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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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