Rising edge false positives

Discussion in 'The Projects Forum' started by greg.brabeck, Sep 5, 2012.

  1. greg.brabeck

    Thread Starter New Member

    Sep 5, 2012
    I'm using a synchronous 4-bit binary counter (http://www.ti.com/lit/ds/symlink/sn74ls193.pdf ) to keep track of pulses from an external clock. It's pretty straightforward: count down from a set number to zero then reset. And it works great, unless I want to use any 1s in my set number. When the set value corresponding with a certain bit is high, that bit occasionally triggers on a falling edge in addition to triggering on the rising edge which causes a miscount.

    The yellow trace is external clock. The blue trace is the output of the first bit of the counter when counting down from 0100, and it should look like this:

    But when the input for that bit is high (e.g. 0101) is randomly looks like this:

    So what's going on here? Have y'all ever seen this before?

    Thanks for your help!