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

Discussion in 'Embedded' started by robb, Dec 13, 2007.

  1. robb

    robb Guest

    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, Dec 13, 2007
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  2. robb

    Gilles Kohl Guest

    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)

    Gilles Kohl, Dec 13, 2007
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  3. robb

    Eeyore Guest

    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

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

    Eeyore, Dec 13, 2007
  4. 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.

    Jim Granville, Dec 13, 2007
  5. robb

    robb Guest

    robb, Dec 19, 2007
  6. robb

    robb Guest

    thanks for help graham
    robb, Dec 19, 2007
  7. robb

    robb Guest

    thanks for info and help Jim,
    robb, Dec 19, 2007
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