Bipolar PROM replacement (was Re: More Info)

From: Eric Smith <eric_at_brouhaha.com>
Date: Fri Feb 4 14:26:11 2005

I wrote about the problems of bipolar PROM replacement.

> Why not use a simple PIC with some of the static
> RAM that was used for cache RAM of the 486 time frame.
> These RAMs were quite fast. You could then use a single
> flash RAM to hold the images and select which one to
> load to the high speed RAM with a switch or such.

Aside from needing a lot of chips (multiplexers for the address, tristate
buffers for the data, to isolate the RAM from the target while the PIC is
loading the RAM), the main problem is that many systems with biploar PROM
depend on the PROM contents being accessible *immediately* (within
nanoseconds or microseconds) from power-up. For instance, processor
microcode. Though I suppose if you battery-backed the RAM it would be OK.

Ferroelectric RAM (FRAM) would be ideal since it is inherently nonvolatile,
and the latest chips have effectively eliminated the read endurance
problem, but they generally aren't fast enough.

The parts that combine EEPROM/Flash and SRAM, with fast transfers between
the two (automatically at power up, or under manual control) might be
suitable. Simtek makes those for 5V logic in sizes up to 128K*8 and speeds
down to 25 ns, e.g., STK11C88. The automatic recall at power-up takes
up to 550 uS, which might be fast enough for some systems.

Eric
Received on Fri Feb 04 2005 - 14:26:11 GMT