Next EPROM Question - Sizes
ard at p850ug1.demon.co.uk
Mon Sep 17 17:41:05 CDT 2007
> So the question of the minute is this, is it possible to substitute a
> 27C256 for a smaller 27C128?
The only real difference betwen those 2 parts is that the former has
twice as many locatiuons, and therefore 1 extra address pin.
There's nothing to stop you holding any of the address pins in a contant
logic state for all time. So if you program the 16K byte image in the
first half of a 27256 and hold the A14 pin low, it'll behave as a 27128.
Or program the image in the second hald and hold A14 high.
Or, of course, program 2 images into the chip and select between them
with a jumper conencted to the A14 pin.
But please don't just program the same image into both halves of the
27156 and leave A14 floating, assuming it'll always apprar as a 0 or a 1
to the chip, and since the 2 halves contain the same data, it doesn't
matter which it appears as. You _will_ have hard-to-find glitches if you
do somehting like that.
Other than the A14 input, the pinouts of the 2 devices are much the same
in read mode. So there's not even mouch PCB modification to do.
Also, IIRC, the pinout for programming a 27128 and a 27256 are rather
more different, with Vpp moving to a different pin. But if you use a
commercial programmer, that's taken care of.
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