Atmel ATmega8U2, ATmega16U2, ATmega32U2 Datasheet
Features
• High Performance, Low Power AVR
• Advanced RISC Architecture
– 125 Powerful Instructions – Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16 MHz
• Non-volatile Program and Data Memories
– 8K/16K/32K Bytes of In-System Self-Programmable Flash
– 512/512/1024 EEPROM
– 512/512/1024 Internal SRAM
– Write/Erase Cycles: 10,000 Flash/ 100,000 EEPROM
– Data retention: 20 years at 85°C/ 100 years at 25°C
– Optional Boot Code Section with Independent Lock Bits
In-System Programming by on-chip Boot Program hardware-activated after
reset
True Read-While-Write Operation
– Programming Lock for Software Security
• USB 2.0 Full-speed Device Module with Interrupt on Transfer Completion
– Complies fully with Universal Serial Bus Specification REV 2.0
– 48 MHz PLL for Full-speed Bus Operation : data transfer rates at 12 Mbit/s
– Fully independant 176 bytes USB DPRAM for endpoint memory allocation
– Endpoint 0 for Control Transfers: from 8 up to 64-bytes
– 4 Programmable Endpoints:
IN or Out Directions
Bulk, Interrupt and IsochronousTransfers
Programmable maximum packet size from 8 to 64 bytes
Programmable single or double buffer
– Suspend/Resume Interrupts
– Microcontroller reset on USB Bus Reset without detach
– USB Bus Disconnection on Microcontroller Request
• Peripheral Features
– One 8-bit Timer/Counters with Separate Prescaler and Compare Mode (two 8-bit
PWM channels)
– One 16-bit Timer/Counter with Separate Prescaler, Compare and Capture Mode
(three 8-bit PWM channels)
– USART with SPI master only mode and hardware flow control (RTS/CTS)
– Master/Slave SPI Serial Interface
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
– Interrupt and Wake-up on Pin Change
• On Chip Debug Interface (debugWIRE)
• Special Microcontroller Features
– Power-On Reset and Programmable Brown-out Detection
– Internal Calibrated Oscillator
– External and Internal Interrupt Sources
– Five Sleep Modes: Idle, Power-save, Power-down, Standby, and Extended Standby
• I/O and Packages
– 22 Programmable I/O Lines
– QFN32 (5x5mm) / TQFP32 packages
• Operating Voltages
– 2.7 - 5.5V
• Operating temperature
– Industrial (-40°C to +85°C)
• Maximum Frequency
– 8 MHz at 2.7V - Industrial range
– 16 MHz at 4.5V - Industrial range
Note: 1. See “Data Retention” on page 6 for details.
®
8-Bit Microcontroller
(1)
8-bit
Microcontroller
with
8/16/32K Bytes
of ISP Flash
and USB
Controller
ATmega8U2
ATmega16U2
ATmega32U2
7799D–AVR–11/10
1. Pin Configurations
UVCC
QFN32
(PCINT11 / AIN2 ) PC2
(OC.0B / INT0) PD0
VCC
XTAL1
(INT5/ AIN3) PD4
(TXD1 / INT3) PD3
(XCK / AIN4 / PCINT12) PD5
PB3 (PDO / MISO / PCINT3)
GND
(PC0) XTAL2
UGND
PB4 (T1 / PCINT4)
282726
1
2
3
4
5
6
7
24
23
22
21
20
19
18
1211109131415
(AIN0 / INT1) PD1
8
16
17
PB6 (PCINT6)
D-
D+
2529303132
PB7 (PCINT7 / OC.0A / OC.1C)
PB5 (PCINT5)
PC7 (INT4 / ICP1 / CLKO)
PC6 (OC.1A / PCINT8)
Reset
(PC1 / dW)
PC5 ( PCINT9/ OC.1B)
PC4 (PCINT10)
UCAP
(RXD1 / AIN1 / INT2) PD2
(RTS / AIN5 / INT6) PD6
(CTS
/ HWB / AIN6 / T0 / INT7) PD7
(SS
/ PCINT0) PB0
(SCLK / PCINT1) PB1
(PDI / MOSI / PCINT2) PB2
AVCC
UVCC
VQFP32
(PCINT11 /AIN2 ) PC2
(OC.0B / INT0) PD0
VCC
XTAL1
(INT5/ AIN3) PD4
(TXD1 / INT3) PD3
(XCK AIN4 / PCINT12) PD5
PB3 (PDO / MISO / PCINT3)
GND
(PC0) XTAL2
UGND
PB4 (T1 / PCINT4)
282726
1
2
3
4
5
6
7
24
23
22
21
20
19
18
1211109131415
(AIN0 / INT1) PD1
8
16
17
PB6 (PCINT6)
D-
D+
2529303132
PB7 (PCINT7 / OC.0A / OC.1C)
PB5 (PCINT5)
PC7 (INT4 / ICP1 / CLKO)
PC6 (OC.1A / PCINT8)
Reset
(PC1 / dW)
PC5 ( PCINT9/ OC.1B)
PC4 (PCINT10)
UCAP
(RXD1 / AIN1 / INT2) PD2
(RTS / AIN5 / INT6) PD6
/ HWB
/ AIN6 / T0 / INT7) PD7
(SS
/ PCINT0) PB0
(SCLK / PCINT1) PB1
(PDI / MOSI / PCINT2) PB2
AVCC
Figure 1-1. Pinout
ATmega8U2/16U2/32U2
1.1 Disclaimer
Note: The large center pad underneath the QFN package should be soldered to ground on the board to
ensure good mechanical stability.
Typical values contained in this datasheet are based on simulations and characterization of
other AVR microcontrollers manufactured on the same process technology. Min and Max values
will be available after the device is characterized.
7799D–AVR–11/10
2
ATmega8U2/16U2/32U2
PROGRAM
COUNTER
STACK
POINTER
PROGRAM
FLASH
MCU CONTROL
REGISTER
SRAM
GENERAL
PURPOSE
REGISTERS
INSTRUCTION
REGISTER
TIMER/
COUNTERS
INSTRUCTION
DECODER
DATA DIR.
REG. PORTC
DATA REGISTER
PORTC
INTERRUPT
UNIT
EEPROM
USART1
STATUS
REGISTER
Z
Y
X
ALU
PORTC DRIVERS
PORTD DRIVERS
PORTB DRIVERS
PC7 - PC0 PD7 - PD0
RESET
VCC
GND
XTAL1
XTAL2
CONTROL
LINES
ANALOG
COMPARATOR
PB7 - PB0
D+/SCK
D-/SDATA
INTERNAL
OSCILLATOR
WATCHDOG
TIMER
8-BIT DA TA BUS
USB
PS/2
TIMING AND
CONTROL
OSCILLATOR
CALIB. OSC
DATA DIR.
REG. PORTB
DATA REGISTER
PORTB
ON-CHIP DEBUG
Debug-Wire
PROGRAMMING
LOGIC
DATA DIR.
REG. PORTD
DATA REGISTER
PORTD
POR - BOD
RESET
PLL
+
-
SPI
ON-CHIP
3.3V
REGULATOR
UVcc
UCap
1uF
2. Overview
The ATmega8U2/16U2/32U2 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 ATmega8U2/16U2/32U2 achieves throughputs approaching
1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed.
2.1 Block Diagram
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
7799D–AVR–11/10
3
ATmega8U2/16U2/32U2
architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.
The ATmega8U2/16U2/32U2 provides the following features: 8K/16K/32K Bytes of In-System
Programmable Flash with Read-While-Write capabilities, 512/512/1024 Bytes EEPROM,
512/512/1024 SRAM, 22 general purpose I/O lines , 32 general purpose work ing registers, two
flexible Timer/Counters with compare modes and PWM, one USART, a prog rammable Watchdog Timer with Internal Oscillator, an SPI serial port, debugWIRE interface, also used for
accessing the On-chip Debug system and programmi n g and five software selectable power saving modes. The Idle mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port,
and interrupt system to continue functioning. The Power-down mode saves the register contents
but freezes the Oscillator, disabling all other chip functions until the next interrupt or Hardware
Reset. In Standby mode, the Crystal/Resonator Oscillator is running while the rest of the device
is sleeping. This allows very fast start-up combined with low power consumption. In Extended
Standby mode, the main Oscillator continues to run.
The device is manufactured using Atmel’s high -dens ity no nvola tile memo ry tec hnol ogy. The onchip ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial
interface, by a conventional nonvolatile memory programmer, or by an on-chip Boot program
running on the AVR core. The boot program can use any interface to download the application
program in the application Flash memory. Software in the Boot Flash section will continue to run
while the Application Flash section is updated, providing true Read-While-Write operation. By
combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip,
the Atmel ATmega8U2/16U2/32U2 is a powerful microcontroller that provides a highly flexible
and cost effective solution to many embedded control applications.
The ATmega8U2/16U2/32U2 are 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.
2.2 Pin Descriptions
2.2.1 VCC
Digital supply voltage.
2.2.2 GND
Ground.
2.2.3 AVCC
AVCC is the supply voltage pin (input) for all analog features (Analog Comparator, PLL). It
should be externally connected to VCC through a low-pass filter.
2.2.4 Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port wit h intern al pull-up r esistors ( selecte d 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 co ndition becomes active,
even if the clock is not running.
7799D–AVR–11/10
Port B also serves the functions of various special features of the ATmega8U2/16U2/32U2 as
listed on
page 74.
4
2.2.5 Port C (PC7..PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port C output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port C pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port C also serves the functions of various special features of the ATmega8U2/16U2/32U2 as
listed on page 77.
2.2.6 Port D (PD7..PD0)
Port D serves as analog inputs to the analog comparator.
Port D also serves as an 8-bit bi-directional I/O port, if the analog comparator is not used (con-
cerns PD2/PD1 pins). Port pins can provide internal pull-up resistors (selected for each bit). The
Port D output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port D pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
2.2.7 D-
USB Full Speed Negative Data Upstream Port
ATmega8U2/16U2/32U2
2.2.8 D+
2.2.9 UGND
2.2.10 UVCC
2.2.11 UCAP
2.2.12 RESET/PC1/dW
2.2.13 XTAL1
USB Full Speed Positive Data Upstream Port
USB Ground.
USB Pads Internal Regulator Input supply voltage.
USB Pads Internal Regulator Output supply voltage. Should be connected to an external capacitor (1µF).
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 “System Control and
Reset” on page 47. Shorter pulses are not guaranteed to generate a reset. This pin alternatively
serves as debugWire channel or as generic I/O. The configuration depen ds on the fuses RSTDISBL and DWEN.
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
2.2.14 XTAL2/PC0
7799D–AVR–11/10
Output from the inverting Oscillator amplifier if enabled by Fuse. Also serves as a generic I/O.
5
3. Resources
A comprehensive set of development tools, application notes and datasheets are available for
download on http://www.atmel.com/avr.
4. Code Examples
This documentation contains simple code examples that briefly sh ow how to use vari ous parts of
the device. Be aware that not all C compiler vendors include bit def initions in the header files
and interrupt handling in C is compiler dependent. Plea se con firm with th e C com piler d ocumentation for more details.
These code examples assume that the part spe cific header file is included before compilation.
For I/O registers located in extended I/O map, "IN", "OUT", "SBIS", "SBI C", "CBI", and "SBI"
instructions must be replaced with instructions that allow access to extended I/O. Typically
"LDS" and "STS" combined with "SBRS", "SBRC", "SBR", and "CBR".
5. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less
than 1 PPM over 20 years at 85°C or 100 years at 25°C.
ATmega8U2/16U2/32U2
7799D–AVR–11/10
6
6. AVR CPU Core
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
SPI
Unit
Watchdog
Timer
Analog
Comparator
I/O Module 2
I/O Module1
I/O Module n
6.1 Introduction
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.
6.2 Architectural Overview
Figure 6-1. Block Diagram of the AVR Architecture
ATmega8U2/16U2/32U2
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 pipe lining. While one instruc tion 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.
7799D–AVR–11/10
7
ATmega8U2/16U2/32U2
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 these address pointers can
also be used as an address pointer for look up tables in Flash program memory. These added
function registers are the 16-bit X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and
a register. Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to
directly address the whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot Program section and the
Application Program section. Both sections have dedicated Lock bits for write and read/write
protection. The SPM instruction that writes into the Application Flash memory section must
reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the
Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack
size is only limited by the total SRAM size and the usage of the SRAM. All user programs must
initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack
Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed
through the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global
Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the
Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other I/O functions. The I/O Memory can be acces sed directly, or as the Data
Space locations following those of the Register File, 0x20 - 0x5F. In addition, the
ATmega8U2/16U2/32U2 has Extended I/O space from 0x60 - 0xFF in SRAM where only the
ST/STS/STD and LD/LDS/LDD instructions can be used.
6.3 ALU – 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 opera tions are divided
into three main categories – arithmetic, logical, and bit-functions. See the “Instruction Set” section for a detailed description.
6.4 Status Register
7799D–AVR–11/10
The Status Register contains information about the r esult of th e most recen tly executed arith metic instruction. This information can be used for altering program flow in order to perform
8
conditional operations. Note that the Status Register is updated after all ALU operations, as
specified in the Instruction Set Reference. This will in man y case s re move th e 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 ha nd le d by software.
6.4.1 SREG – Status Register
Bit 76543210
0x3F (0x5F) ITHSVNZCSREG
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the Global Interrupt Enable
Register is cleared, none of the interrupts are enabled independent of the individual interrupt
enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by
the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by
the application with the SEI and CLI instructions, as described in the instruction set reference.
ATmega8U2/16U2/32U2
• 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.
7799D–AVR–11/10
9
• 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.
6.5 General 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 6-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 6-2. AVR CPU General Purpose Working Registers
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
ATmega8U2/16U2/32U2
70Addr.
R0 0x00
R1 0x01
R2 0x02
…
R13 0x0D
…
R26 0x1A X-register Low Byte
R27 0x1B X-register High Byte
R28 0x1C Y-register Low Byte
R29 0x1D Y-register High Byte
R30 0x1E Z-register Low Byte
R31 0x1F Z-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 6-2, 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.
6.5.1 The X-register, Y-register, and Z-register
The registers R26..R31 have some a dded 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 6-3.
7799D–AVR–11/10
10
6.6 Stack Pointer
ATmega8U2/16U2/32U2
Figure 6-3. The X-, Y-, and Z-registers
15 XH XL 0
X-register 7 0 7 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 7 0 7 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7 0 7 0
R31 (0x1F) R30 (0x1E)
In the different addressing modes these address registers have functions as fixed d isplacem ent,
automatic increment, and automatic decrement (see the instruction set reference fo r details).
The Stack is mainly used for storing temporary data, for storing local variables and for storing
return addresses after interrupts and subroutine calls. Note that the Stack is implemented as
growing from higher to lower memory locations. The Stack Pointer Register always points to the
top of the Stack. The Stack Pointer points to the data SRAM Stack area where the Subroutine
and Interrupt Stacks are located. A Stack PUSH command will decrease the Stack Pointer.
The Stack in the data SRAM must be defined by the program before any subroutine calls are
executed or interrupts are enabled. Initial Stack Pointer value equals the last address of the
internal SRAM and the Stack Pointer must be set to point above start of the SRAM, see Figure
7-2 on page 18.
See Table 6-1 for Stack Pointer details.
Table 6-1. Stack Pointer instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
RCALL
POP Incremented by 1 Data is popped from the stack
RET
RETI
Decremented by 2
Incremented by 2 Return address is popped from the stack with return from
Return address is pushed onto the stack with a subroutine call or
interrupt
subroutine or return from interrupt
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.
7799D–AVR–11/10
11
6.6.1 SPH and SPL – Stack Pointer High and Low Register
clk
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1 T2 T3 T4
CPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
clk
CPU
Bit 1514131211109 8
0x3E (0x5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
0x3D (0x5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL
76543210
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 1 0 0 0 0 0
11111111
6.7 Instruction 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 6-4 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
ATmega8U2/16U2/32U2
Figure 6-4. The Parallel Instruction Fetches and Instruction Executions
Figure 6-5 shows the internal timing concept for the Register File. In a single clock cycl e an ALU
operation using two register operands is executed, and the result is stored back to the destination register.
Figure 6-5. Single Cycle ALU Operation
7799D–AVR–11/10
12
6.8 Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset
Vector each have a separate program vector in the program memory space. All interrupts are
assigned individual enable bits which must be written logic one together with th e Global Interrupt
Enable bit in the Status Register in order to enable the interrupt. Depending on the Program
Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12
are programmed. This feature improves software security. See the section “Memory Program-
ming” on page 246 for details.
The lowest addresses in the program memory space are by default defined as the Reset and
Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page 64. The list also
determines the priority levels of the different interrupts. The lower the address the higher is the
priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request
0. The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL
bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 64 for more information.
The Reset Vector can also be moved to the start of the Boot Flash section by programming the
BOOTRST Fuse, see “Memory Programming” on page 246.
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.
ATmega8U2/16U2/32U2
There are basically two types of interrupts. The first type is triggered by an event that sets the
Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and hardware clears the corresponding
Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s)
to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is
cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is
cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt
Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the
Global Interrupt Enable bit is set, and will then be executed by order of priority.
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 disap pears befo re 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
7799D–AVR–11/10
13
ATmega8U2/16U2/32U2
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, EEMPE ; start EEPROM write
sbi EECR, EEPE
out SREG, r16 ; restore SREG value (I-bit)
C Code Example
char cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
__disable_interrupt();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
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.
7799D–AVR–11/10
14
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
__enable_interrupt(); /* set Global Interrupt Enable */
__sleep(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
6.8.1 Interrupt Response Time
The interrupt execution response for all the enabl ed AVR interrupts is five clock cycles minimum.
After five clock cycles the program vector address for the actual interrupt handling ro utine is executed. During these five 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 five clock cycles. This increase comes in addition to the
start-up time from the selected sleep mode.
ATmega8U2/16U2/32U2
A return from an interrupt handling routine takes five clock cycles. During these five clock cycles,
the Program Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in SREG is set.
7799D–AVR–11/10
15
7. AVR Memories
This section describes the different memories in the ATmega8U2/16U2/32U2. The AVR architecture has two main memory spaces, the Data Memory and the Program Memory space. In
addition, the ATmega8U2/16U2/32U2 features an EEPROM Memory for data storage. All three
memory spaces are linear and regular.
7.1 In-System Reprogrammable Flash Program Memory
The ATmega8U2/16U2/32U2 contains 8K/16K/32K bytes On-chip In-System Reprogrammable
Flash memory for program storage . Since all AVR instructio ns are 16 or 32 bits wide, the Flas h
is organized as 4K x 16, 8K x 16. For software security, the Flash Program memory space is
divided into two sections, Boot Program section and Application Program section.
The Flash memory has an endurance of at least 100,000 write/erase cycles. The
ATmega8U2/16U2/32U2 Program Counter (PC) is 16 bits wide, thus addressing the
8K/16K/32K program memory locations. The operation of Boot Program section and associated
Boot Lock bits for software protection are described in detail in “Memory Programming” on page
246. “Memory Programming” on page 246 contains a detailed description on Flash data serial
downloading using the SPI pins or the debugWIRE interface.
Constant tables can be allocated within the entire program memory address space (see the LPM
– Load Program Memory instruction description and ELPM - Ex tended Load Program Me mory
instruction description).
ATmega8U2/16U2/32U2
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Tim-
ing” on page 12.
7799D–AVR–11/10
16
Figure 7-1. Program Memory Map
0x00000
0x1FFF (8KBytes)
0x3FFF (16KBytes)
Program Memory
Application Flash Section
Boot Flash Section
0x7FFF (32KBytes)
ATmega8U2/16U2/32U2
7.2 SRAM Data Memory
Figure 7-2 shows how the ATmega8U2/16U2/32U2 SRAM Memory is organized.
The ATmega8U2/16U2/32U2 is a complex microcontroller with more peripheral units than can
be supported within the 64 location reserved in the Opcode for the IN and OUT instructions. For
the Extended I/O space from $060 - $0FF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
The first 768 Data Memory locations address the Register File, the I/O Memory, Extended I/O
Memory, and the internal data SRAM. The first 32 locations address the Register file, the next
64 location the standard I/O Memory, then 160 locations of Extended I/O memory, and the 512
locations of 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 Postincrement. 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 addres s given
by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and post-incre-
ment, the address registers X, Y, and Z are decremented or incremented.
7799D–AVR–11/10
17
The 32 general purpose working registers, 64 I/O registers, and the 512/512/1024bytes of inter-
32 Registers
64 I/O Registers
Internal SRAM
(512/512/1024 x 8)
$0000 - $001F
$0020 - $005F
$2FF/$2FF/$4FF (8U2/16U2/32U2)
$0060 - $00FF
Data Memory
160 Ext I/O Reg.
$0100
clk
WR
RD
Data
Data
Address
Address valid
T1 T2 T3
Compute Address
Read
Write
CPU
Memory Access Instruction
Next Instruction
nal data SRAM in the ATmega8U2/16U2/32U2 are all accessible through all these addressing
modes. The Register File is described in “General Purpose Register File” on page 10.
Figure 7-2. Data Memory Map
7.2.1 Data Memory Access Times
This section describes the general access timing concepts for internal memory access. The
internal data SRAM access is performed in two clk
ATmega8U2/16U2/32U2
cycles as described in Figure 7-3.
CPU
7.3 EEPROM Data Memory
7799D–AVR–11/10
Figure 7-3. On-chip Data SRAM Access Cycles
The ATmega8U2/16U2/32U2 contains 512/512/1024 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/er ase 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.
18
For a detailed description of SPI, debugWIRE and Parallel data downloading to the EEPROM,
see page 259, page 244, and page 250 respectively.
7.3.1 EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 7-2 on page 22. 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 take n. In heavily filtered
power supplies, V
some period of time to run at a voltage lower th an sp ecified as min imum for the clo ck freq uency
used. See “Preventing EEPROM Corruption” on page 19. for details on how to avoid problems in
these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.
Refer to the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is
executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next
instruction is executed.
7.3.2 Preventing EEPROM Corruption
During periods of low V
too low for the CPU and the EEPROM to operate properly. These issues a re the same as for
board level systems using EEPROM, and the same design solutions should be applied.
ATmega8U2/16U2/32U2
is likely to rise or fall slowly on power-up/down. This causes the device for
CC
the EEPROM data can be corrupted because the supply voltage is
CC,
7.4 I/O Memory
An EEPROM data corruption can be caused by two situ at ion s wh en the vo lt age 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-ou t Detector (BOD). If the detection level of the internal
BOD does not match the needed detection level, an external low V
reset Protection circuit can
CC
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.
The I/O space definition of the ATmega8U2/16U2/32U2 is shown in “Register Summary” on
page 288.
All ATmega8U2/16U2/32U2 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. The
ATmega8U2/16U2/32U2 is a complex microcontroller with more peripheral units than can be
supported within the 64 location reserved in Op code for the IN and OUT instructions. For the
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19
Extended I/O space from 0x60 - 0x1FF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
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, unlike most
other AVRs, 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.
7.4.1 General Purpose I/O Registers
The ATmega8U2/16U2/32U2 contains three General Purpose I/O Registers. These registers
can be used for storing any information, and they are particularly useful for storing global variables and Status Flags. General Purpose I/O Registers within the address range 0x00 - 0x1F
are directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.
7.5 Register Description
ATmega8U2/16U2/32U2
7.5.1 EEARH and EEARL – The EEPROM Address Register
Bit 1514131211 10 9 8
0x22 (0x42) – – – – EEAR11 EEAR10 EEAR9 EEAR8 EEARH
0x21 (0x41) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL
76543 210
Read/Write R R R R R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 X X X X
XXXXX XXX
• Bits 15:12 – Res: Reserved Bits
These bits are reserved and will always read as zero.
• Bits 11:0 – EEAR[8:0]: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the
512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and
512. The initial value of EEAR is undefined. A proper value must be written before the EEPROM
may be accessed.
7.5.2 EEDR – The EEPROM Data Register
Bit 76543210
0x20 (0x40) MSB LSB EEDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
7799D–AVR–11/10
• Bits 7:0 – EEDR[7:0]: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to b e 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.
20
7.5.3 EECR – The EEPROM Control Register
Bit 76543210
0x1F (0x3F) – – EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR
Read/Write R R R/W R/W R/W R/W R/W R/W
Initial Value 0 0 X X 0 0 X 0
• Bits 7:6 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits
The EEPROM Programming mode bit 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
operations. The Programming times for th e differen t modes are shown in Table 7- 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.
ATmega8U2/16U2/32U2
Table 7-1. EEPROM Mode Bits
Programming
EEPM1 EEPM0
0 0 3.4 ms Erase and Write in one operation (Atomic Operation)
0 1 1.8 ms Erase Only
1 0 1.8 ms Write Only
1 1 – Reserved for future use
Time Operation
• 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 EEPE is cleared.
• Bit 2 – EEMPE: EEPROM Master Programming Enable
The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written.
When EEMPE is set, setting EEPE within four clock cycles will write data to 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. See the
description of the EEPE bit for an EEPROM write procedure.
• Bit 1 – EEPE: EEPROM Programming Enable
The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address
and data are correctly set up, the EEPE bit must be written to one to write the value into the
EEPROM. The EEMPE bit must be written to one before a logical one is written to EEPE, otherwise no EEPROM write takes place. The following procedure should be followed when writing
the EEPROM (the order of steps 3 and 4 is not essential):
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21
ATmega8U2/16U2/32U2
1. Wait until EEPE becomes zero.
2. Wait until SELFPRGEN in SPMCSR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR.
6. Within four clock cycles after setting EEMPE, write a logical one to EEPE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software
must check that the Flash programming is completed before initiating a new EEPROM write.
Step 2 is only relevant if the software contains a Boot Loader allowing the CPU to program the
Flash. If the Flash is never being updated by the CPU, step 2 can be omitted. See “Me mory Pr o-
gramming” on page 246 for details about Boot programming.
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the
EEPROM Master Write Enable will time-out. If an interrupt routine accessing the EEPROM is
interrupting another EEPROM access, the EEAR or EEDR Register will be modified, causing the
interrupted EEPROM access to fail. It is recommended to have the Global Interrupt Flag cleared
during all the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this bit and wait for a zero before writing the next byte. 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 a log ic one to trig ger 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.
The calibrated Oscillator is used to time the EEPROM accesses. Table 7-2 lists the typic al programming time for EEPROM access from the CPU.
Table 7-2. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time
EEPROM write
(from CPU)
The following code examples show one assembly and one C function for writing to 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 examples also
assume that no Flash Boot Loader is present in the software. If such code is present, the
EEPROM write function must also wait for any ongoing SPM command to finish.
26,368 3.3 ms
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22
ATmega8U2/16U2/32U2
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_write
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Write data (r16) to Data Register
out EEDR,r16
; Write logical one to EEMPE
sbi EECR,EEMPE
; Start eeprom write by setting EEPE
sbi EECR,EEPE
ret
C Code Example
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address and Data Registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
(1)
(1)
7799D–AVR–11/10
Note: 1. See “Code Examples” on page 6.
23
ATmega8U2/16U2/32U2
The next code examples show assembly and C functions for reading the EEPROM. The examples assume that interrupts are controlled so that no interrupts will occur during execution of
these functions.
Assembly Code Example
(1)
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_read
; Set up address (r18:r17) in address register
out EEARH, r18
out EEARL, r17
; Start eeprom read by writing EERE
sbi EECR,EERE
; Read data from Data Register
in r16,EEDR
ret
C Code Example
(1)
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from Data Register */
return EEDR;
}
Note: 1. See “Code Examples” on page 6.
7.5.4 GPIOR2 – General Purpose I/O Register 2
Bit 76543210
0x2B (0x4B) MSB LSB GPIOR2
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
7.5.5 GPIOR1 – General Purpose I/O Register 1
Bit 76543210
0x2A (0x4A) MSB LSB GPIOR1
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
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24
7.5.6 GPIOR0 – General Purpose I/O Register 0
Bit 76543210
0x1E (0x3E) MSB LSB GPIOR0
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
ATmega8U2/16U2/32U2
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25
8. System Clock and Clock Options
8.1 Clock Systems and their Distribution
Figure 8-1 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 42. The clock systems are detailed below.
Figure 8-1. Clock Distribution
ATmega8U2/16U2/32U2
8.1.1 CPU Clock – clk
CPU
USB
clk
USB PLL
X6
clk
PLL Clock
Prescaler
clk
USB (48MHz)
Pllin (8MHz)
XTAL (2-16 MHz)
Crystal
Oscillator
General I/O
Modules
clk
CPU Core RAM
I/O
AVR Clock
Control Unit
Source clock
System Clock
Prescaler
Clock
Multiplexer
External
Clock
clk
CPU
clk
FLASH
Watchdog TimerReset Logic
Watchdog clock
Watchdog
Oscillator
Flash and
EEPROM
Calibrated RC
Oscillator
The CPU clock is routed to parts of the system concerned with operation of the AVR core.
Examples of such modules are the General Purpose Register File, the Status Register and the
data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing
general operations and calculations.
8.1.2 I/O Clock – clk
I/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART.
The I/O clock is also used by the External Interrupt module, but note that some external interrupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O
clock is halted.
8.1.3 Flash Clock – clk
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock.
7799D–AVR–11/10
FLASH
26
ATmega8U2/16U2/32U2
USB
CPU Clock
External
Oscillator
RC oscillator
Ext RC Ext
non-Idle Idle
(Suspend)
non-Idle
3ms
resume
1
1
Resume from Host
Watchdog wake-up
from power-down
8.1.4 USB Clock – clk
8.2 Clock Switch
8.2.1 Exemple of use
USB
The USB is provided with a dedicated clock domain. This clock is generated with an on-chip PLL
running at 48 MHz. The PLL always multiply its input frequency by 6. Thus the PLL clock register
should be programmed by software to generate a 8 MHz clock on the PLL input.
In the ATmega8U2/16U2/32U2 product, the Clock Multiplexer and the System Clock Presca ler
can be modified by software.
The modification can occur when the device enters in USB Suspend mode. It th en switches from
External Clock to Calibrated RC Oscillator in order to reduce consumption. In such a configuration, the External Clock is disabled.
The firmware can use the watchdog timer to be woken-up from power-down in order to check if
there is an event on the application.
If an event occurs on the application or if the USB controller signals a non-idle state on the USB
line (Resume for example), the firmware switches the Clock Multiplexer from the Calibrated RC
Oscillator to the External Clock.
Figure 8-2. Example of clock switching with wake-up from USB Host
7799D–AVR–11/10
27
Figure 8-3. Example of clock switching with wake-up from Device
USB
CPU Clock
External
Oscillator
RC oscillator
Ext RC Ext
non-Idle Idle
(Suspend)
non-Idle
3ms
upstream-resume
2
2
Upstream Resume from device
Watchdog wake-up
from power-down
8.2.2 Clock switch Algorythm
8.2.2.1 Swith from external clock to RC clock
if (Usb_suspend_detected()) // if (UDINT.SUSPI == 1)
{
Usb_ack_suspend(); // UDINT.SUSPI = 0;
Usb_freeze_clock(); // USBCON.FRZCLK = 1;
Disable_pll(); // PLLCSR.PLLE = 0;
Enable_RC_clock(); // CLKSEL0.RCE = 1;
while (!RC_clock_ready()); // while (CLKSTA.RCON != 1);
Select_RC_clock(); // CLKSEL0.CLKS = 0;
Disable_external_clock(); // CLKSEL0.EXTE = 0;
}
ATmega8U2/16U2/32U2
8.2.2.2 Switch from RC clock to external clock
if (Usb_wake_up_detected()) // if (UDINT.WAKEUPI == 1)
{
Usb_ack_wake_up(); // UDINT.WAKEUPI = 0;
Enable_external_clock(); // CKSEL0.EXTE = 1;
while (!External_clock_ready()); // while (CLKSTA.EXTON != 1);
Select_external_clock(); // CLKSEL0.CLKS = 1;
Enable_pll(); // PLLCSR.PLLE = 1;
Disable_RC_clock(); // CLKSEL0.RCE = 0;
while (!Pll_ready()); // while (PLLCSR.PLOCK != 1);
Usb_unfreeze_clock(); // USBCON.FRZCLK = 0;
}
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28
8.3 Clock Sources
ATmega8U2/16U2/32U2
The device has the following clock source options, selec table 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 8-1. Device Clocking Options Select
Device Clocking Option CKSEL3:0
Low Power Crystal Oscillator 1111 - 1000
Full Swing Crystal Oscillator 0111 - 0110
Reserved 0101 - 0100
Reserved 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
8.3.1 Default Clock Source
The device is shipped with internal RC oscillator at 8.0 MHz and with the fuse CKDIV8 programmed, resulting in 1.0 MHz system clock. The startup time is set to maximum and time-out
period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that
all users can make their desired clock source setting u sing a ny available prog ramming inter face.
8.3.2 Clock Startup Sequence
Any clock source needs a sufficient V
cycles before it can be considered stable.
(1)
to start oscillating and a minimum number of oscillating
CC
To ensure sufficient V
, the device issues an internal reset with a time-out delay (t
CC
TOUT
) after
the device reset is released by all other reset sources. “On-chip Debug System” on page 45
describes the start conditions for the internal reset. The delay (t
) is timed from the Watchdog
TOUT
Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. The
selectable delays are shown in Table 8-2. The frequency of the Watchdog Oscillator is voltage
dependent as shown in “Typical Characteristics” on page 273.
Table 8-2. Number of Watchdog Oscillator Cycles
Ty p Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles
0 ms 0 ms 0
4.1 ms 4.3 ms 512
65 ms 69 ms 8K (8,192)
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum Vcc. The
delay will not monitor the actual voltage and it will be required to select a delay longer than the
Vcc rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be
used. A BOD circuit will ensure sufficient Vcc before it releases the reset, and the time-out delay
can be disabled. Disabling the time-out delay without utilizing a Brown-Out Detection circuit is
not recommended.
7799D–AVR–11/10
29
The oscillator is required to oscillate for a minimum number of cycles before the clock is consid-
XTAL2
XTAL1
GND
C2
C1
ered stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal
reset active for a given number of clock cycles. The reset is then released and the device will
start to execute. The recommended oscillator start-up time is dependent on the clock type, and
varies from 6 cycles for an externally applied clock to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up time when
the device starts up from reset. When starting up from Power-save or Power-down mode, Vcc is
assumed to be at a sufficient level and only the start-up time is included.
8.4 Low Power Crystal Oscillator
Pins 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 8-4. Either a quartz crystal or a
ceramic resonator may be used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the lowest power con sumption, bu t is not capable of driving other clock inputs, and
may be more susceptible to noise in noisy environments.
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 8-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
ATmega8U2/16U2/32U2
Figure 8-4. Crystal Oscillator Connections
The Low Power 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 8-3.
Table 8-3. Low Power Crystal Oscillator Operating Modes
Recommended Range for Capacitors C1
Frequency Range
0.4 - 0.9 100
0.9 - 3.0 101 12 - 22
3.0 - 8.0 110 12 - 22
8.0 - 16.0 111 12 - 22
(1)
(MHz) CKSEL3..1
(2)
(3)
and C2 (pF)
–
7799D–AVR–11/10
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. This option should not be used with crystals, only with ceramic resonators.
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