CD4094 Shift register STROBE problem

Discussion in 'Digital Circuit Design' started by Sensacell, Sep 14, 2017.

  1. Sensacell

    Thread Starter Moderator

    Jun 19, 2012
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    I am working on a system using the CD4094BPWR CMOS shift register as a multiplexer / channel selector.
    The shift register is supposed to select one channel at a time.

    The system uses 5 boards in cascade, making a 40 bit shift register.
    A master CPU clocks in data to enable the outputs, one at a time.
    The trouble is that the outputs do not remain stable with the STROBE LOW, they should not change when STROBE is LOW!!
    Scope Channel 1 is the STROBE (yellow)
    Scope Channel 4 is the CLOCK (green)
    Scope Channel 3 is the DATA (purple)
    Scope Channel 2 is Q1 of the FIRST shift register in the cascade. (turquoise)

    Shift_reg_trouble.png
    The scope image shows two bursts of 40 pulses, the first setting channel 1 LOW, followed by another burst setting channel 40 LOW.
    Channel 2 ( output #1 ) should stay LOW for until the second STROBE pulse, but it goes HIGH after 2 more clocks!!!!???

    Pulling my hair out trying to understand how this could be happening?
    When the STROBE line is solidly LOW, there should be NO change in any output state!!!???
    Googling around, I found one other reference to this problem, but no solutions.

    I need 12 V logic output, so I cannot sub the CD74HC4094PWR.
     
    Last edited: Sep 14, 2017
  2. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Is this happening with the first output only?

    EDIT/ You say "outputs". Sorry /EDIT
     
    Last edited: Sep 14, 2017
  3. Sensacell

    Thread Starter Moderator

    Jun 19, 2012
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    I think I figured this out, it's always some little thing...

    Since the board is all SMT and the chip is a TSSOT package, it's really hard to probe the tiny pins, I has soldered wires to the 120 ohm resistors that are in-line with the main signals to the shift register so as to have something to hang my scope probes onto.

    Long story short, I think the resistor going to STROBE cracked from the stress of the wire soldered to it, I was probing the input side, while the other side was intermittently floating, causing mayhem.
     
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