Date: Sat, 3 Feb 1996 22:03:22 +1030 (CST) From: Michael Smith <msmith@atrad.adelaide.edu.au> To: phk@critter.tfs.com (Poul-Henning Kamp) Cc: msmith@atrad.adelaide.edu.au, jgreco@brasil.moneng.mei.com, hackers@freebsd.org Subject: Re: Watchdog timers (was: Re: Multi-Port Async Cards) Message-ID: <199602031133.WAA01376@genesis.atrad.adelaide.edu.au> In-Reply-To: <5991.823343047@critter.tfs.com> from "Poul-Henning Kamp" at Feb 3, 96 11:24:07 am
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Poul-Henning Kamp stands accused of saying: > > > All this is a little hypothetical just now, as I'm concentrating on the > > watchdog card. If it works out, it should do the basics, and with a TXCO > > on it should give a good, stable frequency reference. Accessing it > > won't be fast, sorry Bruce 8( > > The bit Bruce said about the 64bit counter is important. Can you make > it so that the count is latched when you read the LSB so that the > count read is atomic ? The plan is for an 8254 running off a 1MHz clock with the counters chained. I was going to use the first two as the timer element and the third to time the relay closure, but this can be done with a 555 easily enough. Bruce has the 8254 data, I suspect that you would as well. Yes, you can latch everything inside it all at once. Would having two counters (32 bits) be enough? The tap points on the chain would then be 1KHz and 1Hz, or 100Hz and 1Hz (either is possible), with resolution (by latching the counts) back to 1us. If I were to generate an interrupt on the first tap, you'd still be at interrupt latency's mercy for your final accuracy. With a cheap oscillator, this would be accurate to ~50ppm, and a TXCO would take it down to ~5ppm with very low drift. > Also, consider putting it on an IDE cable, it may be faster actually... The 'basic' device is currently a small amount of shrapnel (245,541,688,74) the timer, a 555, a crystal and a relay. I don't intend to make this any fancier than it _really_ has to be. > Poul-Henning Kamp | phk@FreeBSD.ORG FreeBSD Core-team. -- ]] Mike Smith, Software Engineer msmith@atrad.adelaide.edu.au [[ ]] Genesis Software genesis@atrad.adelaide.edu.au [[ ]] High-speed data acquisition and (GSM mobile) 0411-222-496 [[ ]] realtime instrument control (ph/fax) +61-8-267-3039 [[ ]] "wherever you go, there you are" - Buckaroo Banzai [[
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