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Interface 4x4 matrix keypad to AT91EB55 board

Discussion in 'Electronic Basics' started by RaMbO, Nov 17, 2005.

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  1. RaMbO

    RaMbO Guest

    Hi all,
    I'm new to this group and I'm hoping someone could please help with
    this issue.
    I'm trying to interface a 4x4 matrix keypad to Atmel's AT91EB55 board.
    The issue that I'm facing is that this board does not have a keypad
    port but it does have many ADCs on it. I'm also trying to limit the
    number of IOs I have to use.

    This keypad is the common 4x4 keypad; that is, you provide voltages to
    the columns (or rows) and then scan the rows. Anytime a button is
    pushed, a short takes place and one of the four row pins goes high.

    I have looked into a resistor network that will give me unique voltage
    values such that I can input them to the ADC to give me a unique
    digital output. This didn't work too well for me as a couple of
    button's voltages were too close to differentiate; this was also
    because I only have access to 5% resistors.

    Would someone be able to help me out with this?
    Thank you very much!
  2. John Fields

    John Fields Guest

    If you can spare 4 I/Os you can do it like this: (View in Courier)

    | | | |
    +-------+ [R] [R] [R] [R]
    |__ __| | | | |
    OUT0>----------O|LE Y0|O-----X---X---X---X---
    | __| | | | |
    OUT1>-----+-----|A0 Y1|O-----X---X---X---X---
    | | __| | | | |
    OUT2>--+--|-----|A1 Y2|O-----X---X---X---X---
    | | | __| | | | |
    | | +--|A2 Y3|O-----X---X---X---X---
    | | | +-------+ | | | |
    | | | 137 | | | |
    | | GND | | | |
    | | | | | |
    | | +----+---+---+---+--+
    | +---------------|S0 IN0 IN1 IN2 IN3 |
    | | |
    +------------------|S1 Y|--+
    |_ | |
    +--|E | |
    | +-------------------+ |
    GND 153 |

    The chips are duals, so the unused inputs of the spared circuits
    need to be connected to either Vcc or GND.
  3. Jasen Betts

    Jasen Betts Guest

    :) normal practice is to use 8 pins to reado one of them.

    assuming a 5V supply you'd want arounsd 0.3V steps between buttons

    germanium diodes have Vf of 0.2V, maybe you could use a bunch of them?

    +--------------------------- output
    | 4x germanium diode
    | | | | |
    | | | | +---+
    | | | | |
    | | | | |
    | | | | +-|<-|<-+
    | | | | |
    | | | | | 15x germanium diiode
    | | | | +-|<-|<-+
    | | | | |
    | | | | |
    | | | | +-|<-|<-+
    | | | | |
    | | | | |
    | | | | +-|<-|<-+
    | | | | |
    ---|-----|-----|-----|--+->|->|->|--- 0V
    | | | |

    4.6 V with no key pressed.

    3.8 4.0 4.2 4.4 approximate output voltage.
    3.0 3.2 3.4 3.6 by key.
    2.2 2.4 2.6 2.8
    1.4 1.6 1.8 2.0
    0.6 0.8 1.0 1.2

    OTOH you could replace the diodes with resistors and use a fixed current
    source in place of the resistor for pretty-much the same result.
    even with 5% parts if they are all from the same batch their correltation
    is usually better than 1%

  4. RaMbO

    RaMbO Guest

    Hi John,
    Thanks for your reply!
    I had a few questions regarding your suggestion, as I'm quite new the
    realm of microcontrollers.

    1) Where would I find these "137" and "153" chips? I've tried looking
    on the web but I couldn't find anything. At least if there's a
    specification for what these chips do, that would be great!

    2) Since I don't know what the chips do yet, where do the "OUTx"
    signals come from and where would "IN0" go to?

    Your help is greatly appreciated.
  5. John Fields

    John Fields Guest

  6. mike

    mike Guest

    This will get you down to 4 pins:
    You can also mux the port with a LCD display if you want.

    In general, I've found that using a part with more pins is easier
    than adding ANY additional hardware. You may save a pin or two by
    adding two extra parts, but that's counter to the idea of using
    the UC to do everything. If you're building one, it's cost effective
    to add a uC to do the peripheral functions. Fewer parts, less space,
    more flexibility.

    YOu have several choices with resistors.
    You can select from 5% resistors to guarantee sufficient voltage
    difference. Assume you're only gonna build one???
    You can add a calibration method.
    You can split into two resistor networks to get more difference.

    Another interesting possibility is to use RS-232 and an external
    keyboard. A $10 garage sale HP200LX makes an excellent keyboard/display
    for uC projects. Can also do the same with a PDA, but I've not tried it.

    Wanted, Serial cable for Dell Axim X5 PDA.
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Bunch of stuff For Sale and Wanted at the link below.
  7. RaMbO

    RaMbO Guest

    Hi John,
    Thanks for the great idea.
    Everything looks good from a theoretical perspective; now I'm actually
    going to try and build version in addition to the others.

    Thanks again!
  8. RaMbO

    RaMbO Guest

    Hi Jasen,
    I'm not sure if I understand this correctly and perhaps, you can
    clarify a few things for me.

    1) On the following row, I would expect the rightmost column to be at
    4.6 (which it is) but the voltage should be increasing as we move to
    the left. According to your matrix of values for the first row (3.8 4.0
    4.2 4.4) seems to be the reverse.


    Also, I didn't mention this but I'm forced to use a 3.3V supply and
    with a 4x4 matrix keypad that basically scans all rows (columns) to
    determine which button was pushed.

    In your diagram, I got confused probably because I couldn't see where
    the connection point are to be. Maybe you can specify this for me?

    Also, there seems to be more than 15x diodes along the vertical side.
    Is this a typo or by intention?

    Thanks for your help as well.
  9. RaMbO

    RaMbO Guest

    Hi Mike,
    Thanks for the feedback and the link to the site. This procedure is
    quite similar to the case Jasen mentioned but using a PIC instead. I
    will probably investigate this route as well if time permits.

    The reason I'm using the 4x4 keypad (pre-built from Grayhills) is
    because I'm forced to use them. I'm currently helping make labs for a
    uC course more robust.

    Thanks again!
  10. Jasen Betts

    Jasen Betts Guest

    that matrix was upside down too. pushing top left button bypasses most of
    the diodes and pulls the output down to O.6V
    you may have to swap the diodes for 1% resitors, 16x0.2 is 3.2 V which is way
    too close to 3.3 to do anything with.

    that'd require a reasonably constant current source to feed the chain.

    if the resistors were all 1K a 15K resistor might do for a current source
    but it'd compress the output range in a non-linear way to 0.2 - 1.6V

    use 16x 1K resistors instead of mixing sizes (where there are some in series)
    if they all come from the same batch (off the same strip of paper)
    they will more closely matched than 1%.
    at the top just after the 10K resistor.
    typo... I think I forgot to count the ones going to ground.
    this is probably not a good design, none of the experts here
    mentioned enything like it. The could be in awe of my brilliance,
    but I think not more likely they're ignoring me.

    some problem that I see with it are

    1> it wastes electricity (this may not be a problem for you)
    2> it's hard to read, - you need to debounce an analogue signal.
    3> it's hard to maintain - dirty contacts will result in incorrect or
    eratic reads.

    I think their advice to use a micro with more pins is probablly the wisest
  11. RaMbO

    RaMbO Guest

    Thanks for the follow-up on this issue Jasen.

    I'm currently looking into the uC option.
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