ATtiny85 WDT sleep

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The following code puts the ATtiny85 into sleep mode and it is woken up by an overflow of the Watchdog Timer (WDT). It should be largely compatible with the ATtiny84 series, although you may get an error about WDTC(S)R. Note that disabling the ADC functions (ADC and Comparator) are probably in different registers (e.g. ADCSRA or ADCSRB).

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

// these define cbi and sbi, for as far they are not known yet
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

void setup_watchdog(int ii) 
  // 0=16ms, 1=32ms,2=64ms,3=128ms,4=250ms,5=500ms
  // 6=1 sec,7=2 sec, 8=4 sec, 9= 8sec

  uint8_t bb;
  if (ii > 9 ) ii=9;
  bb=ii & 7;
  if (ii > 7) bb|= (1<<5);
  bb|= (1<<WDCE);

  MCUSR &= ~(1<<WDRF);
  // start timed sequence
  WDTCR |= (1<<WDCE) | (1<<WDE);
  // set new watchdog timeout value
  WDTCR = bb;

// system wakes up when watchdog is timed out
void system_sleep() 
  cbi(ADCSRA,ADEN);                    // switch Analog to Digitalconverter OFF
  setup_watchdog(9);                   // approximately 8 seconds sleep
  set_sleep_mode(SLEEP_MODE_PWR_DOWN); // sleep mode is set here
  sei();                               // Enable the Interrupts so the wdt can wake us up

  sleep_mode();                        // System sleeps here

  sleep_disable();                     // System continues execution here when watchdog timed out 
  sbi(ADCSRA,ADEN);                    // switch Analog to Digitalconverter ON

int main( void ) 


  while ( 1 ) 

     <your code goes here>		
     <your code goes here>		
     <your code goes here>		
     system_sleep();  // start power down sequence

  return 0;

// Watchdog Interrupt Service / is executed when watchdog timed out
 // nothing here