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Re: [avr-chat] Interrupts from assync timer in sleepmode

From: Bjarne Laursen
Subject: Re: [avr-chat] Interrupts from assync timer in sleepmode
Date: Fri, 23 Sep 2005 11:07:39 +0200
User-agent: Mozilla Thunderbird 0.7.3 (Windows/20040803)

Nils Springob wrote:

AS0 is 1, that is what make it an ansyncrone timer. The timer runs from the 32768 Hz crystal as it should.

do you write to the OCR0 or TCNT0 just before entering sleep mode?

Yes I do.
As you can see belove, I update the OCR0 to a new value every time.

The code belove will show the problem. Every 6th time the mainloop will wait until the interrupt flag have been set before entering sleep. When this result in that the output compare match is not serviced until the next time there is an ocr match. Since the OCR0 haven't been updated, it will count another 256, insteed of 32.

If the sleepmode is idle, every thing works as expected. The interrupt is serviced right after the sleep instruction.

The following code is for IAR compiler, but it should be clear enough. (We are helping a customer that use that compiler, sorry)

  #include <iom64.h>         // same behavior on mega64 and mega128
  #include <ina90.h>

unsigned int b;

#define DEBUG_PORT PORTB      //interrupt uses BIT0
                             //main loop uses BIT1

#pragma vector=TIMER0_COMP_vect
__interrupt void ISR_Timer0_Compare(void)
  OCR0 += 32;
DEBUG_PORT |= 0x01; while (ASSR&0x02); DEBUG_PORT &= ~0x01; }

void AsyncTimer_Init(void)
ASSR=0x08; // Set AS0 (use async 32kHz clk source)
  OCR0=0;                              // Set output compare register to 0
TCCR0=0x04; // TCN0 standart mode, clk=32768/64=512Hz
  while (ASSR&0x01);                   //wait for update
  TIFR=0x03;                           // Clear intr. flags
  TIMSK = 0x00;                        //clear all interrupts
  TIMSK |= 0x02;                       // Enable compare interrupt OCR0

void main( void)
DDRB = 0xFF; //PORTB set to output
  PORTA = 0x00;
  PORTB = 0x00;
  PORTC = 0x00;
  PORTD = 0x00;
  PORTE = 0x00;
  PORTF = 0x00;
  PORTG = 0x00;

  AsyncTimer_Init();            // Initialize timer0
  asm("sei");                   // enable interrupt
for( ;;)
//      MCUCR = 0x20;   //idle mode
     MCUCR = 0x38;     //power save mode

     asm("sei");          //enable intr.
     asm("cli");          //diable intr.
     MCUCR = 0x00;

DEBUG_PORT |= 0x02; if (!(b%6)) // every 6th time we will wait until the interupt is already there before sleep
        for(;!(TIFR&0x02););   //wait for interrupt flag to be set
     DEBUG_PORT &= ~0x02;

TIFF image

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