Re: [ECOS] periodic threads with kapi?

[Bart Veer <bartv at ecoscentric dot com>]

>>>>> "Ryan" == Ryan Boder <icanoop@bitwiser.org> writes:

    Ryan> On Sun, Aug 17, 2003 at 07:43:16PM +0100, Bart Veer wrote:
    >> >>>>> "Ryan" == Ryan Boder <icanoop@bitwiser.org> writes:
    >> 
    Ryan> Is there a clean way to do periodic threads with absolute
    Ryan> timing, as opposed to the relative timing you get with
    Ryan> cyg_thread_delay()?
    >> 
    >> This should be easy enough using something like:
    >> 
    >> void
    >> my_thread_delay_absolute(cyg_tick_count_t abs_time)
    >> {
    >>     cyg_tick_count_t delay = abs_time - cyg_current_time();
    >>     if (delay > 0) {
    >>         cyg_thread_delay(delay);
    >>     }
    >> }
    >> 
    >> There is a possible problem if the current thread gets descheduled
    >> between the cyg_current_time() and cyg_thread_delay() calls, causing
    >> the thread to get delayed for longer than intended. You could avoid
    >> that by e.g. temporarily boosting the current thread's priority.

    Ryan> The problem still exists doing it that way. In fact that
    Ryan> would even make it worse.

    Ryan> The point is that I don't want to read the current time each
    Ryan> period in my thread. That causes drift.

There is no drift. The calling code should calculate the new value of
abs_time by adding the appropriate number of ticks to the previous
value of abs_time, thus moving from one absolute time to the next. For
example adding 100 each time will give absolute intervals of 1 second
(assuming the usual 10ms clock).

If instead the calling code calculated the new value of abs_time by
adding 100 to cyg_current_time() rather than to the previous value of
abs_time then you would get drift - that would be equivalent to
calling cyg_thread_delay().

Bart

-- 
Bart Veer                       eCos Configuration Architect
http://www.ecoscentric.com/     The eCos and RedBoot experts

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