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More EEPROM / EPROM Questions

Discussion in 'Hobby Electronics' started by Adam, Oct 17, 2006.

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  1. Adam

    Adam Guest

    Hi Again Everyone,

    I just wanted to say a quick thanks for the reponses so far, i'm new to
    all this and am full of questions.

    I was wanting to make some changes to the ROM of a Commodore 64 as I am
    quite comfortable with assembly language, All the sites which give
    guides on this mention 2764 & 27256 chips which are EPROMs only, I was
    wanting to use EEPROMs insted.

    I am able to get myself a smallish supply of Winbond W27C512-45 chips
    which look ok for what I want to try and do. I just have a couple of
    questions.. Looking at the datasheets for these chips they state that
    they have 16 address lines (A0-A15), but other 27Cxxx have lines marked
    A0-A13. Will these Windbond chips work in place in a system that only
    uses A0-A13? I'm not quite clear on what the lines do.

    Thanks once again for all the help,


  2. Franc Zabkar

    Franc Zabkar Guest

    Just tie the unused address lines to ground. Otherwise connect them to
    a 2-way header and jumper them high or low. The two extra bits will
    allow you to partition your EEPROM/EPROM into four sections, each with
    different code, effectively giving you four C64 machines in one. Note
    that the C64 uses ROMs (23xx) so you will need to make an adapter
    socket. I've actually replaced all three (?) ROMs in a Commodore FDD
    with a single EPROM in this way.

    BTW, how can you be "comfortable" with assembly language if you don't
    understand addressing?

    - Franc Zabkar
  3. Adam

    Adam Guest

    Hi Franc,

    I am comfortable in the software side i.e, I can code in assembly, but
    I have no idea what address lines do on an EPROM having never touched
    the hardware side before. BTW, I should mention that the only assembly
    I have done is on finished products such as a C64 an a PC, I have not
    done any low level stuff such as prototype boards and the like since

    Thanks for your response.

  4. Adam

    Adam Guest

    Hi Chris,

    Thanks! That is really quite simple!

    So if the EPROM chip does have extra address lines then they can be just
    pulled to ground because the CPU can't use them as it doesn't have enough
    address lines anyway, is that right?


  5. Lord Garth

    Lord Garth Guest

    That is correct, otherwise their state is undetermined.
  6. Lionel

    Lionel Guest

    The newer chips have more memory, so they have more address lines. You
    don't need the extra space, so as long as you ground (or pullup) the
    unneeded lines (A14-A15 or whatever), it'll work correctly. Just make
    sure that you program your code into the part of the ROM that matches
    however you hadwire the extra address lines.
    Alternatively, you could replace two 27256s or eight 2764s with one of
    your 512s, which would be a lot more elegant.
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