Flip flop Circuits: Initialization Questions

Thread Starter


Joined May 19, 2020
Hello everyone. I'm constructing a T flip flop from SN74HCS74, using RC to set the Q output low at startup. I think it works to have RC directly at the clear input in this case because the device has Schmitt trigger inputs.

I also need a monostable, so I'm thinking of saving a few parts by doing it with the other half of the 7474. Problem is, the Q output also needs to be low at startup, and already has an RC circuit at the clear input for monostable operation. Can I use something like a simple diode AND gate to override any inputs or an undefined startup condition?



Joined Jun 8, 2013
This should work.
U1B is self initializing. It will initialize to Qbar high on power up.
The RC values i've shown are random but you can use substitute the values you want to use.



Joined Jun 8, 2013
Thanks. Why do you want C2 at Vcc and not ground?
Because after startup Qb will be high.
Qb will immediately go low after the first rising clock edge, causing C2 to begin charging while CLR pin remains high (C2 will look like a short circuit to VCC for a few milliseconds allowing Q to remain high). As C2 continues to charge, the logic threshold of the CLR pin is exceeded and the FF will reset. This is what produces the "one-shot" behavior.