SRAM chips

[Fraser, Colin J]

> -----Original Message-----
> From: Jeroen Proveniers [mailto:J.Proveniers at orga.nl]
> Sent: Friday, August 18, 2000 2:08 PM
> To: 'Synth DIY'
> Subject: RE: SRAM chips
> 
> 
> An DALLAS DS1244 32K SRAM with backupbattery could also be 
> considered. This
> is a module slighty bigger dan DIP-28 and has an onboard 
> lithium battery
> that should last 10 years. Advantage of such module is that 
> it contains
> protection circuits to prevent inadvert writes at power 
> up/down. These units
> are reliable, but rather expensive. One thing, you know it's 
> guaranteed to
> work.

I find the intergrated battery backed RAM chips to be very expensive.
I prefer to use the Dallas DS1210 battery backed RAM controller.
This is an 8 pin device that provides automatic switching of the RAM Vcc on
power down.
It also buffers the CE pin for the RAM access, and write protects the RAM as
soon as the main power supply drops below 4.75v
Further, it has inputs for 2 batteries to provides resilient backup in the
event of a battery failure.
It also has another neat trick - if the battery voltage is below a nominal
level, after power-up, the second access to the protected RAM device will be
blocked.
This allows you to implement code that writes a byte in memory after
start-up, then writes a different value to the same location.
If the battery is low, the second write will fail, so your can detect in
software that the battery needs replacing and warn the user.
These chips cost about 3ukp, add another typical 3ukp for a 62256 and you're
still a lot cheaper than the equivalent integrated BBSRAM.

Colin f