Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!well!nagle
From: nagle@well.sf.ca.us (John Nagle)
Newsgroups: sci.electronics
Subject: Re: Dallas Semi Intelligent Sockets?
Message-ID: <22984@well.sf.ca.us>
Date: 3 Feb 91 07:47:10 GMT
References: <3222@naucse.cse.nau.edu> <1991Jan28.083000.28601@ims.alaska.edu>
Lines: 23

>     All of these devices (and EEPROM's as well) share a common problem:  the
>microprocessor accessing them can go berzerk on power-up and especially power-
>down conditions.  I have even seen this with serial EEPROM's that have an
>extremely complicated access protocol.  The Motorola 68HC11 single chip
>microcomputer with internal EEPROM also suffers from this problem.

>     The only solution is to make sure the microprocessor is reset whenever
     Yes, the M68HC11 is particularly prone to this problem, enough so
that there's a Motorola application note on it.  The M68HC11 has the
unusual property that as the supply voltage drops, the processor
stops branching but continues to execute instructions.  So, for a
few milliseconds, the processor cycles through its 64K address space,
executing every word in the machine as an instruction.  If there are
any instructions in memory that write EEPROM, trigger peripherals, or
do something obnoxious that can still be done with power failing, they
will be executed.  Since this part is intended as a single-chip microcontroller
(it has an A/D, timers, and such) for minimum-parts-count applications,
it really should have had the appropriate on-chip circuitry to prevent
this.  But it doesn't.  Motorola recommends adding circuitry to
detect power-low and force RESET until power falls to the point that the
clock stops.

					John Nagle