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Date:      Fri, 9 Jul 1999 21:34:38 +1000
From:      Bruce Evans <bde@zeta.org.au>
To:        dfr@nlsystems.com, julian@whistle.com
Cc:        freebsd-current@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG, tanimura@naklab.dnj.ynu.ac.jp
Subject:   Re: Rewriting pca(4) using finetimer(9) (was: Re: MPU401 now works under New Midi Driver Framework with a Fine Timer)
Message-ID:  <199907091134.VAA14655@godzilla.zeta.org.au>

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>this is problematic.
>
>you cannot add a new element before the pending firing because you can't
>tell how far into the present trigger you are.

This is not a problem for readable counters like the i8254.  The problem
for the i8254 is that reading and writing it takes a long time (perhaps
5 usec each).  Every write to it will decrease its accuracy (perhaps by 1
usec after careful calibration).  Thus if the i8254 is used for generating
non-periodic interrupts with an average period of 333 usec, it may take
15/333 of the CPU (estimate another 5 usec for interrupt handling), and if
it is also used for timecounting then it may drift by 333 usec/second.

Bruce


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