Rainbow Electronics ATtiny861V User Manual
Features
• High Performance, Low Power AVR
• Advanced RISC Architecture
– 123 Powerful Instructions – Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
• Non-volatile Program and Data Memories
– 2/4/8K Byte of In-System Programmable Program Memory Flash
(ATtiny261/461/861)
Endurance: 10,000 Write/Erase Cycles
– 128/256/512 Bytes In-System Programmable EEPROM (ATtiny261/461/861)
Endurance: 100,000 Write/Erase Cycles
– 128/256/512 Bytes Internal SRAM (ATtiny261/461/861)
– Programming Lock for Self-Programming Flash Program and EEPROM Data
Security
• Peripheral Features
– 8/16-bit Timer/Counter with Prescaler and Two PWM Channels
– 8/10-bit High Speed Timer/Counter with Separate Prescaler
3 High Frequency PWM Outputs with Separate Output Compare Registers
Programmable Dead Time Generator
– Universal Serial Interface with Start Condition Detector
– 10-bit ADC
11 Single Ended Channels
16 Differential ADC Channel Pairs
15 Differential ADC Channel Pairs with Programmable Gain (1x, 8x, 20x, 32x)
– 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
• I/O and Packages
– 16 Programmable I/O Lines
– 20-pin PDIP, 20-pin SOIC and 32-pad MLF
• Operating Voltage:
– 1.8 - 5.5V for ATtiny261V/461V/861V
– 2.7 - 5.5V for ATtiny261/461/861
• Speed Grade:
– ATtiny261V/461V/861V: 0 - 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V
– ATtiny261/461/861: 0 - 10 MHz @ 2.7 - 5.5V, 0 - 20 MHz @ 4.5 - 5.5V
• Industrial Temperature Range
• Low Power Consumption
– Active Mode: 1 MHz, 1.8V: 380μA
– Power-down Mode: 0.1μA at 1.8V
®
8-Bit Microcontroller
8-bit
Microcontroller
with 2/4/8K
Bytes In-System
Programmable
Flash
ATtiny261/V
ATtiny461/V
ATtiny861/V
Preliminary
2588B–AVR–11/06
1. Pin Configurations
Figure 1-1. Pinout ATtiny261/461/861
PDIP/SOIC
(MOSI/DI/SDA/OC1A/PCINT8) PB0
(MISO/DO/OC1A/PCINT9) PB1
(SCK/USCK/SCL/OC1B/PCINT10) PB2
(OC1B/PCINT11) PB3
(ADC7/OC1D/CLKI/XTAL1/PCINT12) PB4
(ADC8/OC1D/CLKO/XTAL2/PCINT13) PB5
(ADC9/INT0/T0/PCINT14) PB6
(ADC10/RESET/PCINT15) PB7
NC
(OC1B/PCINT11) PB3
NC
VCC
GND
NC
(ADC7/OC1D/CLKI/XTAL1/PCINT12) PB4
(ADC8/OC1D/CLKO/XTAL2/PCINT13) PB5
1
2
3
4
5
VCC
6
GND
7
8
9
10
PB2 (SCK/USCK/SCL/OC1B/PCINT10)
PB1 (MISO/DO/OC1A/PCINT9)
32313029282726
1
2
3
4
5
6
7
8
QFN/MLF
20
19
18
17
16
15
14
13
12
11
PB0 (MOSI/DI/SDA/OC1A/PCINT8)
NCNCNC
PA0 (ADC0/DI/SDA/PCINT0)
PA0 (ADC0/DI/SDA/PCINT0)
PA1 (ADC1/DO/PCINT1)
PA2 (ADC2/INT1/USCK/SCL/PCINT2)
PA3 (AREF/PCINT3)
AGND
AVCC
PA4 (ADC3/ICP0/PCINT4)
PA5 (ADC4/AIN2/PCINT5)
PA6 (ADC5/AIN0/PCINT6)
PA7 (ADC6/AIN1/PCINT7)
PA1 (ADC1/DO/PCINT1)
25
NC
24
PA2 (ADC2/INT1/USCK/SCL/PCINT2)
23
PA3 (AREF/PCINT3)
22
AGND
21
NC
20
NC
19
AVCC
18
PA4 (ADC3/ICP0/PCINT4)
17
9101112131415
NC
NC
(ADC9/INT0/T0/PCINT14) PB6
(ADC10/RESET/PCINT15) PB7
16
NC
(ADC6/AIN1/PCINT7) PA7
(ADC4/AIN2/PCINT5) PA5
(ADC5/AIN0/PCINT6) PA6
Note: The large center pad underneath the QFN/MLF package should be soldered to ground on the board to ensure good mechanical
stability.
1.1 Disclaimer
Typical values contained in this data sheet 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.
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ATtiny261/461/861
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2. Overview
2.1 Block Diagram
ATtiny261/461/861
The ATtiny261/461/861 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
ATtiny261/461/861 achieves throughputs approaching 1 MIPS per MHz allowing the system
designer to optimize power consumption versus processing speed.
Figure 2-1. Block Diagram
GND
Watchdog
Timer
Watchdog
Oscillator
Oscillator
Circuits /
Clock
Generation
EEPROM
Timer/Counter0 A/D Conv.
Powe r
Supervision
POR / BOD &
RESET
Timer/Counter1
VCC
debugWIRE
PROGRAM
CPU
LOGIC
SRAMFlash
AVCC
AGND
AREF
2588B–AVR–11/06
DATA B U S
USI
Analog Comp.
3
PORT A (8)PORT B (8)
PA[0..7]PB[0..7]
Internal
Bandgap
11
RESET
XTAL[1..2]
3
The AVR core combines a rich instruction set with 32 general purpose working registers. All the
32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent
registers to be accessed in one single instruction executed in one clock cycle. The resulting
architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.
The ATtiny261/461/861 provides the following features: 2/4/8K byte of In-System Programmable
Flash, 128/256/512 bytes EEPROM, 128/256/512 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
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 modes. The Idle mode stops the CPU while allowing 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 ATtiny261/461/861 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.
2.2 Pin Descriptions
2.2.1 VCC
Supply voltage.
2.2.2 GND
Ground.
2.2.3 AVCC
Analog supply voltage.
2.2.4 AGND
Analog ground.
2.2.5 Port A (PA7..PA0)
Port A is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port A output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port A pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port A pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port A also serves the functions of various special features of the ATtiny261/461/861 as listed
on page 65.
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ATtiny261/461/861
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2.2.6 Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port B output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port B pins are 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 ATtiny261/461/861 as listed
on page 61.
ATtiny261/461/861
2.2.7 R
ESET
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 23-3 on page
189. Shorter pulses are not guaranteed to generate a reset.
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5
3. Resources
A comprehensive set of development tools, application notes and datasheets are available for
download on http://www.atmel.com/avr.
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ATtiny261/461/861
2588B–AVR–11/06
4. 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.
ATtiny261/461/861
2588B–AVR–11/06
7
5. AVR CPU Core
5.1 Overview
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.
Figure 5-1. Block Diagram of the AVR Architecture
Data Bus 8-bit
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Control Lines
Program
Counter
Direct Addressing
Status
and Control
32 x 8
General
Purpose
Registrers
ALU
Indirect Addressing
Data
SRAM
EEPROM
Interrupt
Unit
Watchdog
Timer
Analog
Comparator
I/O Module1
I/O Module 2
I/O Module n
I/O Lines
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.
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ATtiny261/461/861
2588B–AVR–11/06
ATtiny261/461/861
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data
Space addressing – enabling efficient address calculations. One of the these address pointers
can also be used as an address pointer for look up tables in Flash Program memory. These
added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and
a register. Single register operations can also be executed in the ALU. After 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.
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.
5.2 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 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.
5.3 Status 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.
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9
5.3.1 SREG – AVR Status Register
The AVR Status Register – SREG – is defined as:
Bit 76543210
0x3F (0x5F) I T H S V N Z C SREG
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.
• 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.
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ATtiny261/461/861
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5.4 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 5-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 5-2. AVR CPU General Purpose Working Registers
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
ATtiny261/461/861
7 0 Addr.
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 5-2, 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.
5.4.1 The 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 5-3 on page 12.
2588B–AVR–11/06
11
5.5 Stack Pointer
Figure 5-3. The X-, Y-, and Z-registers
15 XH XL 0
X-register 707 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 707 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7070
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
5.5.1 SPH and SPL – Stack Pointer Register
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 RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
12
ATtiny261/461/861
2588B–AVR–11/06
5.6 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 5-4 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.
Figure 5-4. The Parallel Instruction Fetches and Instruction Executions
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
clk
CPU
ATtiny261/461/861
, directly generated from the selected clock source for the
CPU
T1 T2 T3 T4
Figure 5-5 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 5-5. Single Cycle ALU Operation
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
5.7 Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset
Vector each have a separate Program Vector in the Program memory space. All interrupts are
assigned individual enable bits which must be written logic one together with the Global Interrupt
Enable bit in the Status Register in order to enable the interrupt.
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 48. 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.
clk
T1 T2 T3 T4
CPU
2588B–AVR–11/06
13
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled
interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a
Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the
Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and hardware clears the corresponding
Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s)
to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is
cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is
cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt
Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the
Global Interrupt Enable bit is set, and will then be executed by order of priority.
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, 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 */
_CLI();
EECR |= (1<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
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ATtiny261/461/861
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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) */
5.7.1 Interrupt Response Time
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.
ATtiny261/461/861
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.
2588B–AVR–11/06
15
6. AVR Memories
This section describes the different memories in the ATtiny261/461/861. The AVR architecture
has two main memory spaces, the Data memory and the Program memory space. In addition,
the ATtiny261/461/861 features an EEPROM Memory for data storage. All three memory
spaces are linear and regular.
6.1 In-System Re-programmable Flash Program Memory
The ATtiny261/461/861 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
ATtiny261/461/861 Program Counter (PC) is 10/11/12 bits wide, thus addressing the
1024/2048/4096 Program memory locations. ”Memory Programming” on page 168 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 13.
Figure 6-1. Program Memory Map
6.2 SRAM Data Memory
Figure 6-2 shows how the ATtiny261/461/861 SRAM Memory is organized.
The lower 224/352/608 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.
Program Memory
0x0000
0x03FF/0x07FF/0x0FFF
16
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.
ATtiny261/461/861
2588B–AVR–11/06
ATtiny261/461/861
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 ATtiny261/461/861 are all accessible through all these addressing modes.
The Register File is described in ”General Purpose Register File” on page 11.
Figure 6-2. Data Memory Map
Data Memory
6.2.1 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
Figure 6-3. On-chip Data SRAM Access Cycles
clk
CPU
Address
Data
WR
Data
RD
32 Registers
64 I/O Registers
Internal SRAM
(128/256/512 x 8)
T1 T2 T3
Compute Address
0x0000 - 0x001F
0x0020 - 0x005F
0x0060
0x0DF/0x15F/0x25F
cycles as described in Figure 6-3.
CPU
Address valid
Write
Read
6.3 EEPROM Data Memory
The ATtiny261/461/861 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 181.
2588B–AVR–11/06
Memory Access Instruction
Next Instruction
17
6.3.1 EEPROM 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 6-1. 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, V
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 18 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.
6.3.2 Atomic 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 EEARL 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 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.
CC
is likely to rise or fall slowly on Power-up/down. This causes the device for some
6.3.3 Split 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).
6.3.4 Erase
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 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.
6.3.5 Write
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 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.
18
ATtiny261/461/861
2588B–AVR–11/06
ATtiny261/461/861
The calibrated Oscillator is used to time the EEPROM accesses. Make sure the Oscillator frequency is within the requirements described in ”OSCCAL – Oscillator Calibration Register” on
page 32.
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.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
rjmp EEPROM_write
; Set Programming mode
ldi r16, (0<<EEPM1)|(0<<EEPM0)
out EECR, r16
; 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 char ucAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set Programming mode */
EECR = (0<<EEPM1)|(0<<EEPM0);
/* Set up address and data registers */
EEAR = ucAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
2588B–AVR–11/06
19
The next code examples show assembly and C functions for reading the EEPROM. The examples assume that interrupts are controlled so that no interrupts will occur during execution of
these functions.
Assembly Code Example
EEPROM_read:
; Wait for completion of previous write
sbic EECR,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
unsigned char EEPROM_read(unsigned char ucAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = ucAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from data register */
return EEDR;
}
6.3.6 Preventing EEPROM Corruption
During periods of low V
too low for the CPU and the EEPROM to operate properly. These issues are the same as for
board level systems using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First,
a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can
be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal
BOD does not match the needed detection level, an external low V
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
ATtiny261/461/861
, the EEPROM data can be corrupted because the supply voltage is
CC
reset protection circuit can
CC
2588B–AVR–11/06
6.4 I/O Memory
The I/O space definition of the ATtiny261/461/861 is shown in ”Register Summary” on page 218.
All ATtiny261/461/861 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, 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.
6.4.1 General Purpose I/O Registers
The ATtiny261/461/861 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.
ATtiny261/461/861
6.5 Register Description
6.5.1 EEARH and EEARL – EEPROM Address Register
Bit 76543210
0x1F (0x3F) -------EEAR8EEARH
0x1E (0x3E) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL
Bit 76543210
Read/Write RRRRRRRR/W
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value0000000X
Initial Value XXXXXXXX
• Bit 7:1 – Res6:0: Reserved Bits
These bits are reserved for future use and will always read as 0 in ATtiny261/461/861.
• Bits 8:0 – EEAR8:0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL – 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.
2588B–AVR–11/06
21
6.5.2 EEDR – EEPROM Data Register
Bit 76543210
0x1D (0x3D) EEDR7 EEDR6 EEDR5 EEDR4 EEDR3 EEDR2 EEDR1 EEDR0 EEDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
• Bits 7:0 – EEDR7:0: EEPROM Data
For the EEPROM write operation the EEDR Register contains the data to be written to the
EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the
EEDR contains the data read out from the EEPROM at the address given by EEAR.
6.5.3 EECR – EEPROM Control Register
Bit 76543210
0x1C (0x3C) – – 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
• Bit 7 – Res: Reserved Bit
This bit is reserved for future use and will always read as 0 in ATtiny261/461/861. 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 ATtiny261/461/861 and will always read as zero.
• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits
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
operations. The Programming times for the different modes are shown in Table 6-1. 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 6-1. 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 Non-volatile memory is ready for programming.
22
ATtiny261/461/861
2588B–AVR–11/06
ATtiny261/461/861
• Bit 2 – EEMPE: EEPROM Master Program Enable
The EEMPE bit determines whether writing EEPE to one will have effect or not.
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.
6.5.4 GPIOR2 – General Purpose I/O Register 2
Bit 76543210
0x0C (0x2C) MSB LSB GPIOR2
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
6.5.5 GPIOR1 – General Purpose I/O Register 1
Bit 76543210
0x0B (0x2B) MSB LSB GPIOR1
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
6.5.6 GPIOR0 – General Purpose I/O Register 0
Bit 76543210
0x0A (0x2A) MSB LSB GPIOR0
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
2588B–AVR–11/06
23
7. System Clock and Clock Options
7.1 Clock Systems and their Distribution
Figure 7-1 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 34. The clock systems are detailed below.
Figure 7-1. Clock Distribution
7.1.1 CPU Clock – clk
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.
7.1.2 I/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.
7.1.3 Flash Clock – clk
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock.
7.1.4 ADC Clock – clk
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.
24
ATtiny261/461/861
CPU
FLASH
ADC
2588B–AVR–11/06
ATtiny261/461/861
7.1.5 Internal PLL for Fast Peripheral Clock Generation - clk
The internal PLL in ATtiny261/461/861 generates a clock frequency that is 8x or 4x multiplied
from a source input depending on the Low Speed Mode (LSM) bit. 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 or 32 MHz. The fast peripheral clock, or a clock
prescaled from that, can be selected as the clock source for Timer/Counter1. See the Figure 7-2
on page 25.
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.
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 the register PLLCSR is set or the CKSEL fuses are programmed to ‘0001’.
The bit PLOCK from the register PLLCSR is set when PLL is locked.
Both internal RC oscillator and PLL are switched off in power down and stand-by sleep modes.
Figure 7-2. PCK Clocking System
PCK
2588B–AVR–11/06
25
7.2 Clock 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.
(1)
Table 7-1. Device Clocking Options Select
Device Clocking Option CKSEL3..0 PB4 PB5
External Clock 0000 XTAL1 I/O
PLL Clock 0001 I/O I/O
Calibrated Internal RC Oscillator 8.0 MHz 0010 I/O I/O
Watchdog Oscillator 128 kHz 0011 I/O I/O
External Low-frequency Oscillator 01xx XTAL1 XTAL2
External Crystal/Ceramic Resonator (0.4 - 0.9 MHz) 1000 XTAL1 XTAL2
External Crystal/Ceramic Resonator (0.4 - 0.9 MHz) 1001 XTAL1 XTAL2
External Crystal/Ceramic Resonator (0.9 - 3.0 MHz) 1010 XTAL1 XTAL2
External Crystal/Ceramic Resonator (0.9 - 3.0 MHz) 1011 XTAL1 XTAL2
External Crystal/Ceramic Resonator (3.0 - 8.0 MHz) 1100 XTAL1 XTAL2
External Crystal/Ceramic Resonator (3.0 - 8.0 MHz) 1101 XTAL1 XTAL2
External Crystal/Ceramic Resonator (8.0 - 20.0 MHz) 1110 XTAL1 XTAL2
External Crystal/Ceramic Resonator (8.0 - 20.0 MHz) 1111 XTAL1 XTAL2
vs. PB4 and PB5 Functionality
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 7-
2.
Table 7-2. Number of Watchdog Oscillator Cycles
7.3 Default 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.
7.4 External Clock
To drive the device from an external clock source, CLKI should be driven as shown in Figure 7-
3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Typ Time-out Number of Cycles
4 ms 512
64 ms 8K (8,192)
26
ATtiny261/461/861
2588B–AVR–11/06
Figure 7-3. External Clock Drive Configuration
ATtiny261/461/861
EXTERNAL
CLOCK
SIGNAL
CLKI
GND
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-3.
Table 7-3. Start-up Times for the External Clock Selection
Start-up Time from Power-
SUT1..0
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
down and Power-save
Additional Delay from
Reset Recommended 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
31 for details.
7.5 High 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 7-4. When this clock source is selected, start-up times are determined by the
SUT fuses as shown in Table 7-5. See also ”PCK Clocking System” on page 25.
Table 7-4. PLLCK Operating Modes
CKSEL3..0 Nominal Frequency
0001 16 MHz
Table 7-5. Start-up Times for the PLLCK
Start-up Time from Power
SUT1..0
00 1K (1024) + 4 ms 14CK + 4 ms BOD enabled
01 16K (16384) + 4 ms 14CK + 4 ms Fast rising power
10 1K (1024) + 64 ms 14CK + 4 ms Slowly rising power
11 16K (16384) + 64 ms 14CK + 4 ms Slowly rising power
CLK
Down
Additional Delay from
Reset (VCC = 5.0V) Recommended usage
2588B–AVR–11/06
27
7.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0 MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. See Table
23-1 on page 188 and ”Internal Oscillator Speed” on page 211 for more details. The device is
shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on page 31 for
more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 7-6 on page 28. If selected, it will operate with no external components. During reset,
hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby
automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
calibration in Table 23-1 on page 188.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 32, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 23-1 on page 188.
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 ”Calibration Byte” on page 170.
Table 7-6. Internal Calibrated RC Oscillator Operating Modes
Frequency Range
7.3 - 8.1 0010
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on V
Fuse can be programmed in order to divide the internal frequency by 8.
(2)
(MHz) CKSEL3..0
(1)(3)
), the CKDIV8
CC
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-7 on page 28.
Table 7-7. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
Start-up Time
SUT1..0
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
(1)
10
11 Reserved
Note: 1. The device is shipped with this option selected.
from Power-down
6 CK 14CK + 64 ms Slowly rising power
Additional Delay from
Reset (VCC = 5.0V) Recommended Usage
28
ATtiny261/461/861
2588B–AVR–11/06
7.7 128 kHz Internal Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The frequency is nominal at 3V and 25°C. This clock may be select as the system clock by
programming the CKSEL Fuses to “0011”.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-8.
Table 7-8. Start-up Times for the 128 kHz Internal Oscillator
ATtiny261/461/861
Start-up Time from Power-
SUT1..0
00 6 CK 14CK BOD enabled
01 6 CK 14CK + 4 ms Fast rising power
10 6 CK 14CK + 64 ms Slowly rising power
11 Reserved
down and Power-save
7.8 Low-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 ‘0100’. The crystal should be connected
as shown in Figure 7-4. 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 7-9.
Table 7-9. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Start-up Time from
Power Down and Power
SUT1..0
00 1K (1024) CK
01 1K (1024) CK
10 32K (32768) CK 64 ms Stable frequency at start-up
Save
Additional Delay from
Reset Recommended Usage
Additional Delay from
Reset (VCC = 5.0V) Recommended usage
(1)
(1)
4 ms
64 ms Slowly rising power
Fast rising power or BOD
enabled
Notes: 1. These options should only be used if frequency stability at start-up is not important for the
7.9 Crystal 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 7-4. 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 7-10. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
2588B–AVR–11/06
11 Reserved
application.
29
Figure 7-4. Crystal Oscillator Connections
C2
C1
XTAL2
XTAL1
GND
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 7-10.
Table 7-10. Crystal Oscillator Operating Modes
Recommended Range for Capacitors C1 and
CKSEL3..1 Frequency Range (MHz)
(1)
100
101 0.9 - 3.0 12 - 22
110 3.0 - 8.0 12 - 22
111 8.0 - 12 - 22
Notes: 1. This option should not be used with crystals, only with ceramic resonators.
0.4 - 0.9 –
C2 for Use with Crystals (pF)
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table
7-11.
Table 7-11. Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
CKSEL0 SUT1..0
0 00 258 CK
0 01 258 CK
0 10 1K (1024) CK
0 11 1K (1024)CK
1 00 1K (1024)CK
1 01 16K (16384) CK 14CK
1 10 16K (16384) CK 14CK + 4.1 ms
1 11 16K (16384) CK 14CK + 65 ms
Power-save
(1)
(1)
(2)
(2)
(2)
Additional Delay
from Reset
(VCC = 5.0V) Recommended Usage
14CK + 4.1 ms
14CK + 65 ms
14CK
14CK + 4.1 ms
14CK + 65 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
30
ATtiny261/461/861
2588B–AVR–11/06
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