74HC595 Qh' floater

[PerranOak]

I am creating a PCB involving a 74HC595. I don't really get the function of Qh' - it seems to be the inverse of Qh but ahead of it.

Anyway, I'm not using Qh' (obviously!) so I wondered if it should be left floating or should I use a pull-up/pull-down. It is an output so I don't know.

Cheers.

[oPossum]

It is before the output latch and is used to cascade shift registers. It is an output so there is no need to connect anything to it.

[mrkev]

This allows you to chain multiple shift registers together. Just use Qh' of the first register as input of another, connect all driving signals of both devices and you have a 16b register. This was very handy back in the day, when uC were really expensive and you didn't have enough logic outputs. Now it's mostly used when you wanna drive some slow signalization (f.e. panel of LEDs).

[PerranOak]

Cheers both.

BTW does it need a 100nF cap between Vcc and ground?

[Siwastaja]

BTW does it need a 100nF cap between Vcc and ground?

Very highly recommended unless you have knowledge or measurements to prove doing otherwise works out.