Putting the ATmega168 into Power Save mode, and then waking it with a pin-change interrupt.

atmega168-power-save.c

//**********************************************************************
//
// Putting the ATmega168 into Power Save mode, and then waking it
// with a pin-change interrupt.
//
// electronut.in
//**********************************************************************

/*
  build commands on OS X with CrossPack:

  avr-gcc -Wall -Os -DF_CPU=8000000 -mmcu=atmega168 -c main.c -o main.o
  avr-gcc -Wall -Os -DF_CPU=8000000 -mmcu=atmega168 -o main.elf main.o
  rm -f main.hex
  avr-objcopy -j .text -j .data -O ihex main.elf main.hex
  avr-size --format=avr --mcu=atmega168 main.elf
  avrdude -c usbtiny  -p atmega168 -U flash:w:main.hex:i
  avrdude -c usbtiny  -p atmega168 -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m -U efuse:w:0x01:m 

 */

#include <avr/io.h>
#include <string.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#include <avr/sleep.h> 
 
#define F_CPU 8000000
 
// for sleep
volatile int goToSleep = 0;

int main (void)
{
  // Put an LED on PD4 (pin 6)
  // PD4 as output
  DDRD |= (1<<4);
  //Set high
  PORTD |= (1<<4); 
  
  //
  // 16-bit timer setup:
  //

  // turn off interrupts
  cli();

  // 16 bit timer - every 3 seconds:

  // Count cycles - 3*8000000/1024 
  OCR1A = 23437;                    
  // Put Timer/Counter1 in CTC mode
  TCCR1B |= 1<<WGM12;               
  // enable 16-bit timer interrupt
  TIMSK1 |= 1<<OCIE1A;

  // start 16-bit timer if PCINT0 pin-14 PB0 is low 
  if((!PINB & (1 << PB0))) {
    TCCR1B |= (1<<CS12) | (1<<CS10);  // Divide by 1024
  }

  // enable Pin change interrupt enable 0
  // this is PCINT0 - pin 14
  PCICR |= (1 << PCIE0);
  PCMSK0 |= (1 << PCINT0);

  // turn on interrupts
  sei();

  // loop 
  while (1) {

    // delay
    _delay_ms(100);   
    // toggle led pin on/off
    PORTD ^= (1<<4);     

    // sleep section
    if(goToSleep) {

      // turn LED pin off
      PORTD &= ~(1<<4);

      // set sleep mode
      set_sleep_mode(SLEEP_MODE_PWR_SAVE);
      // disable global interrupts
      cli();

      // enable sleep flag
      sleep_enable();

      // enable global interrupts
      sei();

      // actually sleep
      sleep_cpu();

      // ...
      // just awake here
      // ...

      // disable global interrupts
      cli();

      // enable Pin change interrupt enable 0
      // this is PCINT0 - pin 14
      PCICR |= (1 << PCIE0);
      PCMSK0 |= (1 << PCINT0);

      // unset sleep flag
      goToSleep = 0;
      // disable sleep flag
      sleep_disable();
      
      // enable global interrupts
      sei();
    }

  }
 
  return 1;
}

// pin change interrupt
ISR (PCINT0_vect)
{
  // read PCINT0 (PB0 - pn 14):
  if(PINB & (1 << PB0)) { 

    // rising edge 

    // disable sleep - wake up!
    sleep_disable();

    // stop clock for 16-bit timer
    TCCR1B &= ~((1<<CS12) | (1<<CS11) | (1<<CS10));
  }
  else{
    // falling edge

    // clear counter in 16-bit timer
    TCNT1 = 0;

    // start 16-bit timer
    TCCR1B |= (1<<CS12) | (1<<CS10);
  }
  
}

// 16-bit timer CTC handler
ISR(TIMER1_COMPA_vect)
{
  // set sleep flag
  goToSleep = 1;
}