From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from majordomo by infradead.org with local (Exim 3.03 #1) id 11tyNf-0001dT-00 for mtd-list@infradead.org; Fri, 03 Dec 1999 19:28:35 +0000 Received: from [209.184.180.169] (helo=bob.faxtel.net) by infradead.org with esmtp (Exim 3.03 #1) id 11tyNe-0001dM-00 for mtd@imladris.mvhi.com; Fri, 03 Dec 1999 19:28:34 +0000 Message-ID: <384819EA.5AEB1F19@go2fax.com> Date: Fri, 03 Dec 1999 13:28:42 -0600 From: Bob Canup MIME-Version: 1.0 To: mtd@imladris.mvhi.com Subject: power down Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-mtd@imladris.demon.co.uk List-ID: The reason that I said that expecting anything to work during power down is wishful thinking is this: once the voltage to a digital chip goes below the minimum specification of the chip, the behavior of the chip becomes indeterminate. For example: the old Western Digital 1791 double density disk controller chip would sometimes glitch in such a way during power down that it would write to the floppy - you could see the floppy light blink when this happened. Unless chips are specifically designed to handle power down conditions this sort of thing happens. For example - any competently designed Flash memory has to refuse to write if the voltage is below spec. As to flushing the buffers and doing a shutdown when a power fail condition occurs - I believe that Linux already has code to handle a power down such as I described. What I have described is very similar to a UPS signaling the kernel that power is going down. Linux can do an ordered shutdown when it receives the signal. Qualifying digital circuitry with a POWER GOOD signal is very similar to protecting the circuitry with a typical 'SCR over voltage crowbar circuit': it makes the engineer feel good - but it doesn't actually do much of anything. Why doesn't the crowbar work? After all, it is a text book circuit. The answer is that the SCR is a power device which takes on the order of 10 microseconds to turn on while the delicate chips are destroyed by a few nanoseconds of over voltage. The result is that the SCR never turns on - the fuse blows because the weakest digital chip shorts the power supply to ground. One could "protect" SCR's with digital chips, but not the other way around. Another example of "feel good engineering" is the power on self test which most computers have. One can only test non critical sections of the machine: if anything critical is broken the POST won't run - and a tech will have to figure out what is wrong. It's a bit like asking yourself "Am I alive?" If you can ask the question the answer is always "Yes". To unsubscribe, send "unsubscribe mtd" to majordomo@infradead.org