AVR sleep with PWM output

Using Arduino Microcontrollers
#1

I have a application that generates PWM to lit a LED.
to maximize the battery life, I would like the CPU to sleep instead of a busy-loop (delay()) to delay.

this is my current setup:

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

ISR(WDT_vect)
{
  wdt_disable();
  sleep_disable();
}

void sleep(uint8_t mode, uint8_t wdto)
{
  // set WDCE, allows update for 4 clock cycles
  WDTCSR |= _BV(WDCE) | _BV(WDE);
  
  // set WDT timeout and enable interrupt
  WDTCSR = (wdto << WDP0) | _BV(WDIE);

  // set the sleep mode
  set_sleep_mode(mode);
  sleep_enable();
  
  // go to sleep
  sleep_mode();
}

void setup()
{
  pinMode(13, OUTPUT);
}

uint8_t xxx = 0;
void loop()
{
  analogWrite(13, xxx);
  sleep(SLEEP_MODE_IDLE, WDTO_30MS);
  ++xxx;
}

It seems like the code is functioning correctly, at least my LEDs are changing brightness.
but does my CPU actually really sleeps?

I suspect it should work like this:

  1. Active: sets the PWM duty and WDT timeout, then go to SLEEP_MODE_IDLE.
  2. Idle: …
  3. Active: woken up by timer interrupt, update output, then goto 2.
  4. Active: woken up by watchdog interrupt, goto 1

I have no instrument to really measure the power consumption, but will my code work as what I thought?
or when the CPU is woken up by the timer ISR, it’s just gonna stay active after the ISR is served?

#2

You can check out my tiny85 PWM project that sleeps when the temperature dies not change:
http://www.hackster.io/rayburne/hot-yet

Ray

#3