Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!well!nagle
From: nagle@well.UUCP (John Nagle)
Newsgroups: sci.electronics
Subject: Re: SRAM Battery backup
Message-ID: <11731@well.UUCP>
Date: 18 May 89 16:04:43 GMT
References: <657@serene.UUCP> <1989May15.043845.25572@utzoo.uucp>
Reply-To: nagle@well.UUCP (John Nagle)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 20


     I've used a New Micros M68HC11 microprocessor board with a "smart
socket" keeping the static RAM alive.  An interesting implication of this
is the behavior of the system when power is low.  I was powering the board
with a battery and a 7805 regulator.  When the battery voltage dropped
below about 6v, the output of the regulator fell from the regulated 5v
level.  This didn't bother the microprocessor, an all-CMOS device that will
run quite happily on 3v.  But the "smart socket" detected the loss of power
and took the RAM offline.  This caused the processor to crash and restart.
This M68HC11 had a Forth interpreter mask-programmed onto the chip, and
the Forth interpreter, at startup, searched RAM for a special pattern
indicating the start of the program to be run.  Not finding it, it simply
offered a Forth prompt to the terminal.  

     So, unexpectedly, the embedded application stopped running and the
Forth interpreter's prompt appeared, with a working Forth system available
using only the 1K of RAM on the M68HC11 chip, and the 32Kb on the "protected"
chip offline.  This puzzled us for a while.

					John Nagle