EPROM emulator

[Ludis Langens]

Ron Gregory wrote:
> 
> I'm not an EE by any means, but why not try
> to use a Dual-port RAM?
> I heard they aren't too expensive any more...
> at least the "slow" ones we'd use...

I hope it is intended to use dual port RAM.  A dual bank 27C512 emulator
w/EPROM built with conventional RAM would require:

Each RAM bank needs to be a seperate 64KByte SRAM.  Each SRAM's 16
address lines need to be muxed between the ECM or PIC.  This will take
four "quad 2to1 muxes".  The RAM control lines also need to be muxed, so
add a fifth mux.  The ECM needs to be able to read (only) from either
bank, so add a pair of "quad 2to1 tristate muxes".  The PIC needs to
read and write either bank, so add a pair of octal transceivers.  Place
the EPROM on the ECM side of all this logic.  Two D flip flops can
syncronize the bank switching, one for a bank select, the other being an
EPROM select.  Clock the F/F's with the OR of the ~CS and ~OE signals
from the ECM.  The tristate mux can be enabled by ORing Q of the EPROM
select F/F with (~CS OR ~OE).  The EPROM can be selected similarly,
except using the notQ output.  The bank select F/F's Q and notQ can
drive all the mux selector inputs.

The chip count is:
 10  '157    muxes  (remember, two banks!)
  2  '257    muxes
  2  '245    transceivers
  1  '74     F/Fs
  1  '32     OR gates
  1  PIC     microcontroller
  1  MAX232  RS232 driver/receiver

Gee, this would be quite impressive, even when using SOP surface mount
parts!  Perhaps there are some DRAM controllers that could replace the
sea of '157's.


FYI, the P4's ('165, '727, '730, '748, '749) wire the EPROM's ~CS input
to ground.  The ~OE input is driven by the MPU.  It is asserted for
reads in the upper 32K of memory.  It is conditioned by an (MPU
internal) VMA, so location $FFFF will not be read during address
computation cycles.  ~OE asserts in the middle of the E low time.  It
deasserts upon the E high-to-low transition.  Thus, ~OE is high for only
about 120nS between back to back PROM reads (with a 2^21 Hz E rate.)

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/