[ECOS] ecos idle function--A correction and clarification.

Andrew Lunn andrew.lunn@ascom.ch
Tue Jan 21 10:15:00 GMT 2003


>     But as a sequel, I would like to know why
> sometimes, the idle_loops can turn out to be more than
> the calibration. in other words, when we calibrate, we
> only allow the idle_thread_main function to run with
> the highest priority. Hence whatever value is recorded
> for those 100ms by idle_thread_loops should be the
> maximum possible. But when the code runs, there are
> instances when the idle_thread_loops has a higher
> value than the previously calibrated value. You also
> have a provision in your code to check for this
> condition. I am curious as to why this condition could
> crop up.

One reason i can think of is that the 100ms is not 100ms.

cyg_alarm_initialize(alarmH,cyg_current_time()+10,0);
cyg_alarm_enable(alarmH);

This creates an alarm part way through a tick. So the actual alarm
will be 100m + remainder of a tick. This could in fact be 109.9999 ms
etc. Thus the error is in range of 10%. 

I did not think of this at the time. A simple solution would be to put
a cyg_thread_delay(1) before this. This should, in most cases, put the
initialization at the beginning of a time slice and so increase the
accuracy. 

          Andrew

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