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[Der Herr Hofrat <der.herr@hofr.at>]

>From owner-rtl Mon May 21 01:39:43 2001
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Date: Mon, 21 May 2001 09:37:16 +0200
From: =?iso-8859-1?Q?S=E9bastien?= GAUME <sgaume@b2i-toulouse.com>
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To: rtl@fsmlabs.com
Subject: Re: [rtl] gethrtime
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Thank you for this clue.

Sebastien

Klaas Gadeyne a ecrit :

> > oneshot mode. Periodically, the next wake up date is calculated in clock
> > units. Because the period in clock unit is not equal to the period in
> > nanoseconds, a drift should be seen, especially after a large number of
> > periods.
> >
> > In fact, after having done this test, I did not see such a drift. Do not
> > think I am disapointed, I'm not ! I would like to know wether I am wrong
> > in my assumptions or not, and if not, how the gethrtime function manages
> > not to create a drift.
>
> I don't know wether you're wrong in your assumptions but a couple of weeks
> ago I was working on some steppermotor software and there there was
> obviously some drift when just using a call (for constant speed)
>
> while(1)
> {
>         pthread_make_periodic_np(pthread_self(),gethrtime()+period,0);
>         pthread_wait_np();
>         outb(...);
> }
>
> I don't know wether the drift is due to the rounding process, but the
> jitter alone is responsible for drift.
>
> To avoid drifting errors you can use "absolute timing", ie something like
>
> next_fire_time = somewhere in the future;
> while(1)
> {
>         pthread_make_periodic_np(pthread_self(),next_fire_time,0);
>         pthread_wait_np();
>         outb(...);
>         next_fire_time += period;
> }
>
> Hope this was somewhat an answer to your question,
>
> regards
>
> klaas
>
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