Skip site navigation (1)Skip section navigation (2)
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>

next in thread | previous in thread | raw e-mail | index | archive | help

> 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
> 




Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.BSF.3.96.971018174744.265A-100000>