Can you replace a small EEPROM with a larger one?

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Can you replace a small EEPROM with a larger one?

Postby keenerb » 2018-2-05 @ 14:46

I've got a handful of 28C256 chips, and I need a 28C64 for a board.

From looking at the datasheet I see the pinouts are basically the same, except 28C256's A13 and A14 are NC/not connected on the 28C64 part.

I assume that those signals would be used to select addresses > 8kb? Addressing reads to the first 8kb shouldn't use those pins, so the 28c256 part should work well, I'll just be wasting the additional space.

My programmer seems to bear this out; I can read the first 8kb of a 28C256 by specifying 28C64 as the chip type, but I'd like some validation that this will work reliably.
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Re: Can you replace a small EEPROM with a larger one?

Postby keropi » 2018-2-05 @ 15:02

Yes it is possible for the eeproms you mention... I recommend to fill the C256 eeprom completely with the C64 data:

copy /b C64.BIN + C64.BIN + C64.BIN + C64.BIN C256.BIN

the above command assumes your original 28C64 dump is called "C64.BIN" and C256.BIN is the output file to be written on the 28C256 eeprom
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Re: Can you replace a small EEPROM with a larger one?

Postby SSTV2 » 2018-2-06 @ 02:13

keenerb wrote:From looking at the datasheet I see the pinouts are basically the same, except 28C256's A13 and A14 are NC/not connected on the 28C64 part.

I assume that those signals would be used to select addresses > 8kb? Addressing reads to the first 8kb shouldn't use those pins, so the 28c256 part should work well, I'll just be wasting the additional space.
You are correct. If you wish to substitute a smaller capacity eeprom with a larger, you have to write your binary data to the last 8kb space of the 28c256, starting from: 32768-8192=24576 or 6000H. Keropi's suggestion would solve such problem.
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