Atmel ATtiny25-V, ATtiny45-V, ATtiny85-V User Manual

Atmel 8-bit AVR Microcontroller with 2/4/8K

Bytes In-System Programmable Flash

ATtiny25/V / ATtiny45/V / ATtiny85/V

Features

•High Performance, Low Power AVR® 8-Bit Microcontroller

•Advanced RISC Architecture

–120 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 Bytes of In-System Programmable Program Memory Flash

•Endurance: 10,000 Write/Erase Cycles

–128/256/512 Bytes In-System Programmable EEPROM

•Endurance: 100,000 Write/Erase Cycles

–128/256/512 Bytes Internal SRAM

–Programming Lock for Self-Programming Flash Program and EEPROM Data Security

•Peripheral Features

–8-bit Timer/Counter with Prescaler and Two PWM Channels

–8-bit High Speed Timer/Counter with Separate Prescaler

•2 High Frequency PWM Outputs with Separate Output Compare Registers

•Programmable Dead Time Generator

–USI – Universal Serial Interface with Start Condition Detector

–10-bit ADC

•4 Single Ended Channels

•2 Differential ADC Channel Pairs with Programmable Gain (1x, 20x)

•Temperature Measurement

–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

–Six Programmable I/O Lines

–8-pin PDIP, 8-pin SOIC, 20-pad QFN/MLF, and 8-pin TSSOP (only ATtiny45/V)

•Operating Voltage

–1.8 - 5.5V for ATtiny25V/45V/85V

–2.7 - 5.5V for ATtiny25/45/85

•Speed Grade

–ATtiny25V/45V/85V: 0 – 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V

–ATtiny25/45/85: 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: 300 µA

–Power-down Mode:

• 0.1 µA at 1.8V	Rev. 2586Q–AVR–08/2013

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1. Pin Configurations

Figure 1-1. Pinout ATtiny25/45/85

PDIP/SOIC/TSSOP

							8		VCC
(PCINT5/RESET/ADC0/dW) PB5						1
				PB3		2	7		PB2 (SCK/USCK/SCL/ADC1/T0/INT0/PCINT2)
(PCINT3/XTAL1/CLKI/OC1B/ADC3)
(PCINT4/XTAL2/CLKO/OC1B/ADC2) PB4						3	6		PB1 (MISO/DO/AIN1/OC0B/OC1A/PCINT1)
				GND		4	5
									PB0 (MOSI/DI/SDA/AIN0/OC0A/OC1A/AREF/PCINT0)

NOTE: TSSOP only for ATtiny45/V

QFN/MLF

						1
(PCINT5/RESET/ADC0/dW) PB5
				PB3		2
(PCINT3/XTAL1/CLKI/OC1B/ADC3)
				DNC		3
				DNC		4
(PCINT4/XTAL2/CLKO/OC1B/ADC2) PB4						5

DNC	DNC	DNC	DNC
20	19	18	17

6	7	8	9
DNC	DNC	GND	DNC

16 DNC

DNC 10

15	VCC
14	PB2 (SCK/USCK/SCL/ADC1/T0/INT0/PCINT2)
13	DNC
12	PB1 (MISO/DO/AIN1/OC0B/OC1A/PCINT1)
11
	PB0 (MOSI/DI/SDA/AIN0/OC0A/OC1A/AREF/PCINT0)

NOTE: Bottom pad should be soldered to ground.

DNC: Do Not Connect

1.1Pin Descriptions

1.1.1VCC

Supply voltage.

1.1.2GND

Ground.

1.1.3Port B (PB5:PB0)

Port B is a 6-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a reset condition becomes active, even if the clock is not running.

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Port B also serves the functions of various special features of the ATtiny25/45/85 as listed in “Alternate Functions of Port B” on page 60.

On ATtiny25, the programmable I/O ports PB3 and PB4 (pins 2 and 3) are exchanged in ATtiny15 Compatibility Mode for supporting the backward compatibility with ATtiny15.

1.1.4RESET

Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running and provided the reset pin has not been disabled. The minimum pulse length is given in Table 21-4 on page 165. Shorter pulses are not guaranteed to generate a reset.

The reset pin can also be used as a (weak) I/O pin.

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2. Overview

The ATtiny25/45/85 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the ATtiny25/45/85 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed.

2.1Block Diagram

Figure 2-1.	Block Diagram
		8-BIT DATABUS
				CALIBRATED
				INTERNAL
				OSCILLATOR
	PROGRAM	STACK	WATCHDOG	TIMING AND
	COUNTER	POINTER	TIMER	CONTROL
VCC
			MCU CONTROL
	PROGRAM	SRAM	REGISTER
	FLASH
			MCU STATUS
			REGISTER
GND	INSTRUCTION	GENERAL
	REGISTER	PURPOSE
		REGISTERS	TIMER/
		X	COUNTER0
	INSTRUCTION	Y
	DECODER	Z	TIMER/
			COUNTER1
	CONTROL	ALU	UNIVERSAL
			SERIAL
	LINES
			INTERFACE
		STATUS	INTERRUPT
			UNIT
		REGISTER
	PROGRAMMING		DATA	OSCILLATORS
	LOGIC		EEPROM
		DATA REGISTER	DATA DIR.	ADC /
		PORT B	REG.PORT B	ANALOG COMPARATOR
		PORT B DRIVERS
				RESET
		PB[0:5]

The AVR core combines a rich instruction set with 32 general purpose working registers. All 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.

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The ATtiny25/45/85 provides the following features: 2/4/8K bytes of In-System Programmable Flash, 128/256/512 bytes EEPROM, 128/256/256 bytes SRAM, 6 general purpose I/O lines, 32 general purpose working registers, one 8-bit Timer/Counter with compare modes, one 8-bit high 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. Idle mode stops the CPU while allowing the SRAM, Timer/Counter, ADC, Analog Comparator, and Interrupt system to continue functioning. Power-down mode saves the register contents, disabling all chip functions until the next Interrupt or Hardware Reset. ADC Noise Reduction mode stops the CPU and all I/O modules except ADC, to minimize switching noise during ADC conversions.

The device is manufactured using Atmel’s high density non-volatile memory technology. The On-chip ISP Flash allows the Program memory to be re-programmed In-System through an SPI serial interface, by a conventional non-volatile memory programmer or by an On-chip boot code running on the AVR core.

The ATtiny25/45/85 AVR is supported with a full suite of program and system development tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators and Evaluation kits.

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3. About

3.1Resources

A comprehensive set of development tools, application notes and datasheets are available for download on http://www.atmel.com/avr.

3.2Code 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.

For I/O Registers located in the extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically, this means “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”. Note that not all AVR devices include an extended I/O map.

3.3Capacitive Touch Sensing

Atmel QTouch Library provides a simple to use solution for touch sensitive interfaces on Atmel AVR microcontrollers. The QTouch Library includes support for QTouch® and QMatrix® acquisition methods.

Touch sensing is easily added to any application by linking the QTouch Library and using the Application Programming Interface (API) of the library to define the touch channels and sensors. The application then calls the API to retrieve channel information and determine the state of the touch sensor.

The QTouch Library is free and can be downloaded from the Atmel website. For more information and details of implementation, refer to the QTouch Library User Guide – also available from the Atmel website.

3.4Data 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.

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4. AVR CPU Core

4.1Introduction

This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure correct program execution. The CPU must therefore be able to access memories, perform calculations, control peripherals, and handle interrupts.

4.2Architectural Overview

Figure 4-1. Block Diagram of the AVR Architecture

			Data Bus 8-bit
Flash	Program		Status
	Counter		and Control
Program
Memory
			32 x 8
Instruction			General
Register			Purpose	Interrupt
			Registrers
				Unit
Instruction				Watchdog
Decoder	AddressingDirect	AddressingIndirect		Timer
			ALU	Analog
Control Lines				Comparator
				I/O Module1
			Data	I/O Module 2
			SRAM
				I/O Module n
			EEPROM
			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 oper-

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ands are output from the Register File, the operation is executed, and the result is stored back in the Register File

– in one clock cycle.

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing – enabling efficient address calculations. One of the these address pointers can also be used as an address pointer for look up tables in Flash Program memory. These added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.

The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single register operations can also be executed in the ALU. After 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, but there are also 32-bit instructions.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps.

A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other I/O functions. The I/O memory can be accessed directly, or as the Data Space locations following those of the Register File, 0x20 - 0x5F.

4.3ALU – Arithmetic Logic Unit

The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bitfunctions. Some implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description.

4.4Status Register

The Status Register contains information about the result of the most recently executed arithmetic instruction. This information can be used for altering program flow in order to perform conditional operations. Note that the Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases remove the need for using the dedicated compare instructions, resulting in faster and more compact code.

The Status Register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt. This must be handled by software.

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4.4.1SREG – AVR Status Register

The AVR Status Register – SREG – is defined as:

Bit	7	6	5	4	3	2	1	0
0x3F									SREG
	I	T	H	S	V	N	Z	C
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

• 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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4.5General Purpose Register File

The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required performance and flexibility, the following input/output schemes are supported by the Register File:

•One 8-bit output operand and one 8-bit result input

•Two 8-bit output operands and one 8-bit result input

•Two 8-bit output operands and one 16-bit result input

•One 16-bit output operand and one 16-bit result input

Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.

Figure 4-2. AVR CPU General Purpose Working Registers

7		0	Addr.
			0x00
		R0
		R1	0x01
		R2	0x02
		…
		R13	0x0D
General		R14	0x0E
Purpose		R15	0x0F
Working			0x10
		R16
Registers		R17	0x11
		…
		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 4-2, each register is also assigned a Data memory address, mapping them directly into the first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory organization provides great flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to index any register in the file.

4.5.1The X-register, Y-register, and Z-register

The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are defined as described in Figure 4-3.

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Figure 4-3. 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 displacement, automatic increment, and automatic decrement (see the instruction set reference for details).

4.6Stack Pointer

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.

4.6.1SPH and SPL — Stack Pointer Register

Bit	15	14	13	12	11	10	9	8
0x3E									SPH
	SP15	SP14	SP13	SP12	SP11	SP10	SP9	SP8
0x3D	SP7	SP6	SP5	SP4	SP3	SP2	SP1	SP0	SPL
	7	6	5	4	3	2	1	0
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Read/Write	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
Initial Value	RAMEND	RAMEND	RAMEND	RAMEND	RAMEND	RAMEND	RAMEND	RAMEND

4.7Instruction Execution Timing

This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used.

Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit.

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Figure 4-4. The Parallel Instruction Fetches and Instruction Executions

T1

T2

T3

T4

clkCPU

1st Instruction Fetch

1st Instruction Execute 2nd Instruction Fetch 2nd Instruction Execute 3rd Instruction Fetch 3rd Instruction Execute 4th Instruction Fetch

Figure 4-5 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using two register operands is executed, and the result is stored back to the destination register.

Figure 4-5. Single Cycle ALU Operation

T1

T2

T3

T4

clkCPU Total Execution Time

Register Operands Fetch

ALU Operation Execute

Result Write Back

4.8Reset 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.

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.

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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) */

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) */

4.8.1Interrupt 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.

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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.

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5. AVR Memories

This section describes the different memories in the ATtiny25/45/85. The AVR architecture has two main memory spaces, the Data memory and the Program memory space. In addition, the ATtiny25/45/85 features an EEPROM Memory for data storage. All three memory spaces are linear and regular.

5.1In-System Re-programmable Flash Program Memory

The ATtiny25/45/85 contains 2/4/8K bytes On-chip In-System Reprogrammable Flash memory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 1024/2048/4096 x 16.

The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATtiny25/45/85 Program Counter (PC) is 10/11/12 bits wide, thus addressing the 1024/2048/4096 Program memory locations. “Memory Programming” on page 147 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 Timing” on page 11.

Figure 5-1. Program Memory Map

Program Memory

0x0000

0x03FF/0x07FF/0x0FFF

5.2SRAM Data Memory

Figure 5-2 shows how the ATtiny25/45/85 SRAM Memory is organized.

The lower 224/352/607 Data memory locations address both the Register File, the I/O memory and the internal data SRAM. The first 32 locations address the Register File, the next 64 locations the standard I/O memory, and the last 128/256/512 locations address the internal data SRAM.

The five different addressing modes for the Data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register File, registers R26 to R31 feature the indirect addressing pointer registers.

The direct addressing reaches the entire data space.

The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z- register.

When using register indirect addressing modes with automatic pre-decrement and post-increment, the address registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O Registers, and the 128/256/512 bytes of internal data SRAM in the ATtiny25/45/85 are all accessible through all these addressing modes. The Register File is described in “General Purpose Register File” on page 10.

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Figure 5-2. Data Memory Map

Data Memory

32 Registers	0x0000 - 0x001F
64 I/O Registers	0x0020 - 0x005F
	0x0060
Internal SRAM
(128/256/512 x 8)
	0x0DF/0x015F/0x025F

5.2.1Data Memory Access Times

This section describes the general access timing concepts for internal memory access. The internal data SRAM access is performed in two clkCPU cycles as described in Figure 5-3.

Figure 5-3. On-chip Data SRAM Access Cycles

T1 T2 T3

clkCPU

Address Compute Address Address valid

Data

WR

Data

RD

Read Write

Memory Access Instruction

Next Instruction

5.3EEPROM Data Memory

The ATtiny25/45/85 contains 128/256/512 bytes of data EEPROM memory. It is organized as a separate data space, in which single bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the CPU is described in the following, specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control Register. For details see “Serial Downloading” on page 151.

5.3.1EEPROM Read/Write Access

The EEPROM Access Registers are accessible in the I/O space.

The write access times for the EEPROM are given in Table 5-1 on page 21. A self-timing function, however, lets the user software detect when the next byte can be written. If the user code contains instructions that write the EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC is likely to rise or fall slowly on Power-up/down. This causes the device for some period of time to run at a voltage lower than specified as minimum for the clock frequency used. See “Preventing EEPROM Corruption” on page 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 “Atomic Byte Programming” on page 17 and “Split Byte Programming” on page 17 for details on this.

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When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed.

5.3.2Atomic Byte Programming

Using Atomic Byte Programming is the simplest mode. When writing a byte to the EEPROM, the user must write the address into the EEAR Register and data into EEDR Register. If the EEPMn bits are zero, writing EEPE (within four cycles after EEMPE is written) will trigger the erase/write operation. Both the erase and write cycle are done in one operation and the total programming time is given in Table 5-1 on page 21. 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.

5.3.3Split Byte Programming

It is possible to split the erase and write cycle in two different operations. This may be useful if the system requires short access time for some limited period of time (typically if the power supply voltage falls). In order to take advantage of this method, it is required that the locations to be written have been erased before the write operation. But since the erase and write operations are split, it is possible to do the erase operations when the system allows doing time-critical operations (typically after Power-up).

5.3.4Erase

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 5-1 on page 21). The EEPE bit remains set until the erase operation completes. While the device is busy programming, it is not possible to do any other EEPROM operations.

5.3.5Write

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 5-1 on page 21). The EEPE bit remains set until the write operation completes. If the location to be written has not been erased before write, the data that is stored must be considered as lost. While the device is busy with programming, it is not possible to do any other EEPROM operations.

The calibrated Oscillator is used to time the EEPROM accesses. Make sure the Oscillator frequency is within the requirements described in “OSCCAL – Oscillator Calibration Register” on page 31.

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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 (r19) to data register out EEDR, r19

; 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);

}

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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;

}

5.3.6Preventing EEPROM Corruption

During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the CPU and the EEPROM to operate properly. These issues are the same as for board level systems using EEPROM, and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the needed detection level, an external low VCC reset protection circuit can be used. If a reset occurs while a write operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.

5.4I/O Memory

The I/O space definition of the ATtiny25/45/85 is shown in “Register Summary” on page 200.

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All ATtiny25/45/85 I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose working registers and the I/O space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the instruction set section for more details. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these addresses.

For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses should never be written.

Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.

The I/O and Peripherals Control Registers are explained in later sections.

5.5Register Description

5.5.1EEARH – EEPROM Address Register

Bit		7	6	5	4	3	2	1	0
0x1F										EEARH
		–	–	–	–	–	–	–	EEAR8
Read/Write		R	R	R	R	R	R	R	R/W
Initial Value		0	0	0	0	0	0	0	X/0

• Bits 7:1 – Res: Reserved Bits

These bits are reserved for future use and will always read as zero.

• Bits 0 – EEAR8: EEPROM Address

This is the most significant EEPROM address bit of ATtiny85. In devices with less EEPROM, i.e. ATtiny25/ATtiny45, this bit is reserved and will always read zero. The initial value of the EEPROM Address Register (EEAR) is undefined and a proper value must therefore be written before the EEPROM is accessed.

5.5.2EEARL – EEPROM Address Register

Bit
0x1E									EEARL
	EEAR7	EEAR6	EEAR5	EEAR4	EEAR3	EEAR2	EEAR1	EEAR0
Rear/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Initial Value	X	X	X	X	X	X	X	X

• Bit 7 – EEAR7: EEPROM Address

This is the most significant EEPROM address bit of ATtiny45. In devices with less EEPROM, i.e. ATtiny25, this bit is reserved and will always read zero. The initial value of the EEPROM Address Register (EEAR) is undefined and a proper value must therefore be written before the EEPROM is accessed.

• Bits 6:0 – EEAR[6:0]: EEPROM Address

These are the (low) bits of the EEPROM Address Register. The EEPROM data bytes are addressed linearly in the range 0...(128/256/512-1). The initial value of EEAR is undefined and a proper value must be therefore be written before the EEPROM may be accessed.

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5.5.3EEDR – EEPROM Data Register

Bit		7	6	5	4	3	2	1	0
0x1D		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 Value		0	0	0	0	0	0	0	0

• Bits 7:0 – EEDR[7:0]: EEPROM Data

For the EEPROM write operation the EEDR Register contains the data to be written to the EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out from the EEPROM at the address given by EEAR.

5.5.4EECR – EEPROM Control Register

Bit	7	6	5	4	3	2	1	0
0x1C	–	–	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 ATtiny25/45/85. For compatibility with future AVR devices, always write this bit to zero. After reading, mask out this bit.

• Bit 6 – Res: Reserved Bit

This bit is reserved in the ATtiny25/45/85 and will always read as zero.

• Bits 5:4 – EEPM[1:0]: 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 5-1. While EEPE is set, any write to EEPMn will be ignored. During reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming.

Table 5-1.	EEPROM Mode Bits
		Programming
EEPM1	EEPM0	Time	Operation
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

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

Writing EERIE to one enables the EEPROM Ready Interrupt if the I-bit in SREG is set. Writing EERIE to zero disables the interrupt. The EEPROM Ready Interrupt generates a constant interrupt when Non-volatile memory is ready for programming.

• Bit 2 – EEMPE: EEPROM Master Program Enable

The EEMPE bit determines whether writing EEPE to one will have effect or not.

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.

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• 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.

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6. System Clock and Clock Options

6.1Clock Systems and their Distribution

Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be active at a given time. In order to reduce power consumption, the clocks to modules not being used can be halted by using different sleep modes, as described in “Power Management and Sleep Modes” on page 34. The clock systems are detailed below.

Figure 6-1. Clock Distribution

ADC

General I/O

CPU Core

RAM

Flash and

Modules

EEPROM

clkPCK

clkI/O

AVR Clock

clkCPU

Control Unit

clkFLASH

clk

ADC

Reset Logic

Watchdog Timer

Source clock

Watchdog clock

System Clock

PCK

Prescaler

clk

Clock

Watchdog

PLL

Multiplexer

Oscillator

Oscillator

	External Clock	CalibratedCrystal RC	Low-Frequency	Calibrated RC
		Oscillator	Crystal Oscillator	Oscillator

6.1.1CPU Clock – clkCPU

The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such modules are the General Purpose Register File, the Status Register and the Data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing general operations and calculations.

6.1.2I/O Clock – clkI/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counter. The I/O clock is also used by the External Interrupt module, but note that some external interrupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O clock is halted.

6.1.3Flash Clock – clkFLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock.

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6.1.4ADC Clock – clkADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results.

6.1.5Internal PLL for Fast Peripheral Clock Generation - clkPCK

The internal PLL in ATtiny25/45/85 generates a clock frequency that is 8x multiplied from a source input. By default, the PLL uses the output of the internal, 8.0 MHz RC oscillator as source. Alternatively, if bit LSM of PLLCSR is set the PLL will use the output of the RC oscillator divided by two. Thus the output of the PLL, the fast peripheral clock is 64 MHz. The fast peripheral clock, or a clock prescaled from that, can be selected as the clock source for Timer/Counter1 or as a system clock. See Figure 6-2. The frequency of the fast peripheral clock is divided by two when LSM of PLLCSR is set, resulting in a clock frequency of 32 MHz. Note, that LSM can not be set if PLLCLK is used as system clock.

Figure 6-2.	PCK Clocking System.
	OSCCAL	LSM	PLLE	CKSEL[3:0]	CLKPS[3:0]
				LOCK
				DETECTOR	PLOCK
		1/2
		4 MHz			PCK
	8.0 MHz
	OSCILLATOR	8 MHz	PLL
			8x	1/4
			64 / 32 MHz
				16 MHz
XTAL1				8 MHz	PRESCALER
					SYSTEM
XTAL2	OSCILLATORS				CLOCK

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 when:

•The PLLE bit in the register PLLCSR is set.

•The CKSEL fuse is programmed to ‘0001’.

•The CKSEL fuse is programmed to ‘0011’.

The PLLCSR bit PLOCK is set when PLL is locked.

Both internal RC oscillator and PLL are switched off in power down and stand-by sleep modes.

6.1.6Internal PLL in ATtiny15 Compatibility Mode

Since ATtiny25/45/85 is a migration device for ATtiny15 users there is an ATtiny15 compatibility mode for backward compatibility. The ATtiny15 compatibility mode is selected by programming the CKSEL fuses to ‘0011’.

In the ATtiny15 compatibility mode the frequency of the internal RC oscillator is calibrated down to 6.4 MHz and the multiplication factor of the PLL is set to 4x. See Figure 6-3. With these adjustments the clocking system is ATtiny15-compatible and the resulting fast peripheral clock has a frequency of 25.6 MHz (same as in ATtiny15).

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Figure 6-3. PCK Clocking System in ATtiny15 Compatibility Mode.

OSCCAL

PLLE

PLL

25.6 MHz

PCK

8x

1/2

3.2 MHz

LOCK

6.4 MHz

DETECTOR

PLOCK

OSCILLATOR

1/4

1.6 MHz

SYSTEM

CLOCK

Note that low speed mode is not implemented in ATtiny15 compatibility mode.

6.2Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock from the selected source is input to the AVR clock generator, and routed to the appropriate modules.

Table 6-1.	Device Clocking Options Select
Device Clocking Option			CKSEL[3:0](1)
External Clock (see page 26)			0000
High Frequency PLL Clock (see page 26)			0001
Calibrated Internal Oscillator (see page 27)			0010(2)
Calibrated Internal Oscillator (see page 27)			0011(3)
Internal 128 kHz Oscillator (see page 28)			0100
Low-Frequency Crystal Oscillator (see page 29)			0110
Crystal Oscillator / Ceramic Resonator (see page 29)			1000 – 1111
Reserved			0101, 0111
Note: 1.	For all fuses “1” means unprogrammed while “0” means programmed.

2.The device is shipped with this option selected.

3.This will select ATtiny15 Compatibility Mode, where system clock is divided by four, resulting in a 1.6 MHz clock frequency. For more inormation, see “Calibrated Internal Oscillator” on page 27.

The various choices for each clocking option is given in the following sections. When the CPU wakes up from Power-down, the selected clock source is used to time the start-up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts from reset, there is an additional delay allowing the power to reach a stable level before commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table 6-2.

Table 6-2.	Number of Watchdog Oscillator Cycles
	Typ Time-out		Number of Cycles
	4 ms		512
	64 ms		8K (8,192)

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6.2.1External Clock

To drive the device from an external clock source, CLKI should be driven as shown in Figure 6-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “00”.

Figure 6-4. External Clock Drive Configuration

EXTERNAL

CLOCK CLKI

SIGNAL

GND

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 6-3.

Table 6-3.	Start-up Times for the External Clock Selection
	Start-up Time from	Additional Delay from
SUT[1:0]	Power-down	Reset	Recommended Usage
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

When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the MCU is kept in Reset during such changes in the clock frequency.

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.

6.2.2High Frequency PLL Clock

There is an internal PLL that provides nominally 64 MHz clock rate locked to the RC Oscillator for the use of the Peripheral Timer/Counter1 and for the system clock source. When selected as a system clock source, by programming the CKSEL fuses to ‘0001’, it is divided by four like shown in Table 6-4.

Table 6-4. High Frequency PLL Clock Operating Modes

CKSEL[3:0]	Nominal Frequency
0001	16 MHz

When this clock source is selected, start-up times are determined by the SUT fuses as shown in Table 6-5.

Table 6-5.	Start-up Times for the High Frequency PLL Clock
	Start-up Time from	Additional Delay from	Recommended
SUT[1:0]	Power Down	Power-On Reset (VCC = 5.0V)	usage
00	14CK + 1K (1024) CK + 4 ms	4 ms	BOD enabled

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	Table 6-5.	Start-up Times for the High Frequency PLL Clock
		Start-up Time from		Additional Delay from	Recommended
	SUT[1:0]	Power Down		Power-On Reset (VCC = 5.0V)	usage
	01	14CK + 16K (16384) CK + 4 ms		4 ms	Fast rising power
	10	14CK + 1K	(1024) CK + 64 ms	4 ms	Slowly rising power
	11	14CK + 16K	(16384) CK + 64 ms	4 ms	Slowly rising power

6.2.3Calibrated Internal 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 “Calibrated Internal RC Oscillator Accuracy” on page 164 and “Internal Oscillator Speed” on page 192 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 6-6 on page 27. 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 21-2 on page 164.

By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on page 31, 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 21-2 on page 164.

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 Bytes” on page 150.

The internal oscillator can also be set to provide a 6.4 MHz clock by writing CKSEL fuses to “0011”, as shown in Table 6-6 below. This setting is reffered to as ATtiny15 Compatibility Mode and is intended to provide a calibrated clock source at 6.4 MHz, as in ATtiny15. In ATtiny15 Compatibility Mode the PLL uses the internal oscillator running at 6.4 MHz to generate a 25.6 MHz peripheral clock signal for Timer/Counter1 (see “8-bit Timer/Counter1 in ATtiny15 Mode” on page 95). Note that in this mode of operation the 6.4 MHz clock signal is always divided by four, providing a 1.6 MHz system clock.

Table 6-6.	Internal Calibrated RC Oscillator Operating Modes
	CKSEL[3:0]		Nominal Frequency
	0010(1)		8.0 MHz
	0011(2)		6.4 MHz
Note: 1.	The device is shipped with this option selected.

2.This setting will select ATtiny15 Compatibility Mode, where system clock is divided by four, resulting in a 1.6 MHz clock frequency.

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When the calibrated 8 MHz internal oscillator is selected as clock source the start-up times are determined by the SUT Fuses as shown in Table 6-7 below.

Table 6-7.	Start-up Times for Internal Calibrated RC Oscillator Clock
	Start-up Time	Additional Delay from
SUT[1:0]	from Power-down	Reset (VCC = 5.0V)	Recommended Usage
00	6 CK	14CK(1)	BOD enabled
01	6 CK	14CK + 4 ms	Fast rising power
10(2)	6 CK	14CK + 64 ms	Slowly rising power
11		Reserved
Note: 1.	If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
	mode can be entered.

2. The device is shipped with this option selected.

In ATtiny15 Compatibility Mode start-up times are determined by SUT fuses as shown in Table 6-8 below.

Table 6-8. Start-up Times for Internal Calibrated RC Oscillator Clock (in ATtiny15 Mode)

	Start-up Time	Additional Delay from
SUT[1:0]	from Power-down	Reset (VCC = 5.0V)	Recommended Usage
00	6 CK	14CK + 64 ms
01	6 CK	14CK + 64 ms
10	6 CK	14CK + 4 ms
11	1 CK	14CK(1)
Note: 1.	If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming
	mode can be entered.

In summary, more information on ATtiny15 Compatibility Mode can be found in sections “Port B (PB5:PB0)” on page 2, “Internal PLL in ATtiny15 Compatibility Mode” on page 24, “8-bit Timer/Counter1 in ATtiny15 Mode” on page 95, “Limitations of debugWIRE” on page 140, “Calibration Bytes” on page 150 and in table “Clock Prescaler Select” on page 33.

6.2.4Internal 128 kHz 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 “0100”.

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 6-9.

Table 6-9.	Start-up Times for the 128 kHz Internal Oscillator
	Start-up Time from	Additional Delay from
SUT[1:0]	Power-down	Reset	Recommended Usage
00	6 CK	14CK(1)	BOD enabled
01	6 CK	14CK + 4 ms	Fast rising power
10	6 CK	14CK + 64 ms	Slowly rising power
11		Reserved

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Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to 14CK + 4 ms to ensure programming mode can be entered.

6.2.5Low-Frequency Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal Oscillator must be selected by setting CKSEL fuses to ‘0110’. The crystal should be connected as shown in Figure 6-5. To find suitable load capacitance for a 32.768 kHz crysal, please consult the manufacturer’s datasheet.

When this oscillator is selected, start-up times are determined by the SUT fuses as shown in Table 6-10.

Table 6-10.		Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
		Start-up Time from	Additional Delay from
SUT[1:0]		Power Down	Reset (VCC = 5.0V)	Recommended usage
00		1K (1024) CK(1)	4 ms	Fast rising power or BOD enabled
01		1K (1024) CK(1)	64 ms	Slowly rising power
10		32K (32768) CK	64 ms	Stable frequency at start-up
11			Reserved

Note: 1. These options should be used only if frequency stability at start-up is not important.

The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 6-11 at each TOSC pin.

Table 6-11. Capacitance of Low-Frequency Crystal Oscillator

Device	32 kHz Osc. Type	Cap (Xtal1/Tosc1)	Cap (Xtal2/Tosc2)
ATtiny25/45/85	System Osc.	16 pF	6 pF

6.2.6Crystal Oscillator / Ceramic Resonator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an On-chip Oscillator, as shown in Figure 6-5. Either a quartz crystal or a ceramic resonator may be used.

Figure 6-5. Crystal Oscillator Connections

C2

XTAL2

C1

XTAL1

GND

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 environ-

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ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 6-12 below. For ceramic resonators, the capacitor values given by the manufacturer should be used.

Table 6-12.	Crystal Oscillator Operating Modes
			Recommended Range for Capacitors C1 and
CKSEL[3:1]	Frequency Range (MHz)		C2 for Use with Crystals (pF)
100(1)	0.4 - 0.9		–
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.

The Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL[3:1] as shown in Table 6-12.

The CKSEL0 Fuse together with the SUT[1:0] Fuses select the start-up times as shown in Table 6-13.

Table 6-13.	Start-up Times for the Crystal Oscillator Clock Selection
		Start-up Time from	Additional Delay
CKSEL0	SUT[1:0]	Power-down	from Reset		Recommended Usage
0	00	258 CK(1)	14CK + 4 ms		Ceramic resonator,
					fast rising power
0	01	258 CK(1)	14CK + 64 ms		Ceramic resonator,
					slowly rising power
0	10	1K (1024) CK(2)	14CK		Ceramic resonator,
					BOD enabled
0	11	1K (1024)CK(2)	14CK + 4 ms		Ceramic resonator,
					fast rising power
1	00	1K (1024)CK(2)	14CK + 64 ms		Ceramic resonator,
					slowly rising power
1	01	16K (16384) CK	14CK		Crystal Oscillator,
					BOD enabled
1	10	16K (16384) CK	14CK + 4 ms		Crystal Oscillator,
					fast rising power
1	11	16K (16384) CK	14CK + 64 ms		Crystal Oscillator,
					slowly rising power

Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if frequency stability at start-up is not important for the application. These options are not suitable for crystals.

2.These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency stability at start-up is not important for the application.

6.2.7Default 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, resulting in 1.0 MHz system clock. This default setting ensures that all users can make their desired clock source setting using an In-System or High-voltage Programmer.

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