DTPS1 is a 2-bit read/write register.
Bits 1 - 0 - DTPS1: Timer/Counter1 Dead Time Prescaler register 1
The dedicated Dead Time prescaler in front of the Dead Time Generator can divide the
Timer/Counter1 clock (PCK or CK) by 1, 2, 4 or 8 providing a large range of dead times that can
be generated. The Dead Time prescaler is controlled by two bits DTPS11..10 from the Dead
Time Prescaler register. These bits define the division factor of the Dead Time prescaler. The
division factors are given in table 46..
Table 15-1.Division factors of the Dead Time prescaler
The dead time value register A is an 8-bit read/write register.
The dead time delay of is adjusted by the dead time value register, DT1A. The register consists
of two fields, DT1AH3..0 and DT1AL3..0, one for each complementary output. Therefore a different dead time delay can be adjusted for the rising edge of OC1A and the rising edge of OC1A.
• Bits 7..4- DT1AH3..DT1AH0: Dead Time Value for OC1A Output
92
ATtiny25/45/85
7598G–AVR–03/08
The dead time value for the OC1A output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the
prescaled time/counter clock period multiplied by 15.
• Bits 3..0- DT1AL3..DT1AL0: Dead Time Value for OC1A Output
The dead time value for the OC1A output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the
prescaled time/counter clock period multiplied by 15.
The dead time value register Bis an 8-bit read/write register.
The dead time delay of is adjusted by the dead time value register, DT1B. The register consists
of two fields, DT1BH3..0 and DT1BL3..0, one for each complementary output. Therefore a different dead time delay can be adjusted for the rising edge of OC1A and the rising edge of OC1A.
ATtiny25/45/85
• Bits 7..4- DT1BH3..DT1BH0: Dead Time Value for OC1B Output
The dead time value for the OC1B output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the
prescaled time/counter clock period multiplied by 15.
• Bits 3..0- DT1BL3..DT1BL0: Dead Time Value for OC1B Output
The dead time value for the OC1B output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the
prescaled time/counter clock period multiplied by 15.
16. Universal Serial Interface – USI
The Universal Serial Interface, or USI, provides the basic hardware resources needed for serial
communication. Combined with a minimum of control software, the USI allows significantly
higher transfer rates and uses less code space than solutions based on software only. Interrupts
are included to minimize the processor load. The main features of the USI are:
•
Two-wire Synchronous Data Transfer (Master or Slave, f
•
Three-wire Synchronous Data Transfer (Master or Slave f
•
Data Received Interrupt
•
Wakeup from Idle Mode
•
In Two-wire Mode: Wake-up from All Sleep Modes, Including Power-down Mode
•
Two-wire Start Condition Detector with Interrupt Capability
SCLmax
SCKmax
= fCK/16)
= fCK/4)
16.1Overview
7598G–AVR–03/08
A simplified block diagram of the USI is shown on Figure 16-1. For the actual placement of I/O
pins, refer to “Pinout ATtiny25/45/85” on page 2. CPU accessible I/O Registers, including I/O
93
bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed
DATA BUS
USIPF
USITC
USICLK
USICS0
USICS1
USIOIFUSIOIE
USIDC
USISIF
USIWM0
USIWM1
USISIEBit7
Two-wire Clock
Control Unit
DO
(Output only)
DI/SDA
(Input/Open Drain)
USCK/SCL
(Input/Open Drain)
4-bit Counter
USIDR
USISR
D Q
LE
USICR
CLOCK
HOLD
TIM0 COMP
Bit0
[1]
3
0
1
2
3
0
1
2
0
1
2
USIDB
in the “USI Register Descriptions” on page 100.
Figure 16-1. Universal Serial Interface, Block Diagram
The 8-bit Shift Register is directly accessible via the data bus and contains the incoming and
outgoing data. The register has no buffering so the data must be read as quickly as possible to
ensure that no data is lost. The most significant bit is connected to one of two output pins
depending of the wire mode configuration. A transparent latch is inserted between the Serial
Register Output and output pin, which delays the change of data output to the opposite clock
edge of the data input sampling. The serial input is always sampled from the Data Input (DI) pin
independent of the configuration.
The 4-bit counter can be both read and written via the data bus, and can generate an overflow
interrupt. Both the Serial Register and the counter are clocked simultaneously by the same clock
source. This allows the counter to count the number of bits received or transmitted and generate
an interrupt when the transfer is complete. Note that when an external clock source is selected
the counter counts both clock edges. In this case the counter counts the number of edges, and
not the number of bits. The clock can be selected from three different sources: The USCK pin,
Timer/Counter0 Compare Match or from software.
The Two-wire clock control unit can generate an interrupt when a start condition is detected on
the Two-wire bus. It can also generate wait states by holding the clock pin low after a start condition is detected, or after the counter overflows.
16.2Functional Descriptions
16.2.1Three-wire Mode
The USI Three-wire mode is compliant to the Serial Peripheral Interface (SPI) mode 0 and 1, but
does not have the slave select (SS) pin functionality. However, this feature can be implemented
in software if necessary. Pin names used by this mode are: DI, DO, and USCK.
Figure 16-2 shows two USI units operating in Three-wire mode, one as Master and one as
Slave. The two Shift Registers are interconnected in such way that after eight USCK clocks, the
data in each register are interchanged. The same clock also increments the USI’s 4-bit counter.
The Counter Overflow (interrupt) Flag, or USIOIF, can therefore be used to determine when a
transfer is completed. The clock is generated by the Master device software by toggling the
USCK pin via the PORT Register or by writing a one to the USITC bit in USICR.
Figure 16-3. Three-wire Mode, Timing Diagram
The Three-wire mode timing is shown in Figure 16-3. At the top of the figure is a USCK cycle reference. One bit is shifted into the USI Shift Register (USIDR) for each of these cycles. The
USCK timing is shown for both external clock modes. In External Clock mode 0 (USICS0 = 0), DI
is sampled at positive edges, and DO is changed (Data Register is shifted by one) at negative
edges. External Clock mode 1 (USICS0 = 1) uses the opposite edges versus mode 0, i.e., samples d ata at negative and ch anges the output at positive edges. Th e USI clock modes
corresponds to the SPI data mode 0 and 1.
7598G–AVR–03/08
Referring to the timing diagram (Figure 16-3.), a bus transfer involves the following steps:
1.The Slave device and Master device sets up its data output and, depending on the pro-
tocol used, enables its output driver (mark A and B). The output is set up by writing the
95
data to be transmitted to the Serial Data Register. Enabling of the output is done by setting the corresponding bit in the port Data Direction Register. Note that point A and B
does not have any specific order, but both must be at least one half USCK cycle before
point C where the data is sampled. This must be done to ensure that the data setup
requirement is satisfied. The 4-bit counter is reset to zero.
2.The Master generates a clock pulse by software toggling the USCK line twice (C and
D). The bit value on the slave and master’s data input (DI) pin is sampled by the USI on
the first edge (C), and the data output is changed on the opposite edge (D). The 4-bit
counter will count both edges.
3.Step 2. is repeated eight times for a complete register (byte) transfer.
4.After eight clock pulses (i.e., 16 clock edges) the counter will overflow and indicate that
the transfer is completed. The data bytes transferred must now be processed before a
new transfer can be initiated. The overflow interrupt will wake up the processor if it is
set to Idle mode. Depending of the protocol used the slave device can now set its output to high impedance.
16.2.2SPI Master Operation Example
The following code demonstrates how to use the USI module as a SPI Master:
The code is size optimized using only eight instructions (+ ret). The code example assumes that
the DO and USCK pins are enabled as output in the DDRE Register. The value stored in register
r16 prior to the function is called is transferred to the Slave device, and when the transfer is completed the data received from the Slave is stored back into the r16 Register.
96
The second and third instructions clears the USI Counter Overflow Flag and the USI counter
value. The fourth and fifth instruction set Three-wire mode, positive edge Shift Register clock,
count at USITC strobe, and toggle USCK. The loop is repeated 16 times.
The following code demonstrates how to use the USI module as a SPI Master with maximum
speed (fsck = fck/4):
The following code demonstrates how to use the USI module as a SPI Slave:
init:
ldir16,(1<<USIWM0)|(1<<USICS1)
stsUSICR,r16
...
SlaveSPITransfer:
stsUSIDR,r16
ldir16,(1<<USIOIF)
stsUSISR,r16
SlaveSPITransfer_loop:
ldsr16, USISR
sbrsr16, USIOIF
rjmpSlaveSPITransfer_loop
ldsr16,USIDR
ret
The code is size optimized using only eight instructions (+ ret). The code example assumes that
the DO is configured as output and USCK pin is configured as input in the DDR Register. The
value stored in register r16 prior to the function is called is transferred to the master device, and
when the transfer is completed the data received from the Master is stored back into the r16
Register.
7598G–AVR–03/08
Note that the first two instructions is for initialization only and needs only to be executed
once.These instructions sets Three-wire mode and positive edge Shift Register clock. The loop
is repeated until the USI Counter Overflow Flag is set.
97
16.2.4Two-wire Mode
MASTER
SLAVE
Bit7 Bit6Bit5 Bit4Bit3 Bit2Bit1 Bit0
SDA
SCL
Bit7 Bit6Bit5 Bit4Bit3 Bit2Bit1 Bit0
Two-wire Clock
Control Unit
HOLD
SCL
PORTxn
SDA
SCL
VCC
The USI Two-wire mode is compliant to the Inter IC (TWI) bus protocol, but without slew rate limiting on outputs and input noise filtering. Pin names used by this mode are SCL and SDA.
Figure 16-4 shows two USI units operating in Two-wire mode, one as Master and one as Slave.
It is only the physical layer that is shown since the system operation is highly dependent of the
communication scheme used. The main differences between the Master and Slave operation at
this level, is the serial clock generation which is always done by the Master, and only the Slave
uses the clock control unit. Clock generation must be implemented in software, but the shift
operation is done automatically by both devices. Note that only clocking on negative edge for
shifting data is of practical use in this mode. The slave can insert wait states at start or end of
transfer by forcing the SCL clock low. This means that the Master must always check if the SCL
line was actually released after it has generated a positive edge.
Since the clock also increments the counter, a counter overflow can be used to indicate that the
transfer is completed. The clock is generated by the master by toggling the USCK pin via the
PORT Register.
The data direction is not given by the physical layer. A protocol, like the one used by the TWIbus, must be implemented to control the data flow.
Referring to the timing diagram (Figure 16-5.), a bus transfer involves the following steps:
1.The a start condition is generated by the Master by forcing the SDA low line while the
SCL line is high (A). SDA can be forced low either by writing a zero to bit 7 of the Shift
Register, or by setting the corresponding bit in the PORT Register to zero. Note that the
Data Direction Register bit must be set to one for the output to be enabled. The slave
device’s start detector logic (Figure 16-6.) detects the start condition and sets the
USISIF Flag. The flag can generate an interrupt if necessary.
2.In addition, the start detector will hold the SCL line low after the Master has forced an
negative edge on this line (B). This allows the Slave to wake up from sleep or complete
its other tasks before setting up the Shift Register to receive the address. This is done
by clearing the start condition flag and reset the counter.
3.The Master set the first bit to be transferred and releases the SCL line (C). The Slave
samples the data and shift it into the Serial Register at the positive edge of the SCL
clock.
4.After eight bits are transferred containing slave address and data direction (read or
write), the Slave counter overflows and the SCL line is forced low (D). If the slave is not
the one the Master has addressed, it releases the SCL line and waits for a new start
condition.
5.If the Slave is addressed it holds the SDA line low during the acknowledgment cycle
before holding the SCL line low again (i.e., the Counter Register must be set to 14
before releasing SCL at (D)). Depending of the R/W bit the Master or Slave enables its
output. If the bit is set, a master read operation is in progress (i.e., the slave drives the
SDA line) The slave can hold the SCL line low after the acknowledge (E).
6.Multiple bytes can now be transmitted, all in same direction, until a stop condition is
given by the Master (F). Or a new start condition is given.
If the Slave is not able to receive more data it does not acknowledge the data byte it has last
received. When the Master does a read operation it must terminate the operation by force the
acknowledge bit low after the last byte transmitted.
The start condition detector is shown in Figure 16-6. The SDA line is delayed (in the range of 50
to 300 ns) to ensure valid sampling of the SCL line. The start condition detector is only enabled
in Two-wire mode.
99
The start condition detector is working asynchronously and can therefore wake up the processor
from the Power-down sleep mode. However, the protocol used might have restrictions on the
SCL hold time. Therefore, when using this feature in this case the Oscillator start-up time set by
the CKSEL Fuses (see “Clock Systems and their Distribution” on page 21) must also be taken
into the consideration. Refer to the USISIF bit description on page 101 for further details.
16.3Alternative USI Usage
When the USI unit is not used for serial communication, it can be set up to do alternative tasks
due to its flexible design.
16.3.1Half-duplex Asynchronous Data Transfer
By utilizing the Shift Register in Three-wire mode, it is possible to implement a more compact
and higher performance UART than by software only.
16.3.24-bit Counter
The 4-bit counter can be used as a stand-alone counter with overflow interrupt. Note that if the
counter is clocked externally, both clock edges will generate an increment.
16.3.312-bit Timer/Counter
Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit
counter.
16.3.4Edge Triggered External Interrupt
By setting the counter to maximum value (F) it can function as an additional external interrupt.
The Overflow Flag and Interrupt Enable bit are then used for the external interrupt. This feature
is selected by the USICS1 bit.
16.3.5Software Interrupt
The counter overflow interrupt can be used as a software interrupt triggered by a clock strobe.
16.4USI Register Descriptions
16.4.1USI Data Register – USIDR
Bit76543210
Read/WriteR/WR/WR/WR/WR/WR/WR/WR/W
Initial Value00000000
When accessing the USI Data Register (USIDR) the Serial Register can be accessed directly. If
a serial clock occurs at the same cycle the register is written, the register will contain the value
written and no shift is performed. A (left) shift operation is performed depe nding of the
USICS1..0 bits setting. The shift operation can be controlled by an external clock edge, by a
Timer/Counter0 Compare Match, or directly by software using the USICLK strobe bit. Note that
even when no wire mode is selected (USIWM1..0 = 0) both the external data input (DI/SDA) and
the external clock input (USCK/SCL) can still be used by the Shift Register.
The output pin in use, DO or SDA depending on the wire mode, is connected via the output latch
to the most significant bit (bit 7) of the Data Register. The output latch is open (transparent) during the first half of a serial clock cycle when an external clock source is selected (USICS1 = 1),
and constantly open when an internal clock source is used (USICS1 = 0). The output will be
MSBLSBUSIDR
100
ATtiny25/45/85
7598G–AVR–03/08
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