The ATtiny85 is a lovely chip for relatively simple applications. More than enough flash, 512 byte SRAM, 5 usable pins, cheap (half the cost of ATmega328 – 10 cost me £10) and most importantly a small form factor (8 pin chip).

Datasheet: http://electronics.arunsworld.com/?attachment_id=116

Google provides an Arduino core for the ATtiny85. Located here. I have it installed and use it when I need some functions like millis rather than write it myself. However, I have also setup my own core that is absolutely barebones and effectively lets me use the Arduino IDE to write & upload sketches using nothing more than AVR Libc.

All the core contains is a main.cpp that calls setup and loop. It also includes one header file: WProgram.h and I have an Arduino.h that is simply including WProgram.

Below is a BareMinimum sketch that doesn’t do anything but can be used as a template to start a sketch. It includes power saving features that can be tweaked based on what’s needed in the sketch. The sketch compiles to only 150 bytes.

#include <avr/wdt.h>
#include <avr/sleep.h>

void setup() {
  // Turn off unnecessary peripherals

  MCUSR = 0;
  wdt_disable(); // watch dog timer
  ACSR |= _BV(ACD); // Disable analog comparator
  ADCSRA &= ~_BV(ADEN); // Ensure ADC is indeed disabled
  PRR |= _BV(PRTIM0) | _BV(PRTIM1) | _BV(PRUSI) | _BV(PRADC); // shut down timers, USI, ADC
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);

  // do something - enable interrupts, whatever
}

void loop() {
  cli();
  sleep_enable();
  sei();
  sleep_cpu();
  sleep_disable();
}

Useful registers on the ATtiny85:

Register	Bits	Purpose
PRR	PRTIM1 PRTIM0 PRUSI PRADC	Writing a logic 1 shuts down power to Timer1, Timer0, USI & ADC. Note: Google core uses Timer1 to implement delay, millis, micros etc.
MCUCR	ICS01 ICS00	Controls the logic to drive the external interrupt 0 (INT0)
GIMSK	INT0 PCIE	INT0: Enables interrupt on INT0 (by writing 1) PCIE: Enables pin change interrupt (PCINT[5:0]). Before setting PCIE though remember to set PCMSK.
PCMSK	PCINT5 PCINT4 … PCINT0	Selects whether pin change interrupt enabled on that pin.
PORTB	PORTB5 PORTB4 … PORTB0	Set PORT to 0 or 1 to control an output pin. Set 1 on an input pin to enable pull-up.
DDRB	DDB5 DDB4 … DDB0	Set 1 to make a pin an output pin. 0 for input.
PINB	PINB5 PINB4 … PINB0	Use to read value of pin. Write 1 to toggle the value, especially if it’s an output pin.
TCCR0A	WGM01 WGM00	WGM00, WGM01 and WGM02 control the timer mode. eg. Normal, CTC, PWM Normal implies it counts up to counter overflow. CTC: clear timer on compare match.
TCCR0B	WGM02 CS02 CS01 CS00	CS: clock select – sets the pre-scaler No prescaling, 8, 64, 256 & 1024.
OCR0A		Value: Output compare register A.
TCNT0		Value: Timer/Counter register.
TIMSK	OCIE0A OCIE0B TOIE0	OCIE: Output compare match interrupt enable. (write 1). TOIE0: overflow interrupt enable (write 1)
ADCSRB	ACME	Write ACME bit to logic 0 to apply AIN1 as negative input to analog comparator. Write logic 1 with ADC off to use ADC multiplexer to comparator
ACSR	ACD ACBG ACIE ACIS[1:0]	ACD: analog comparator disable (write 1 to disable) ACBG: use band gap voltage as positive input. If cleared use AIN0. ACIE: Interrupt enable (write 1). ACIS: Logic to determine whether interrupt based on toggle, falling or rising edge.
ADMUX	REFS[2:0] ADLAR	REFS: Voltage ref selection. Vcc, AREF (PB0), 1.1V, 2.56V ADLAR: ADC Left Adjust Result.
ADCSRA	ADEN ADSC ADATE ADIE ADPS[2:0]	ADEN: ADC Enable (write 1 to enable or 0 to disable). ADSC: start conversion. In free running, this starts the conversion. ADATE: auto-trigger enable. Trigger set by ADTS in ADDSRB. ADIE: Interrupt enable. ADPS: pre-scaler
ADCSRB	ADTS[2:0]	Choice of: Free Running Mode Analog Comparator Ext. interrupt 0, Timer/Counter CTC or overflow, Pin Change request.
ADCL, ADCH		Value: depending on ADLAR either left or right justified 10 bits.
USIDR		Value of USI data. Copy also available at USIBR.
USIBR		Loaded here when transfer completed.
USICR	USISIE USIOIE USIWM[1:0] USICS[1:0] USICLK USITC	USISIE: enables interrupt on start condition (write 1). USIOIE: counter overflow interrupt enable. 4 bit counter USIWM: wire mode selection: 3-wire mode, 2-wire mode. USITC: toggle clock port pin. USICLK: clock strobe.

List of interrupts available:

Interrupt	description
INT0_vect	External Interrupt 0
PCINT0_vect	Pin change Interrupt Request 0
TIMER1_COMPA_vect	Timer/Counter1 Compare Match 1A
TIMER1_OVF_vect	Timer/Counter1 Overflow
TIMER0_OVF_vect	Timer/Counter0 Overflow
ANA_COMP_vect	Analog comparator
ADC_vect	ADC Conversion ready
TIMER1_COMPB_vect	Timer/Counter1 Compare Match B
TIMER0_COMPA_vect	Timer/Counter0 Compare Match A
TIMER0_COMPB_vect	Timer/Counter0 Compare Match B
USI_START_vect	USI START
USI_OVF_vect	USI Overflow

Defining a board for the ATtiny85 so it shows up on the boards list:

Setting up a board helps set the fuses (via Burn Bootloader) and also in calling the appropriate core and then uploading to chip.

Here are my settings in boards.txt under hardware/tiny/

attiny85at8.name=ATtiny85 @ 8 MHz
attiny85at8.upload.using=arduino:arduinoisp
attiny85at8.upload.maximum_size=8192

attiny85at8.bootloader.low_fuses=0xE2
attiny85at8.bootloader.high_fuses=0xDE
attiny85at8.bootloader.extended_fuses=0xFF
attiny85at8.bootloader.path=empty
attiny85at8.bootloader.file=empty

attiny85at8.build.mcu=attiny85
attiny85at8.build.f_cpu=8000000L
attiny85at8.build.core=tiny

################################################

attiny85at8ab.name=ATtiny85 @ 8 MHz w/ ABTiny Core
attiny85at8ab.upload.using=arduino:arduinoisp
attiny85at8ab.upload.maximum_size=8192

attiny85at8ab.bootloader.low_fuses=0xE2
attiny85at8ab.bootloader.high_fuses=0xDE
attiny85at8ab.bootloader.extended_fuses=0xFF
attiny85at8ab.bootloader.path=empty
attiny85at8ab.bootloader.file=empty

attiny85at8ab.build.mcu=attiny85
attiny85at8ab.build.f_cpu=8000000L
attiny85at8ab.build.core=abtiny

Installation of cores and boards

This can be done in the folder that has the sketches (eg. ~/Documents/Arduino/) under a folder called hardware.

Folder structure:

• hardware/tiny/ <– root folder for the files for ATTiny85
• boards.txt (under root folder above)
• cores/ <– under root folder it contains the cores
• cores/tiny <– contains the google core
• cores/abtiny <– contains my core (main.cpp, WProgram.h etc.)