I am trying to put my controller in sleep mode and waking it up on a certain number of timer overflow. The code is as below:

//*******TIMER 2 INITILIZATION  will be used
    TCCR2=0X07;   //clock source select, prescaler
    TIMSK=0X40;  //timer 2 interrupt selected
    TCNT2=0X00;   // timer resistor

    // controller sleep mode
    set_sleep_mode(SLEEP_MODE_PWR_SAVE); // sleep mode selected
    sleep_enable();  // set SE bit
    sleep_cpu(); // sleep mode activated
        if(count>=1000) //  min delay
            sleep_disable();   // SE bit reset
            print("waking up.....");


    } while (1);

the corresponding ISR is as follows



The problem it is not coming out of sleep mode.

I am using internal clock 8MHz and timer2. edit: i found this description in datasheet ,so this means i have to use asynchronous clock for timer 2 (i.e provided by external crystal osc).Is there any way i can do this with the help of internal clock.
power save mode description in datasheet

  • \$\begingroup\$ Why do you return(count) out of interrupt service? It is pretty meaningless. Also, is it declared as volatile? \$\endgroup\$
    – Mike Spark
    Commented Aug 19, 2016 at 9:18
  • \$\begingroup\$ i have declared count as integer type globally with initially value 0.the return function i was using just to check might be using return it will work .but i will remove it. \$\endgroup\$ Commented Aug 19, 2016 at 9:23
  • \$\begingroup\$ remove the return(count); line from ISR and add 'volatile' before 'int count' declaration. \$\endgroup\$
    – Mike Spark
    Commented Aug 19, 2016 at 9:24
  • \$\begingroup\$ Also, where do you reset the 'count' variable? \$\endgroup\$
    – Mike Spark
    Commented Aug 19, 2016 at 9:30
  • \$\begingroup\$ still doesnt work .i am not resetting the count variable.is this the reason code not working? \$\endgroup\$ Commented Aug 19, 2016 at 9:35

1 Answer 1


As you said, timer 2 can only be used with external Xtal in power save mode.

  • Either use the Idle sleep mode (you may slow down MCU frequency before entering sleep to gain consumption)
  • Or you could use the watchdog to wake you up (unsing interrupt and reset mode). Of course this will be a bit less precise but you could use power down sleep mode which is even better.
  • \$\begingroup\$ I did some research and used external xtal of 32.7khz and setting the flag bit accordingly. now it is working properly. \$\endgroup\$ Commented Sep 9, 2016 at 11:43
  • \$\begingroup\$ I was in need of the minimum possible power and power down mode is fulfilling my need <1μA. now its working thankyou \$\endgroup\$ Commented Sep 9, 2016 at 11:52

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