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27cXXX eprom switcher

Discussion in 'Hobby Electronics' started by mark krawczuk, Feb 7, 2007.

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  1. hi , can anyone point me in the right direction for a eprom switcher
    schematic ?

    this is what i want to do :

    i have a engine ecu which contains a 27c128 eprom , i want to be able to
    switch between 2 .bin files (maps) while the engine is running i.e on
    the fly . i presume i will have to use a 27c256 eprom .?

    any help appreciated.

    mark k
  2. Lord Garth

    Lord Garth Guest

    In between chip enable or output enable cycles, you could set or reset the
    order address line. This would effectively bank switch the EPROM.

    I still would not do it on the fly however, I'd feel better if it were
    powered off.
  3. kreed

    kreed Guest

    you will

    you must then switch the highest address line (A14 for a 256) high for
    one map, and low for the other. in practice, a 1k resistor going to
    +5 to A14, and then another from A14 to a switch that goes to ground.
    a 220 ohm resistor should be connected in series with this switch to
    protect the input. (If its a really noisy electrical environment such
    as a car, and the switch is going to be mounted some distance away
    such as on the dashboard, then you should consider using an
    optocoupler near the ROM to do this job and avoid electrical
    interference possibly getting into the A14 line via the length of wire
    that goes to the switch.).

    Switching a ROM (or any device thats connected to a processor Address
    Data bus like this such as RAM etc) while the software is running
    isn't a really good idea, especially if there are changes to the
    operation of the actual CODE running in both roms (as compared to
    changes in data tables within the ROMS).

    When changing software versions, its often the case that the program
    will Clear any NV RAM back to default settings, and if there are any
    changes that you have to make (ie, settings you need to enter for your
    particular options that are then stored in an internal NV RAM). If
    the program learns and calibrates itself over time as the engine is
    used, then all this special (and probably constantly changing) data
    will likely be lost too, leading to less than optimum performance
    until this data is replaced.

    At worst, engine damage may occur, or it might stall, or might fail
    completely (from thinking the computer is stuffed, and shutting down
    to protect the engine, or for safety reasons) needing a service centre
    to reprogram the settings etc. At best, you might generate many ROM
    checksum errors in the system :)
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