Date: Sat, 18 Oct 1997 17:55:45 -0700 (PDT) From: "Jamil J. Weatherbee" <jamil@trojanhorse.ml.org> To: Peter Dufault <dufault@hda.com> Cc: mdean <mdean@best.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: Opinions wanted. Message-ID: <Pine.BSF.3.96.971018174744.265A-100000@trojanhorse.ml.org> In-Reply-To: <199710182313.TAA23016@hda.hda.com>
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> You must pass the normal behavior of the hardware through to the > software. When The 8255 board resets all i/o lines are high impedance inputs this shouldn't trigger anything (like a SSR module). Inputs on this board look would high to anything sensitive enough to use them like outputs. I have 6 device nodes for each port, each control an 8 bit port. Any that are not currently open have their corresponding port set as input (the reset state). However on an 8255 when you set a direction flag to output it automatically defaults to all low on the outputs. My question is whether I should leave that as the default state on an open() call or immediately (in the driver) during the open set all outputs to high initially. I am already doing the later but comptemplating changing it back to all lows on an open because doing all highs on open() call generates a high, low, high pulse of about 1 microsecond width. Which could conceivably (not likely though) damage something (like an SSR) that doesn't like high frequency driving signals. > > The system design must arrange to pull the signals high at start > up (if you can, yes I'm mostly software) so that it doesn't turn > on devices. The hardware must be set up with pullup or pulldown > resistors as appropriate to e.g. avoid turning on the natural gas > solenoid in the oxygen tent at powerup in case the system hangs > while trying to automatically connect to microsoft.com to download > the latest browser. All systems may do this soon. > > Peter > > -- > Peter Dufault (dufault@hda.com) Realtime development, Machine control, > HD Associates, Inc. Safety critical systems, Agency approval >
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