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74HC299 resetting when I clock in a 0 bit (Help, I'm at my wits end here)

Discussion in 'Electronic Basics' started by Anthony Fremont, Mar 2, 2005.

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  1. Hello all,

    I'm tinkering with a 74HC299 and I'm having a bit of trouble. I am just
    using it as a serial-in/parallel-out shift register. Whenever I clock
    in 1 bits, all is fine. As soon as I clock in a single 0 bit, all the
    output registers reset to 0 simultaneously as if I had driven the reset
    pin low.

    I have both OE pins tied low as I don't need the output pins blanked as
    I set them up (maybe this is the problem?)
    The reset(clear) pin is tied permanently high
    I've tried left shifting *and* right shifting with identical results
    Both serial input pins (left and right) are pulled up (I've tried them
    pulled down too)
    The cascade outputs are floating
    ..1uF and 10uF bypass caps on power pins

    The datasheet is not real descriptive, but I think I got it hooked up
    right. I even tried another chip with the same results. Obviously I'm
    doing something stupid here, but I just don't get it. :-(

    In summary: As long as I'm clocking in 1 bits, all is cool. The first
    0 bit resets all the outputs. I hope this sounds familiar to somebody.
  2. I'd guess that your clock is bouncing for some reason. Are you using a
    button to clock it? You are probably getting hundreds of clock
    transitions, so the single 0 you tried to clock in gets clocked into
    every cell.

    If you have a schmitt trigger buffer (or inverter) try using that
    between your clock and your shift register. Or, just debounce the button
    like this:

    | |
    | .-.
    --- | | 100k
    0.1uF--- | |
    N/O | '-'
    Button | |
    | |
    | | |
    --- | |
    .---o o-------o-----o--------- Out
    (created by AACircuit v1.28 beta 10/06/04

    The other thing to check is that you have Vcc connected. You can power
    cmos through inputs, which has confused me before. Seems to work until
    you change a data input.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  3. I'm using the SQW output (1Hz square wave) of a Dallas 1307 real time
    clock chip. That signal is also driving a PIC RB0 interrupt line and
    that is all working fine generating 1 interrupt per second, so I suspect
    the clock signal is ok. Plus, clocking in 1 bits results in the correct
    operation of the first output turning on followed by the next on the
    next clock tick etc... But as soon as I apply a 0 bit and clock it in,
    the outputs all reset. It's too weird, I know it's something dumb, but
    I've checked and rechecked every pin (all 20) I am manually switching
    the data input pin (it has a weak pullup resistor) by attaching a wire
    to ground, but the clock signal should be good.
    BTDT ;-)) I thought there were ghosts in my PIC. But this does
    describe the precise behavior that I am seeing. Right now I'm using a
    pull-down on the data input pin. When I put a jumper wire from the
    input pin to Vcc, 1 LED lights each second in the proper sequence
    exactly as it should. As soon as I remove the jumper letting the pull
    down do it's job, the outputs all reset. Vcc is verified at +5V (pin
    20) and Gnd (pin 10) is at ground potential. I can tell that they all
    reset precisely when the rising clock occurs clocking in the 0, not when
    the data changes.

    Thanks for the help.
  4. Alfred Gemsa

    Alfred Gemsa Guest

    What's about pin 1 and 19? If they are both high the next clock would do
    a parallel load.
  5. One is high, the other is low. I've tried both combinations (shifting
    left and shifting right) with equal results.
  6. Turned out to be too weak of a pull-up on the clock line (100K) 10K was
    too weak still, but 1K did it. The shift register needs rise and fall
    times <500nS at 4.5V Vcc. Apparently the PIC chip is much more
    forgiving on that.

    Thanks for everyone's help.
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