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CD74HC273: Triggering on both the positive and negative edges

Brian Long
Prodigy 20 points
Part Number: CD74HC273

Based on the truth table for this device it should load the contents of Dx to Qx on a positive clock pulse.    The issue that I am having it that Dx gets loaded into Qx every positive and negative clock pulse.  I have a 10k + 1uF going to the clock pin to ensure the data is there before the clock pulse.  The frequency of the clock is around 1Hz but i don't see a minimum frequency spec for the clock so I assume it is okay.  Does anyone have any insight to why the device is triggering on both the positive and negative edges?

RESET CLOCK CP DATA Dx Qx
L X X L
H UP H H
H UP L K
H L X Q0
  • 0
    Guru 271980 points

    The clock triggers only on positive edges. I suspect that you clock input is bouncing, i.e., that there are multiple edges. Look at it on an oscilloscope with some nanoseconds time base.

    Does the clock come from a button, or through a long line? You might be able to add a small RC low-pass filter.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 44540 points

    Hi Brian,

    As Clemens has stated, there might be bouncing on the clock line. I see you've added an RC to the D input and recommend caution when doing so as you will be violating the recommended transition rate spec. This can also be the root cause of the issue as slow edges can cause erroneous data at the output of clocked devices.