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CD74HC4040: Q, Q_bar, and Q'?

Alex Naylor
Prodigy 90 points
Part Number: CD74HC4040
Other Parts Discussed in Thread: SN74HC4040

I'm using CD74HC4040 in a design and in the datasheet I noticed that the first output is different than the rest: It has an apostrophe after its name (Q1'). Initially I thought that this might indicate that it's active low, but I don't see a reference to its operation elsewhere on the datasheet. Looking at the block diagram, I see that the first flip flop has 3 outputs: Q, Q', and Q_bar. What is the meaning of Q'? Is it active high or low?

  • +1
    Guru 317080 points

    The value of Q' is the same as that of Q. This appears to be an implementation detail that you do not need to care about.

    There are separate switching specifications for the first output. But the SN74HC4040 datasheet also has these specifications for QA without needing to introduce an apostrophe.