Fwd: Re: Timing of events in an ISR

Mark VanderVoord markvandervoord at yahoo.com
Tue Oct 19 16:14:43 UTC 2004

> Could the problem be your interrupt source is
> asserts its irq, which
> is inadvertently being masked somewhere such that no
> interrupt occurs,
> then the next time the timer tick comes along the
> vectoring code sees
> the asserted irq and then handles it- essentially 1
> timer tick late?

It certainly seems that way.  I ran the experiment at
a few additionaly rates (changing
MICROSECONDS_PER_TICK) and came up with similar
results.  The delay is better, on the average, as the
tick time gets smaller.  Also, it is always smaller
than one tick time (which matches with your
suggestion, that the ISR is being run on the following
normal timer tick).

If the interrupt is being masked, where would that
occur?  If the hardware interrupt was being masked,
the event would never happen, correct?  So I must be
masking it in RTEMS?  Shouldn't it be disallowed from
running in this case also?

Mark S VanderVoord
Self-Guided Systems, LLC

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