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CD54HC4538: Output state for (Rn=H,A=L,Bn=H) not given in truth table

stevenh
Guru 20045 points
Other Parts Discussed in Thread: CD74HC4538, CD54HC4538


Hello,

Please see CD54HC4538 truth table below and datasheet located at www.ti.com/.../cd74hc4538.pdf.

What is the Q and Qn for the following input conditions.


Rn A Bn Q Qn

H   L  H    ?  ?

Stephen

  • 0
    Guru 74080 points

    Since pulse actions are edge triggered and not level triggered, having A static low and Bn static high will output Q low and Qn high.

  • 0 in reply to Dennis Eichmann
    Guru 20045 points

    Hello Dennis,

    How did you come to that conclusion?

    I had difficulty using the Logic Diagram (Figure 2) and the Flip Flop diagram (Figure 1) to come up with an answer because I wasn't sure what some of the components were.  For example, what is the device that has a P and N in the flip flop diagram?  Also, how does the P-channel FET work without the source connected to the substrate?

    Stephen

  • 0 in reply to stevenh
    Guru 74080 points
    This is only a conclusion based on the information given in the datasheet.
  • 0 in reply to Dennis Eichmann
    Guru 20045 points
    Sorry, I was in the process of changing my response when you replied.

    Did you use the logic and flip flop diagram to come to that conclusion?
  • 0 in reply to stevenh
    TI__Guru 62930 points
    Steven, I agree with Dennis's conclusion.
    Since the outputs on Q/Qn are pulses, you can default state on either side of pulse.
    The last row of the truth table shows this.
    When rising edge of A causes a low to high to low pulse on Q. This triggered device to unstable state (pulse), then returns to stable. This is same state as prior to rising edge, ie. when A was low.
    Regards,
    Wade
  • 0 in reply to stevenh
    Guru 74080 points

    Hi Steven!

    Sorry, did not have a chance to respond earlier. I didn't have a look at the flip flop diagram, only the truth table. The device outputs a high pulse on Q (low pulse on Qn) when A is low and Bn has a high->low transition or when Bn is high and A has a low->high transition. This means that having A low and Bn high will not trigger anything, so Q is low and Qn is high. Bn would have to go from high->low to start a high pulse on Q. Similar for Bn high and A low - Q will be low until A transitions from low->high.

    Dennis

  • 0 in reply to Dennis Eichmann
    TI__Guru 62250 points
    Dennis , Wade thanks for your inputs . I agree to both your inputs. This device is edge triggered and not level triggered . when edge triggered the output has a pulse, the duration depends on the RC values .Otherwise , the output settles to the steady state value.