AntiGuide: AtTiny85Sleep

// selon https://enavarro.me/exploration-des-sleep-mode-du-attiny85.html
#include <avr/io.h>
#include <avr/wdt.h> 
#include <avr/sleep.h>

/* Turn off WDT */
void watchdog_stop() {
  WDTCR |= _BV(WDCE) | _BV(WDE);
  WDTCR = 0x00;
}


volatile uint8_t wdt_count = 0;

void watchdog_start_interrupt(uint8_t wd_prescaler) {
  if(wd_prescaler > 9) wd_prescaler = 9;
  byte _prescaler = wd_prescaler & 0x7;
  if (wd_prescaler > 7 ) _prescaler |= _BV(WDP3); 
  // ^ fourth bit of the prescaler is somewhere else in the register...
  // set new watchdog timer prescaler value
  WDTCR = _prescaler;
  // start timed sequence (and activate interrupt)
  WDTCR |= _BV(WDIE) | _BV(WDCE) | _BV(WDE);
}

// Watchdog Interrupt Service / is executed when watchdog timed out
ISR(WDT_vect) {
  wdt_count++;
  WDTCR |= _BV(WDIE);          // next watchdog will go to interrupt (not reset)
}


// Incremented by watchdog interrupt
// volatile uint8_t wdt_count;

int led = 1;

void blink(uint8_t flash){
  bool state = HIGH;
  for(int i = 0; i<2*flash; i++){
    digitalWrite(led, state);
    state = !state;
    delay(120);
  }
}

void setup() {
  ADCSRA &= ~_BV(ADEN);  // switch ADC OFF
  ACSR  |= _BV(ACD);     // switch Analog Compartaror OFF
  // Configure attiny85 sleep mode
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  // Reset watchdog interrupt counter
  wdt_count = 255; //max value
  // Configure pin modes
  pinMode(led, OUTPUT);
}


void loop() {
  blink(3);
  wdt_count = 0;
  watchdog_start_interrupt(6);      // prescale of 6 ~= 1sec
  while(wdt_count < 10){         // Wait 10 watchdog interupts (~10secs)
    sleep_mode();                   // Make CPU sleep until next WDT interrupt
  }
  watchdog_stop();
}