why address pin 14-15 tied for a 27c256 ROM ?

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my microcontroller board has installed a 23256AC (i.e. 27c256
style) ROM chip with 8031 MCU...

I traced the ROM chip's pins to be (reasonably) sure that i was
selecting a correct/compatible eprom chip setup for ***reading***
this ROM chip on an e(e)prom programmer, as the programmer did
not show direct support for the chip.

When tracing connections i found that (pin 1 - NC) on the chip
was tied to (pin 27 - A14) and these  connected to (Addr 14) on
the MCU

is there a design reason for doing that, maybe prevent using a
27c512 chip for example ?


so for different ROM  chips that means that

Chip      Pin 1   ->   Pin 27    =    ?result/purpose?
--------------------------------------------------------
23128     NC      ->  PGM        =    no prob
27c128    Vprog   ->  PGM        =    ???

23256     NC      ->  A14        =    no prob
27c256    Vprog   ->  A14        =    ???

27c512    A15     ->  A14        =    **???
-------------------------------------------------------

Does this mean i can not use a 27c512 chip ?
or maybe it is a simple  way to prevent a floating address line ?

any ideas on this greatly appreciated,
robb



Re: why address pin 14-15 tied for a 27c256 ROM ?

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Maybe it has some advantages (I'm just a hobbyist and not a hardware
guy), but I guess the same effect could have been achieved by tying
the NC pin to GND?

If the reason for wanting to use a higher capacity chip is that these
are more readily available, you could IMHO use a 27c512 if you keep
within half the capacity (32K) and program those it twice into both
"halves". That is contents from 0x0000 to 0x7fff should be the same as
from 0x8000 to 0xffff.

Half of its memory would be wasted / redundant.

If the reason for wanting to use a 27c512 is the additional 32K, then
you'll have to cut the A15->A14 trace and somehow get the A15 address
line from the CPU bus to the chip. (Make sure that it is not used
otherwise though, e.g. as a simple enable line for some other memory
mapped hardware, to save address decoding hardware)

   Regards,
   Gilles.


Re: why address pin 14-15 tied for a 27c256 ROM ?



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I forget the reason, it's been ages since I used an Eprom rather than
parts with internal flash memory but there was some good reason for it
IIRC.

Yes, it may stop a 27512 being used on that board.

Graham




Re: why address pin 14-15 tied for a 27c256 ROM ?
Quoted text here. Click to load it

Yes, you can use a 27C512, or a MTP equivalent such
as SST27SF512.
All you have to do, is duplicate the 256K code, in both halves.
Then, when A15 flips, an identical code copy is found.

-jg




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