Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!purdue!decwrl!hplabs!hp-sdd!ncr-sd!serene!pnet12!gbell
From: gbell@pnet12.cts.com (Greg Bell)
Newsgroups: sci.electronics
Subject: SRAM Battery backup
Message-ID: <657@serene.UUCP>
Date: 14 May 89 00:36:07 GMT
Sender: root@serene.UUCP
Organization: People-Net [pnet12], Del Mar, CA
Lines: 42

I'm working on a microcontroller project using Intel's 8031.  I have
a 1x16 LCD, a DTMF decoder, and a line voltage detector as peripherals.
 
I have a 2716 EPROM acting as a monitor ROM that loads a .OBJ file from
my AT clone's COM port into the static RAM.  Cuts software developement
time 4x at least! 
 
Alright, enough of the description...  I would like to battery back the
RAM.  Its a 6264LP-15.  I have the appropriate diodes set up so the
battery doesn't try to run the rest of the circuit, and so that it does
not get charged by the power supply.
 
How do I get the circuit to power down in an orderly fashion?  As of now,
the setup works many times, but occasionally the RAM gets corrupted. Its
possible that the RAM is getting corrupted on each power-up/down but just
becoming noticable when certain (instruction holding) locations get corr-
upted.
 
Is this RAM corruption due to a pulse on the WE/ (write enable) line?  Or,
is it because there's a program running in the RAM?  Do I have to jump
to some EPROM location before powering down just to assure the RAM isn't
selected?  
 
I've tried having the setup jump to an EPROM address so the RAM is quiet
at power down, and I still have the problem.  Also, I've tried buffering
the write line so that it can be pulled up when the power to the rest of
the circuit goes down.  Its hard to tell what's making a difference since
I can't tell what, if any, locations get corrupted at any one time.
 
I know Maxim and Dallas Semiconductor make chips that tame this problem, 
but I'm wondering if they're actually necessary.  Does anyone have Dallas
Semi's address?
 
I'd like suggestions on how to track down the actual problem (finding out
when the RAM is getting corrupted w/out printing out all RAM locations!)
and whether a solution can be arrived at without using an entire IC.
 

    Greg Bell_________________________________________________________
      Hardware hacker          |
      Electronics hobbyist     | UUCP:  uunet!serene!pnet12!gbell
      EE major at UC San Diego |