From owner-freebsd-current Fri Jul 9 4:34:53 1999 Delivered-To: freebsd-current@freebsd.org Received: from godzilla.zeta.org.au (godzilla.zeta.org.au [203.26.10.9]) by hub.freebsd.org (Postfix) with ESMTP id 96293154DF; Fri, 9 Jul 1999 04:34:48 -0700 (PDT) (envelope-from bde@godzilla.zeta.org.au) Received: (from bde@localhost) by godzilla.zeta.org.au (8.8.7/8.8.7) id VAA14655; Fri, 9 Jul 1999 21:34:38 +1000 Date: Fri, 9 Jul 1999 21:34:38 +1000 From: Bruce Evans Message-Id: <199907091134.VAA14655@godzilla.zeta.org.au> To: dfr@nlsystems.com, julian@whistle.com Subject: Re: Rewriting pca(4) using finetimer(9) (was: Re: MPU401 now works under New Midi Driver Framework with a Fine Timer) Cc: freebsd-current@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG, tanimura@naklab.dnj.ynu.ac.jp Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG >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-current" in the body of the message