Newsgroups: comp.robotics
Path: brunix!sgiblab!pacbell.com!amdahl!netcomsv!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: Miniboard Kits from SDRTFT
Message-ID: <nagleCo5rIK.9C5@netcom.com>
Organization: NETCOM On-line Communication Services (408 241-9760 guest)
References: <steve-110494223450@fuzbat.pgh.pa.us> <1994Apr12.032534.27701@news.media.mit.edu>
Date: Tue, 12 Apr 1994 18:03:54 GMT
Lines: 21

fredm@media.mit.edu (Fred G Martin) writes:
>  * if IRQ button is not installed, a Dallas Semiconductor DS1233
>    "econo-reset" chip may be mounted instead.  This chip assures
>    correct power-on reset operation of 6811 for "mission critical"
>    applications.  (Most of the time the 6811 boots properly when
>    power is applied, but the DS1233 may be used to insure that it
>    -always- boots properly.)

       Actually, this is backwards.  The problem with 68HC11s is not
power-on reset operation; it's power-OFF reset.  The 68HC11 has the
annoying property that as power drops, the CPU tends to continue to 
execute instructions without executing branches.  So every instruction
in memory gets executed a few times, in address order.  This is why you
don't want to put the code sequence for altering EEPROM in memory in
a sequence which will cause EEPROM to be altered during power-down.
There's a Motorola application note on this, and a copy is supplied with their
EVB board.

      So install the DS1233.  It's cheap.

					John Nagle
