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4X4 matrix keypad-74c922- source code

Discussion in 'Electronic Basics' started by bb, Mar 21, 2006.

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  1. bb

    bb Guest

    im trying to interface a 4x4 matrix keypad using a 74c922 decoder to a
    Atmel micrcontroller c- programm.......

    i would appreciate some pointers in how to create d source code that wil
    read from the output of the decoder...according to key pressed...

  2. Chris

    Chris Guest

    Hi, bb. Assuming you're using external caps for the oscillator and the
    keybounce mask, all you have to do is set your keycheck loop so it
    happens at least twice during the component-selected debounce period.

    Check for Data Available. If it's high, read the keypress data, then
    set a software flag. Don't read data while the flag is set. Wait for
    Data Available to be low again before resetting the flag. Read data
    again when flag is reset and Data Available is high.

    If you don't need to tri-state the '922 data outputs, that's the
    easiest way. If you have bussed the data, clear the bus and then
    toggle output enable to read the data as above. It's all in the data
    sheet, available at the Fairchild Semiconductor website.

    If this isn't an exercise in programming, though, the 74C922 is an
    expensive way to do this job. If your code is so bound up that you
    need a dedicated part to do keypad debounce, it might be better to get
    another small uC to do this job at less cost and fewer pins (data can
    also be sent from one uC to the other in serial format). You can also
    have n-key rollover instead of just 2-key quite easily.

    There's no law that says you can't have more than one uC on a board.

    Good luck
  3. bb

    bb Guest

    thx chris....

    adding uc was an option i thought about earlier but d cct handles lot
    more function than just keypad reading...tis includes alarm,lc
    interface,relays n stuff...n i hv only 6 IO lines to spare.... besides..
    the cct is already made finalize

    wht i still dont get is tis

    a) how to make a source code that will enable uc read d 5 input take
    from 74c922....74c922 takes in 16 values(from 4x4 matrix) and decode i
    into a 4 bit binary code to the uc plus a DA line.....
    do i hv to include d conversion table(from 16 to 5) in the program sourc

    b)how do i know
    whts d approprite delay time for key bounce routine....if im using a there a mathematical calculation involved in estimatin

    c)and finally in c -language....
    is library file-> reg52.h, reg51.h and declaration term'sbit'....ANS

    when i compile d program it gives me error..cannot define reg 52.h an
    term sbit
    im using turbo c++ 4.5

  4. Chris

    Chris Guest

    Hi, bb. In the data sheet, it shows how to calculate the debounce time
    based on the caps selected. Read the data sheet.

    You should be able to read the DA bit directly in C. Just jump on zero
    or not zero. As far as the 4-bit pushbutton data, you can read the
    byte, mask off the bits you don't need, rearrange if necessary (usually
    the case -- somehow, it seems to be trivially easy to transpose bits on
    a PC board), then shift as necessary to make it the low nybble. Then
    just read the data. Or if it takes fewer instructions/time, read the
    four bits individually, then multiply/shift as necessary and add to
    form the data nybble. Your call.

    I don't know the Atmel C compiler you're talking about. If you need
    help with code, you might want to go to one of the Atmel groups.

    I've found it usually helps a lot to read the manuals and data sheets.
    Even if you spend some time looking, that's just a free opportunity to
    learn something else along the way.

    By the way, please take a chance to glance at this:

    and also, please don't use TXT lingo unless you're posting from your
    cell phone. Us older cripples find it somewhat obscure. I know -- our
    problem, and we'll soon enough be dead. But if fossils can't
    understand, fossils can't help, can they?

    And it actually sounds more like the TXT lingo poster can't even take
    the time or show the respect to express himself carefully. Whether
    that's interpreted as panic, extreme hurry, or a lack of respect by the
    person who's taking the time to help and craft a response, it probably
    won't help the OP.

    And a fossil might just answer:

    havnt a clue wot U' spoutin', m8. hOp U git out of d car trunk s%n!

    ( for us fossils out there

    Good luck
  5. bb

    bb Guest


    thank you chris.....understanding the needs of fossil over here( can yo
    see...all lengthy words-ohmygoditiskilling-but its ok )...

    young blokes lik me always in a hurry.....*wink *wink *smile
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