replace an EPROM with an EEPROM?

[Chad Clendening]

I have personally got them from Jameco electronics( p/n 74843), 1-800-831-4242.
I have also been provided some samples from a semicoductor supplier.  The ones I
have bought are the AT28C256-15PC for about $12.00 a piece.  Datasheets are
available at the ATMEL website.  These are true EEPROM devices, as you can write
each byte individually.  There is no erasing needed to over-write a byte.

The flash / eeprom shouldn't be a concern for automotive ECM use as you would
have to program it using a programmer unless you can bootload the GM ecm and then
write to the program chip.  I don't know if this is possible or not?  I do know
that method works great for standard HC11 chips.

The 28F256 IS a 32 pin flash chip.  I have 2 samples but havn't touched them.  I
believe you need to erase the things before writing over the previous program.
This is also a 12 volt programming chip meaning you need to apply +12v to program
or erase the chip.

I personally prefer the AMD29F010 ( 512Kx8) for flash.  It is a 5 volt device,
dosn't require you to write the entire chip to 00's to erase it, and I have been
quoted a price the same or less than the 28F256 in 2000-4000 / yr quantities.

To the other questions on the EEPROM:

1) the eeprom DOES NOT loose memory when power is removed.  It is simply able to
be erased using electricity instead of UV light.

2) You can program it by a variety of methods.  EPROM programmers work, It can be
written in a matter similiar to SRAM with some hitches - the AT28C256 WORKS in
the same pinout as a 62256 SRAM chip ( 256Kx8).

3) you cannot read from the device while writing to it.  It has special logic (
data polling) to tell you when the chip is done writing a location.  Anything
read from the chip while writing can be considered trash.  I have heard of some
new types where this is not the case.

4) It MAY be possible to write it in the the ECM if the write enable decoding is
done on the ECM.  I really doubt this as the EPROMS generally need the program
pin held at +5v when not programming.  I have sucked out the contents of a EPROM
from the car ECM using a ribbon cable extending the MUXed bus of a 6811 running a
monitor.  A  40 pin DIP test clip allows this to be done in about 2 minutes.
This gets tricky as the processor on the ECM must be disabled, set in the proper
mode...   It requires digging very deeply into processor manuals.

I wouldn't suggest this as a first uC project!

Chad