Re: [avr-gcc-list] Problem with timing

[Anton Erasmus]

On 2004/02/05 at 07:19 Juraj wrote:

>Hi Keith, and all!
>
>ms_timer is once set to 0 in definition in timer.c:
>volatile uint32_t ms_timer = 0;
>
>and is incremented by one every 1 milisecond in interupt.
>The main idea was:
>variable ms_timer is 32bit long unsigned, so that max. value is
>4.29497e+09.
>4.29497e+09 miliseconds is about 49 days, and because of it, this
>variable never overruns in my application.
>
>Variable ms_timer is used just as real time running in background, and
>its value is used when needed.
>
>I'll try to use compare match, as wrote Andras Nagy.
>
>Thanks all..
>
>Juraj.
>
>
>
>On Tue, 2004-02-03 at 21:18, Keith Gudger wrote:
>> Where is ms_timer set and reset to 0?  Shouldn't your code be:
>>
>>      while( ms_timer < koniec );
>>         ms_timer = 0 ;
>> }
>>
>> Keith
>>
>>
>> On Tue, 3 Feb 2004, Juraj wrote:
>>
>> > Hi all..
>> >
>> > Perhaps, it is trivial problem, but I cannot find the bug.
>> > Please help.
>> >
>> > I have LED, connected to AVR. I want to get blinking LED, with 1:1
>> > alternation:
>> > LED_ON - 500ms delay - LED_OFF - 500ms delay - LED_ON ...
>> >
>> > Problem is - times are not always 500ms, some of periodes are shorter.
>> >
>> >
>> > Thank you very much.
>> >
>> > Juraj.
>> >

[Code Snipped]

You must protect the access to the multibyte timer variable.

void ms_delay(uint16_t ms )
 {
   uint32_t koniec,tmp;

ATOMIC_START();
   koniec = ms_timer;
ATOMIC_END();
   koniec+=(uint32_t)ms;

  do
  {
    ATOMIC_START();
     tmp=ms_timer;
    ATOMIC_END();
  }
   while( tmp < koniec );
 }

The simple implimentation for ATOMIC_START and ATOMIC_END is
disable interrupts in ATOMIC_START and enable interrupts in ATOMIC_END.
For some applications you might have to only disable/enable the actual timer 
interrupt
used for the 1ms interrupt.

Regards
   Anton Erasmus