Newsgroups: comp.robotics
Path: brunix!uunet!wupost!csus.edu!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: A1 glitching on 6.270 boards
Message-ID: <1993Jan18.051907.22813@netcom.com>
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
References: <1993Jan18.032624.28824@athena.mit.edu>
Date: Mon, 18 Jan 1993 05:19:07 GMT
Lines: 33

spedhead@athena.mit.edu (Pankaj Oberoi) writes:
>I talked with a Motorola engineer about this problem. of the glitchiness.
>He thought that because the EEPROM memory space of the A1 coincided with
>the RAM on the 6.270 board that when the chip was being reset into the 
>wrong state, it would access the EEPROM and wipe out some of the RAM.
>The problem never existed on the A0 because the EEPROM is unaccessible
>on that version, and if the processor was to be reset into the strange 
>mode, then it would access the RAM anyway.

       There's an old Motorola note that explains what's going on.
It's "MC68HC11A8/A1/A0 Errata (Rev B. - 7/17/86) A38P and A49N Mask
Sets", which shipped with the old EVB boards.  There's an additional
sheet, titled "Notes on EEPROM and RESET", which explains the subject
in great detail.  The right solution is forcing RESET on power low.
As Motorola puts it; "The decay of Vdd from operating level to zero
volts can often be measured in seconds.  During this time the CPU can
execute millions of instructions, not necessarily correctly."
Motorola also writes "At a very low level of Vdd the branch instructions
fail to branch while other instructions operate more or less correctly.  As
a result, the program memory is executed in a more or less linear fashion
ignoring branches. Eventually the programming or erase sequence (for EEPROM)
is encountered."

       A software workaround for this problem is in the BUFFALO monitor
versions 2.5 and up.  (Motorola BBS at 512-440-FREE (haven't called
recently) should have it).  The old EVB boards don't force RESET on power
low, which caused Motorola to develop a software workaround.

					John Nagle

					John Nagle


