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D flip-flops SN74HC74 unused pins termination

Xiaoxiao Zhang
Prodigy 85 points
Other Parts Discussed in Thread: SN74HC74

Dear experts,

I am using D Flip Flop SN74HC74. There are two D Flip Flops inside one chip. However I only need to use one. What should I do with the unused pins?

Datasheet said all unused inputs of the device must be held at Vcc or GND to ensure proper device operation.

Can I just tie all pins directly to ground or do I need to pull them down through pull down resistors?

For each pin PRE', CLK, D, CLR', Q, Q', should I tie them high or low? Do I need to use pull up or down resistors?

Thanks,

Best Regards,

Xiaoxiao

  • 0
    TI__Guru 62040 points

    Hi Xiaoxiao,

    We always recommend to have unused inputs tied to some known logic level instead of being floating . There are consequences of having it floating on the device by going to unknown state which might start pulling large current affecting the device negatively . I have attached an app note discussing the same .

    A pulldown resistor on PRE , CLK,D can be pulldown to gnd using pulldown resistor & CLR can be tied high using pullup resistor to Vcc to avoid race conditions on the outputs . Q and Q' can be left open .

    4760.scba004c_slownfloatingCMOS.pdf