Next EPROM Question - Sizes
dave06a at dunfield.com
Sun Sep 16 23:00:18 CDT 2007
> > So the question of the minute is this, is it possible to substitute a
> > 27C256 for a smaller 27C128?
> IIRC the difference between a 27256 and a 27128 is that 128 puts VPP
> on pin 1 - this is normally held at +5V for read applications. The
> 256 puts A14 on pin 1.
> So a 256 will usually work as a 128 (reading - programming is
> different) however you will most likely have to program your
> code into the TOP half of the 27256.
Dave you idiot! - that the RAM pinouts you were thinking
about. Yes, as others have noted, Pin 27 is A14 of 27256
and -PGM on a 128. Rest was correct - normally tied high
for read applications, so your code will be read from the
Mirroring in both halfs is always safe, or do what I do
and put a jumper on the extra address pin - lets you have
two selectable ROMS (nor application and a debugger in
my case usually).
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
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