Rainbow Electronics ATtiny28L User Manual

Features

• Utilizes the AVR

• AVR – High-performance and Low-power RISC Architecture

– 90 Powerful Instructions – Most Single Clock Cycle Execution – 32 x 8 General-purpose Working Registers – Up to 4 MIPS Throughput at 4 MHz

• Nonvolatile Program Memory

– 2K Bytes of Flash Program Memory – Endurance: 1,000 Write/Erase Cycles – Programming Lock for Flash Program Data Security

• Peripheral Features

– Interrupt and Wake-up on Low-level Input – One 8-bit Timer/Counter with Separate Prescaler – On-chip Analog Comparator – Programmable Watchdog Timer with On-chip Oscillator – Built-in High-current LED Driver with Programmable Modulation

• Special Microcontroller Features

– Low-power Idle and Power-down Modes – External and Internal Interrupt Sources – Power-on Reset Circuit with Programmable Start-up Time – Internal Calibrated RC Oscillator

• Power Consumption at 1 MHz, 2V, 25°C

– Active: 3.0 mA – Idle Mode: 1.2 mA – Power-down Mode: <1 µA

• I/O and Packages

– 11 Programmable I/O Lines, 8 Input Lines and a High-current LED Driver – 28-lead PDIP, 32-lead TQFP, and 32-pad MLF

• Operating Voltages

–V

: 1.8V - 5.5V for the ATtiny28V

: 2.7V - 5.5V for the ATtiny28L

–V

• Speed Grades

– 0 - 1.2 MHz for the ATtiny28V – 0 - 4 MHz For the ATtiny28L

RISC Architecture

8-bit Microcontroller with 2K Bytes of Flash

ATtiny28L ATtiny28V

Pin Configurations

PDIP

RESET

XTAL1 XTAL2

(AIN0) PB0

PD0 PD1 PD2 PD3 PD4

VCC

GND

PD5 PD6 PD7

1 2 3 4 5 6 7 8 9 10 11 12 13 14

28 27 26 25 24 23 22 21 20 19 18 17 16 15

PA0 PA1 PA3 PA2 (IR) PB7 PB6 GND NC VCC PB5 PB4 (INT1) PB3 (INT0) PB2 (T0) PB1 (AIN1)

PD3 PD4

VCC

GND

NC XTAL1 XTAL2

TQFP/MLF

PD2

PD1

32313029282726

1 2 3 4 5 6 7 8

9101112131415

PD5

PD6

PD0

RESET

PD7

(AIN0) PB0

PA0

PA1

PA3

(T0) PB2

(INT0) PB3

(AIN1) PB1

PA2 (IR)

24 23 22 21 20 19 18 17

(INT1) PB4

PB7 PB6 NC GND NC NC VCC PB5

Rev. 1062E–10/01

Description The ATtiny28 is a low-power CMOS 8-bit microcontroller based on the AVR RISC archi-

tecture. By executing powerful instructions in a single clock cycle, the ATtiny28 achieves throughputs approaching 1 MIPS per MHz, allowing the system designer to optimize power consumption versus processing speed. 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.

Block Diagram Figure 1. The ATtiny28 Block Diagram

VCC

GND

PROGRAM COUNTER

PROGRAM

FLASH

INSTRUCTION

DECODER

CONTROL

LINES

PROGRAMMING

LOGIC

DATA REGISTER

ANALOG

COMPARATOR

PORTB

STACK

POINTER

HARDWARE

STACK

GENERAL PURPOSE

REGISTERS

STATUS

PORTB

8-BIT DATA BUS

ALU

INTERNAL

OSCILLATOR

WATCHDOG

MCU CONTROL

TIMER/

COUNTER

INTERRUPT

DATA REGISTER

PORTD

TIMER

UNIT

PORTD

OSCILLATOR

TIMING AND

CONTROL

DATA DIR

REG. PORTD

XTAL2XTAL1

INTERNAL CALIBRATED OSCILLATOR

HARDWARE

MODULATOR

DATA REGISTER

PORTA

PORTA CONTROL

RESET

The ATtiny28 provides the following features: 2K bytes of Flash, 11 general-purpose I/O lines, 8 input lines, a high-current LED driver, 32 general-purpose working registers, an 8-bit timer/counter, internal and external interrupts, programmable Watchdog Timer with internal oscillator and 2 software-selectable power-saving modes. The Idle Mode stops the CPU while allowing the timer/counter and interrupt system to continue functioning. The Power-down mode saves the register contents but freezes the oscillator, disabling all other chip functions until the next interrupt or hardware reset. The wake-up or inter-

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rupt on low-level input feature enables the ATtiny28 to be highly responsive to external events, still featuring the lowest power consumption while in the power-down modes.

The device is manufactured using Atmel’s high-density, nonvolatile memory technology. By combining an enhanced RISC 8-bit CPU with Flash on a monolithic chip, the Atmel ATtiny28 is a powerful microcontroller that provides a highly flexible and cost-effective solution to many embedded control applications. The ATtiny28 AVR is supported with a full suite of program and system development tools including: macro assemblers, program debugger/simulators, in-circuit emulators and evaluation kits.

Pin Descriptions

VCC Supply voltage pin.

GND Ground pin.

Port A (PA3..PA0) Port A is a 4-bit I/O port. PA2 is output-only and can be used as a high-current LED

driver. At V bi-directional I/O pins with internal pull-ups (selected for each bit). The port pins are tristated when a reset condition becomes active, even if the clock is not running.

= 2.0V, the PA2 output buffer can sink 25 mA. PA3, PA1 and PA0 are

Port B (PB7..PB0) Port B is an 8-bit input port with internal pull-ups (selected for all Port B pins). Port B

pins that are externally pulled low will source current if the pull-ups are activated.

Port B also serves the functions of various special features of the ATtiny28 as listed on page 39. If any of the special features are enabled, the pull-up(s) on the corresponding pin(s) is automatically disabled. The port pins are tri-stated when a reset condition becomes active, even if the clock is not running.

Port D (PD7..PD0) Port D is an 8-bit I/O port. Port pins can provide internal pull-up resistors (selected for

each bit). The port pins are tri-stated when a reset condition becomes active, even if the clock is not running.

XTAL1 Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2 Output from the inverting oscillator amplifier.

RESET

Reset input. An external reset is generated by a low level on the RESET pin. Reset pulses longer than 50 ns will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset.

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Clock Options The device has the following clock source options, selectable by Flash Fuse bits as

shown in Table 1.

Table 1. Device Clocking Option Select

Clock Option CKSEL3..0

External Crystal/Ceramic Resonator 1111 - 1010

External Low-frequency Crystal 1001 - 1000

External RC Oscillator 0111 - 0101

Internal RC Oscillator 0100 - 0010

External Clock 0001 - 0000

Note: “1” means unprogrammed, “0” means programmed.

The various choices for each clocking option give different start-up times as shown in Table 5 on page 14.

Internal RC Oscillator The internal RC oscillator option is an on-chip calibrated oscillator running at a nominal

frequency of 1.2 MHz. If selected, the device can operate with no external components. The device is shipped with this option selected.

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 2. Either a quartz crystal or a ceramic resonator may be used. When the INTCAP fuse is programmed, internal load capacitors with typical values 50 pF are connected between XTAL1/XTAL2 and ground.

Figure 2. Oscillator Connections

MAX 1 HC BUFFER

Note: 1. When using the MCU oscillator as a clock for an external device, an HC buffer should

be connected as indicated in the figure.

XTAL2

XTAL1

GND

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External Clock To drive the device from an external clock source, XTAL2 should be left unconnected

while XTAL1 is driven as shown in Figure 3.

Figure 3. External Clock Drive Configuration

EXTERNAL

OSCILLATOR

SIGNAL

XTAL2

XTAL1

GND

External RC Oscillator For timing insensitive applications, the external RC configuration shown in Figure 4 can

be used. For details on how to choose R and C, see Table 25 on page 54.

Figure 4. External RC Configuration

XTAL2

XTAL1

GND

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Architectural Overview

The fast-access register file concept contains 32 x 8-bit general-purpose working registers with a single clock cycle access time. This means that during one single clock cycle, one ALU (Arithmetic Logic Unit) operation is executed. 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.

Two of the 32 registers can be used as a 16-bit pointer for indirect memory access. This pointer is called the Z-pointer and can address the register file and the Flash program memory.

Figure 5. The ATtiny28 AVR RISC Architecture

Data Bus 8-bit

1K x 16

Program

Flash

Instruction

Decoder

Control Lines

Program

Counter

Status

and Test

32 x 8 General Purpose

Registrers

ALU

Control

Registrers

Interrupts

Unit

8-bit

Timer/Counter

Watchdog

Timer

Analog

Comparator

I/O Lines

The ALU supports arithmetic and logic functions between registers or between a constant and a register. Single register operations are also executed in the ALU. Figure 5 shows the ATtiny28 AVR RISC microcontroller architecture. The AVR uses a Harvard architecture concept – with separate memories and buses for program and data memories. The program memory is accessed with a two-stage pipeline. While one instruction is being executed, the next instruction is pre-fetched from the program memory. This concept enables instructions to be executed every clock cycle. The program memory is reprogrammable Flash memory.

With the relative jump and relative call instructions, the whole 1K address space is directly accessed. All AVR instructions have a single 16-bit word format, meaning that every program memory address contains a single 16-bit instruction.

During interrupts and subroutine calls, the return address program counter (PC) is stored on the stack. The stack is a 3-level-deep hardware stack dedicated for subroutines and interrupts.

The I/O memory space contains 64 addresses for CPU peripheral functions such as Control Registers, Timer/Counters and other I/O functions. 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 the different interrupts have a sepa-

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rate interrupt vector in the interrupt vector table at the beginning of the program memory. The different interrupts have priority in accordance with their interrupt vector position. The lower the interrupt vector address, the higher the priority.

General-purpose Register File

Figure 6 shows the structure of the 32 general-purpose registers in the CPU.

Figure 6. AVR

CPU General-purpose Working Registers

General …

Purpose …

Working R28

Registers R29

R30 (Z-Register low byte)

R31(Z-Register high byte)

All the register operating instructions in the instruction set have direct and single cycle access to all registers. The only exception are the five constant arithmetic and logic instructions SBCI, SUBI, CPI, ANDI and ORI between a constant and a register and the LDI instruction for load immediate constant data. These instructions apply to the second half of the registers in the register file – R16..R31. The general SBC, SUB, CP, AND, OR and all other operations between two registers or on a single register apply to the entire register file.

Registers 30 and 31 form a 16-bit pointer (the Z-pointer), which is used for indirect Flash memory and register file access. When the register file is accessed, the contents of R31 are discarded by the CPU.

ALU – Arithmetic Logic Unit

Downloadable Flash Program Memory

Program and Data Addressing Modes

The high-performance AVR ALU operates in direct connection with all the 32 generalpurpose working registers. Within a single clock cycle, ALU operations between registers in the register file are executed. The ALU operations are divided into three main categories – arithmetic, logic and bit functions. Some microcontrollers in the AVR product family feature a hardware multiplier in the arithmetic part of the ALU.

The ATtiny28 contains 2K bytes of on-chip Flash memory for program storage. Since all instructions are single 16-bit words, the Flash is organized as 1K x 16 words. The Flash memory has an endurance of at least 1,000 write/erase cycles.

The ATtiny28 program counter is 10 bits wide, thus addressing the 1K word Flash program memory. See page 44 for a detailed description of Flash data downloading.

The ATtiny28 AVR RISC microcontroller supports powerful and efficient addressing modes. This section describes the different addressing modes supported in the ATtiny28. In the figures, OP means the operation code part of the instruction word. To simplify, not all figures show the exact location of the addressing bits.

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Figure 7. Direct Single Register Addressing

The operand is contained in register d (Rd).

Register Indirect Figure 8. Indirect Register Addressing

Register Direct, Two Registers Rd and Rr

REGISTERFILE

Z-Register

30 31

The register accessed is the one pointed to by the Z-register (R31, R30).

Figure 9. Direct Register Addressing, Two Registers

Operands are contained in register r (Rr) and d (Rd). The result is stored in register d (Rd).

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I/O Direct Figure 10. I/O Direct Addressing

Operand address is contained in six bits of the instruction word. n is the destination or source register address.

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Relative Program Addressing, RJMP and RCALL

Constant Addressing Using the LPM Instruction

Figure 11. Relative Program Memory Addressing

15 0

15 012 11

PROGRAM MEMORY

$000

$3FF

Program execution continues at address PC + k + 1. The relative address k is -2048 to

2047.

Figure 12. Code Memory Constant Addressing

PROGRAM MEMORY

15 1 0

Z-REGISTER

$000

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$3FF

Constant byte address is specified by the Z-register contents. The 15 MSBs select word address (0 - 1K), and LSB selects low byte if cleared (LSB = 0) or high byte if set (LSB =

1).

Subroutine and Interrupt Hardware Stack

The ATtiny28 uses a 3-level-deep hardware stack for subroutines and interrupts. The hardware stack is 10 bits wide and stores the program counter (PC) return address while subroutines and interrupts are executed.

RCALL instructions and interrupts push the PC return address onto stack level 0, and the data in the other stack levels 1 - 2 are pushed one level deeper in the stack. When a RET or RETI instruction is executed the returning PC is fetched from stack level 0, and the data in the other stack levels 1 - 2 are popped one level in the stack.

If more than three subsequent subroutine calls or interrupts are executed, the first values written to the stack are overwritten.

Memory Access and Instruction Execution Timing

This section describes the general access timing concepts for instruction execution and internal memory access.

The AVR CPU is driven by the System Clock, directly generated from the external clock crystal for the chip. No internal clock division is used.

Figure 13 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-access register file concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per clocks and functions per power unit.

Figure 13. The Parallel Instruction Fetches and Instruction Executions

T1 T2 T3 T4

System Clock Ø

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

Figure 14 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 14. Single Cycle ALU Operation

T1 T2 T3 T4

System Clock Ø

Total Execution Time

ALU Operation Execute

Result Write Back

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I/O Memory The I/O space definition of the ATtiny28 is shown in Table 2.

Table 2. ATtiny28 I/O Space

Address Hex Name Function

$3F SREG Status Register

$1B PORTA Data Register, Port A

$1A PACR Port A Control Register

$19 PINA Input Pins, Port A

$16 PINB Input Pins, Port B

$12 PORTD Data Register, Port D

$11 DDRD Data Direction Register, Port D

$10 PIND Input Pins, Port D

$08 ACSR Analog Comparator Control and Status Register

$07 MCUCS MCU Control and Status Register

$06 ICR Interrupt Control Register

$05 IFR Interrupt Flag Register

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$04 TCCR0 Timer/Counter0 Control Register

$03 TCNT0 Timer/Counter0 (8-bit)

$02 MODCR Modulation Control Register

$01 WDTCR Watchdog Timer Control Register

$00 OSCCAL Oscillator Calibration Register

Note: Reserved and unused locations are not shown in the table.

All ATtiny28 I/O and peripherals are placed in the I/O space. The I/O locations are accessed by the IN and OUT instructions transferring data between the 32 general-purpose working registers and the I/O space. I/O registers within the address range $00 $1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the Instruction Set section for more details.

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

The I/O and peripherals control registers are explained in the following sections.

Status Register – SREG The AVR status register (SREG) at I/O space location $3F is defined as:

Bit 76543210

$3F 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

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• Bit 7 – I: Global Interrupt Enable

The global interrupt enable bit must be set (one) 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 (zero), 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.

• Bit 6 – T: Bit Copy Storage

The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source and 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. See the Instruction Set description for detailed information.

• Bit 4 – S: Sign Bit, S = N

The S-bit is always an exclusive or between the negative flag N and the two’s complement overflow flag V. See the Instruction Set description for detailed information.

• Bit 3 – V: Two’s Complement Overflow Flag

The two’s complement overflow flag V supports two’s complement arithmetic. See the Instruction Set description for detailed information.

• Bit 2 – N: Negative Flag

The negative flag N indicates a negative result from an arithmetical or logical operation. See the Instruction Set description for detailed information.

• Bit 1 – Z: Zero Flag

⊕ V

Reset and Interrupt Handling

The zero flag Z indicates a zero result from an arithmetical or logical operation. See the Instruction Set description for detailed information.

• Bit 0 – C: Carry Flag

The carry flag C indicates a carry in an arithmetical or logical operation. See the Instruction Set description for detailed information.

Note that the status register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt routine. This must be handled by software.

The ATtiny28 provides five different interrupt sources. These interrupts and the reset vector each have a separate program vector in the program memory space. All the interrupts are assigned to individual enable bits. In order to enable the interrupt, both the individual enable bit and the I-bit in the status register (SREG) must be set to one.

The lowest addresses in the program memory space are automatically defined as the Reset and Interrupt vectors. The complete list of vectors is shown in Table 3. The list also determines the priority levels of the different interrupts. The lower the address, the higher the priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request 0.

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Table 3. Reset and Interrupt Vectors

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Vector

No.

1 $000 RESET

2 $001 INT0 External Interrupt Request 0

3 $002 INT1 External Interrupt Request 1

4 $003 Input Pins Low-level Input on Port B

5 $004

6 $005 ANA_COMP Analog Comparator

Program Address Source Interrupt Definition

Hardware Pin, Power-on Reset and Watchdog Reset

TIMER0, OVF0

Timer/Counter0 Overflow

The most typical and general program setup for the Reset and Interrupt vector addresses are:

Address Labels Code Comments

$000 rjmp RESET ; Reset handler

$001 rjmp EXT_INT0 ; IRQ0 handler

$002 rjmp EXT_INT1 ; IRQ1 handler

$003 rjmp LOW_LEVEL ; Low level input handler

$004 rjmp TIM0_OVF ; Timer0 overflow handle

$005 rjmp ANA_COMP ; Analog Comparator handle

;

$006 MAIN: <instr> xxx ; Main program start

…… ……

Reset Sources The ATtiny28 provides three sources of reset:

• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold (V

POT

• External Reset. The MCU is reset when a low level is present on the RESET pin for more than 50 ns.

• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is enabled.

During reset, all I/O registers are then set to their initial values and the program starts execution from address $000. The instruction placed in address $000 must be an RJMP (relative jump) instruction to the reset handling routine. If the program never enables an interrupt source, the interrupt vectors are not used, and regular program code can be placed at these locations. The circuit diagram in Figure 15 shows the reset logic. Table 4 defines the timing and electrical parameters of the reset circuitry.

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Figure 15. Reset Logic

DATA BUS

MCU Control and Status

PORF

WDRF

EXTRF

VCC

100 - 500K

RESET

Power-on

Reset Circuit

SRQ

Watchdog

Timer

CKSEL[3..0]

COUNTER RESET

On-chip

RC Oscillator

Delay Counters

Full

Table 4. Reset Characteristics

Symbol Parameter Min Typ Max Unit

(1)

POT

RST

Power-on Reset Threshold Voltage (rising) 1.0 1.4 1.8 V

Power-on Reset Threshold Voltage (falling) 0.4 0.6 0.8 V

RESET Pin Threshold Voltage 0.6 V

Note: 1. The Power-on Reset will not work unless the supply voltage has been below V

(falling).

INTERNAL RESET

POT

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Table 5. ATtiny28 Clock Options and Start-up Time

CKSEL3..0 Clock Source Start-up Time at 2.7V

1111 External Crystal/Ceramic Resonator

1110 External Crystal/Ceramic Resonator

1101 External Crystal/Ceramic Resonator

1100 External Crystal/Ceramic Resonator 16K CK

1011 External Crystal/Ceramic Resonator 4.2 ms + 16K CK

1010 External Crystal/Ceramic Resonator 67 ms + 16K CK

1001 External Low-frequency Crystal 67 ms + 1K CK

1000 External Low-frequency Crystal 67 ms + 32K CK

0111 External RC Oscillator 6 CK

0110 External RC Oscillator 4.2 ms + 6 CK

0101 External RC Oscillator 67 ms + 6 CK

(1)

1K CK

4.2 ms + 1K CK

67 ms + 1K CK

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Table 5. ATtiny28 Clock Options and Start-up Time (Continued)

CKSEL3..0 Clock Source Start-up Time at 2.7V

0100 Internal RC Oscillator 6 CK

0011 Internal RC Oscillator 4.2 ms + 6 CK

0010 Internal RC Oscillator 67 ms + 6 CK

0001 External Clock 6 CK

0000 External Clock 4.2 ms + 6 CK

Note: 1. Due to limited number of clock cycles in the start-up period, it is recommended that

ceramic resonator be used.

This table shows the start-up times from reset. From Power-down mode, only the clock counting part of the start-up time is used. The Watchdog oscillator is used for timing the real-time part of the start-up time. The number WDT oscillator cycles used for each time-out is shown in Table 6.

Table 6. Number of Watchdog Oscillator Cycles

Time-out Number of Cycles

4.2 ms 1K

67 ms 16K

The frequency of the Watchdog oscillator is voltage-dependent, as shown in the section “Typical Characteristics” on page 55.

The device is shipped with CKSEL = 0010.

Power-on Reset A Power-on Reset (POR) pulse is generated by an on-chip detection circuit. The detec-

tion level is nominally 1.4V. The POR is activated whenever V

is below the detection

level. The POR circuit can be used to trigger the start-up reset, as well as detect a failure in supply voltage.

The Power-on Reset (POR) circuit ensures that the device is reset from power-on. Reaching the Power-on Reset threshold voltage invokes a delay counter, which determines the delay for which the device is kept in RESET after V

rise. The time-out

period of the delay counter can be defined by the user through the CKSEL fuses. The different selections for the delay period are presented in Table 5. The RESET signal is activated again, without any delay, when the V

decreases below detection level. See

Figure 16.

If the built-in start-up delay is sufficient, RESET an external pull-up resistor. By holding the RESET

can be connected to VCC directly or via

pin low for a period after VCC has been applied, the Power-on Reset period can be extended. Refer to Figure 17 for a timing example of this.

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Figure 16. MCU Start-up, RESET Tied to VCC.

VCC

RESET

TIME-OUT

INTERNAL

RESET

POT

RST

TOUT

Figure 17. MCU Start-up, RESET

VCC

RESET

TIME-OUT

INTERNAL

RESET

POT

Controlled Externally

RST

TOUT

External Reset An external reset is generated by a low level on the RESET

than 50 ns will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset. When the applied voltage reaches the Reset Threshold Voltage (V period (t

) on its positive edge, the delay timer starts the MCU after the Time-out

RST

) has expired.

TOUT

Figure 18. External Reset during Operation

VCC

RESET

RST

pin. Reset pulses longer

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TIME-OUT

INTERNAL

RESET

TOUT

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Watchdog Reset When the Watchdog times out, it will generate a short reset pulse of 1 XTAL cycle dura-

tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period

). Refer to page 26 for details on operation of the Watchdog.

TOUT

Figure 19. Watchdog Reset during Operation

MCU Control and Status Register – MCUCS

The MCU Control and Status Register contains control and status bits for general MCU functions.

Bit 76543210

$07 PLUPB – SE SM WDRF – EXTRF PORF MCUCS

Read/Write R/W R R/W R/W R/W R R/W R/W

Initial Value 0 0 0 0 See Bit

Desc.

0 See Bit Description

• Bit 7 – PLUPB: Pull-up Enable Port B

When the PLUPB bit is set (one), pull-up resistors are enabled on all Port B input pins. When PLUPB is cleared, the pull-ups are disabled. If any of the special functions of Port B is enabled, the corresponding pull-up(s) is disabled, independent of the value of PLUPB.

• Bit 6 – Res: Reserved Bit

This bit is a reserved bit in the ATtiny28 and always reads as zero.

• Bit 5 – SE: Sleep Enable

The SE bit must be set (one) to make the MCU enter the sleep mode when the SLEEP instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended to set the Sleep Enable SE bit just before the execution of the SLEEP instruction.

• Bit 4 – SM: Sleep Mode

This bit selects between the two available sleep modes. When SM is cleared (zero), Idle Mode is selected as sleep mode. When SM is set (one), Power-down mode is selected as sleep mode. For details, refer to “Sleep Modes” below.

• Bit 3 – WDRF: Watchdog Reset Flag

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This bit is set if a Watchdog reset occurs. The bit is cleared by a Power-on Reset, or by writing a logical “0” to the flag.

• Bit 2 – Res: Reserved Bit

This bit is a reserved bit in the ATtiny28 and always reads as zero.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an external reset occurs. The bit is cleared by a Power-on Reset, or by writing a logical “0” to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is cleared by writing a logical “0” to the flag.

To make use of the reset flags to identify a reset condition, the user should read and then clear the flag bits in MCUCS as early as possible in the program. If the register is cleared before another reset occurs, the source of the reset can be found by examining the reset flags.

Interrupt Handling The ATtiny28 has one 8-bit Interrupt Control Register (ICR).

When an interrupt occurs, the Global Interrupt Enable I-bit is cleared (zero) and all interrupts are disabled. The user software can set (one) the I-bit to enable nested interrupts. The I-bit is set (one) when a Return from Interrupt instruction (RETI) is executed.

When the program counter is vectored to the actual interrupt vector in order to execute the interrupt handling routine, hardware clears the corresponding flag that generated the interrupt. Some of the interrupt flags can also be cleared by writing a logical “1” to the flag bit position(s) to be cleared.

If an interrupt condition occurs when the corresponding interrupt enable bit is cleared (zero), the interrupt flag will be set and remembered until the interrupt is enabled or the flag is cleared by software.

If one or more interrupt conditions occur when the global interrupt enable bit is cleared (zero), the corresponding interrupt flag(s) will be set and remembered until the global interrupt enable bit is set (one), and will be executed by order of priority.

Note that external level interrupt does not have a flag and will only be remembered for as long as the interrupt condition is active.

Note that the status register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt routine. This must be handled by software.

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 4-clock-cycle period, the program counter (10 bits) is pushed onto the stack. The vector is normally a relative jump to the interrupt routine, and this jump takes two 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.

A return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the program counter (10 bits) is popped back from the stack, and the I-flag in SREG is set. When AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served.

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Interrupt Control Register – ICR

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Bit 76543210

$06 INT1 INT0 LLIE TOIE0 ISC11 ISC10 ISC01 ISC00 ICR

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 – INT1: External Interrupt Request 1 Enable

When the INT1 bit is set (one) and I-bit in the Status Register (SREG) is set (one), the external pin interrupt 1 is enabled. The interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) define whether the external interrupt is activated on rising or falling edge, on pin change or low level of the INT1 pin. The corresponding interrupt of External Interrupt Request 1 is executed from program memory address $002. See also “External Interrupt”.

• Bit 6 – INT0: External Interrupt Request 0 Enable

When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt 0 is enabled. The interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) define whether the external interrupt is activated on rising or falling edge, on pin change or low level of the INT0 pin. The corresponding interrupt of External Interrupt Request 0 is executed from program memory address $001. See also “External Interrupt”.

• Bit 5 – LLIE: Low-level Input Interrupt Enable

When the LLIE is set (one) and the I-bit in the status register (SREG) is set (one), the interrupt on low-level input is activated. Any of the Port B pins pulled low will then cause an interrupt. However, if any Port B pins are used for other special features, these pins will not trigger the interrupt. The corresponding interrupt of Low-level Input Interrupt Request is executed from program memory address $003. See also “Low-level Input Interrupt”.

• Bit 4 – TOIE0: Timer/Counter0 Overflow Interrupt Enable

When the TOIE0 bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter0 Overflow Interrupt is enabled. The corresponding interrupt (at vector $004) is executed if an overflow in Timer/Counter0 occurs, i.e., when the TOV0 bit is set in the Interrupt Flag Register (IFR).

• Bits 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0

The External Interrupt 1 is activated by the external pin INT1 if the SREG I-flag and the corresponding interrupt enable are set. The level and edges on the external INT1 pin that activate the interrupt are defined in Table 7.

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Table 7. Interrupt 1 Sense Control

ISC11 ISC10 Description

0 0 The low level of INT1 generates an interrupt request.

0 1 Any change on INT1 generates an interrupt request.

1 0 The falling edge of INT1 generates an interrupt request.

1 1 The rising edge of INT1 generates an interrupt request.

Note: When changing the ISC11/ISC10 bits, INT1 must be disabled by clearing its Interrupt

Enable bit. Otherwise, an interrupt can occur when the bits are changed.

• Bits 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0

The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding interrupt enable are set. The level and edges on the external INT0 pin that activate the interrupt are defined in Table 8.

Table 8. Interrupt 0 Sense Control

ISC01 ISC00 Description

0 0 The low level of INT0 generates an interrupt request.

0 1 Any change on INT0 generates an interrupt request.

1 0 The falling edge of INT0 generates an interrupt request.

1 1 The rising edge of INT0 generates an interrupt request.

Note: When changing the ISC01/ISC00 bits, INT0 must be disabled by clearing its Interrupt

Enable bit. Otherwise, an interrupt can occur when the bits are changed.

Interrupt Flag Register – IFR

The value on the INT pins are sampled before detecting edges. If edge interrupt is selected, pulses that last longer than one CPU clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low-level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. If enabled, a level-triggered interrupt will generate an interrupt request as long as the pin is held low.

Bit 76543210

$05 INTF1 INTF0 – TOV0 ––––IFR

Read/Write R/W R/W R R/W R R R R

Initial Value 0 0 0 0 0 0 0 0

• Bit 7 – INTF1: External Interrupt Flag1

When an edge on the INT1 pin triggers an interrupt request, the corresponding interrupt flag, INTF1 becomes set (one). If the I-bit in SREG and the corresponding interrupt enable bit, INT1 in GIMSK is set (one), the MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical “1” to it. This flag is always cleared when INT1 is configured as level interrupt.

• Bit 6 – INTF0: External Interrupt Flag0

When an edge on the INT0 pin triggers an interrupt request, the corresponding interrupt flag, INTF0 becomes set (one). If the I-bit in SREG and the corresponding interrupt enable bit, INT0 in GIMSK is set (one), the MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be

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cleared by writing a logical “1” to it. This flag is always cleared when INT0 is configured as level interrupt.

• Bit 5 – Res: Reserved Bit

This bit is a reserved bit in the ATtiny28 and always reads as zero.

• Bit 4 – TOV0: Timer/Counter0 Overflow Flag

The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when executing the corresponding interrupt handling vector. TOV0 is cleared by writing a logical “1” to the flag. When the SREG I-bit, TOIE0 in ICR and TOV0 are set (one), the Timer/Counter0 Overflow interrupt is executed.

• Bit 3..0 - Res: Reserved Bits

These bits are reserved bits in the ATtiny28 and always read as zero.

Note: 1. One should not try to use the SBI (Set Bit in I/O Register) instruction to clear individ-

ual flags in the Register. This will result in clearing all the flags in the register, because the register is first read, then modified and finally written, thus writing ones to all set flags. Using the CBI (Clear Bit in I/O Register) instruction on IFR will result in clearing all bits apart from the specified bit.

External Interrupt The external interrupt is triggered by the INT pins. Observe that, if enabled, the interrupt

will trigger even if the INT pin is configured as an output. This feature provides a way of generating a software interrupt. The external interrupt can be triggered by a falling or rising edge, a pin change or a low level. This is set up as indicated in the specification for the Interrupt Control Register (ICR). When the external interrupt is enabled and is configured as level-triggered, the interrupt will trigger as long as the pin is held low.

The external interrupt is set up as described in the specification for the Interrupt Control Register (ICR).

Low-level Input Interrupt The low-level interrupt is triggered by setting any of the Port B pins low. However, if any

Port B pins are used for other special features, these pins will not trigger the interrupt. For example, if the analog comparator is enabled, a low level on PB0 or PB1 will not cause an interrupt. This is also the case for the special functions T0, INT0 and INT1. If low-level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. When this interrupt is enabled, the interrupt will trigger as long as any of the Port B pins are held low.

Sleep Modes To enter the sleep modes, the SE bit in MCUCS must be set (one) and a SLEEP instruc-

tion must be executed. The SM bit in the MCUCS register selects which sleep mode (Idle or Power-down) will be activated by the SLEEP instruction. If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU awakes. The CPU is then halted for four cycles. It executes the interrupt routine and resumes execution from the instruction following SLEEP. The contents of the register file and I/O memory are unaltered. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset vector.

Idle Mode When the SM bit is cleared (zero), the SLEEP instruction forces the MCU into the Idle

Mode, stopping the CPU but allowing Timer/Counters, Watchdog and the interrupt system to continue operating. This enables the MCU to wake up from external triggered interrupts as well as internal ones like Timer Overflow interrupt and Watchdog reset. If wake-up from the Analog Comparator Interrupt is not required, the analog comparator can be powered down by setting the ACD bit in the Analog Comparator Control and Status register (ACSR). This will reduce power consumption in Idle Mode. Note that the ACD bit is set by default.

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Power-down Mode When the SM bit is set (one), the SLEEP instruction forces the MCU into the Power-

down mode. In this mode, the external oscillator is stopped, while the external interrupts and the Watchdog (if enabled) continue operating. Only an external reset, a Watchdog reset (if enabled), or an external level interrupt can wake up the MCU.

Note that if a level-triggered interrupt is used for wake-up from Power-down mode, the changed level must be held for some time to wake up the MCU. This makes the MCU less sensitive to noise. The wake-up period is equal to the clock-counting part of the reset period (see Table 5). The MCU will wake up from power-down if the input has the required level for two Watchdog oscillator cycles. If the wake-up period is shorter than two Watchdog oscillator cycles, the MCU will wake up if the input has the required level for the duration of the wake-up period. If the wake-up condition disappears before the wake-up period has expired, the MCU will wake up from power-down without executing the corresponding interrupt. The period of the Watchdog oscillator is 2.7 µs (nominal) at

3.0V and 25 shown in the section “Typical Characteristics” on page 55.

When waking up from the Power-down mode, there is a delay from the wake-up condition until the wake-up becomes effective. This allows the clock to restart and become stable after having been stopped.

°C. The frequency of the watchdog oscillator is voltage-dependent as

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Timer/Counter0 The ATtiny28 provides one general-purpose 8-bit Timer/Counter – Timer/Counter0.

Timer/Counter0 has prescaling selection from the 10-bit prescaling timer. The Timer/Counter0 can either be used as a timer with an internal clock time base or as a counter with an external pin connection that triggers the counting.

Timer/Counter Prescaler Figure 20 shows the Timer/Counter prescaler.

Figure 20. Timer/Counter0 Prescaler

COUNT ENABLE

FROM MODULATOR

CS00 CS01 CS02

10-BIT T/C PRESCALER

CK/64

TIMER/COUNTER0 CLOCK SOURCE

TCK0

CK/256

CK/1024

The four different prescaled selections are: the hardware modulator period, CK/64, CK/256 and CK/1028, where CK is the oscillator clock. CK, external source and stop can also be selected as clock sources.

Figure 21 shows the block diagram for Timer/Counter0.

Figure 21. Timer/Counter0 Block Diagram

T/C0 OVER-

FLOW IRQ

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INTERRUPT CONTROL

TOIE0

TIMER/COUNTER0

(TCNT0)

TOV0

INTERRUPT FLAG

TOV0

T/C0 CONTROL

CONTROL

LOGIC

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