SN74LV74A: How to set Tdelay for ^CLR?

Kotaro Yamashita

Part Number: SN74LV74A
Other Parts Discussed in Thread: SN74HCS74

Hi Team,

I think SN74LV74A will enter unstable when PRE = CLR = H at start up. To avoid this, I would like to add some delay circuit at CLR pin.

- Is my understanding correct?

- What is the recomended value for delay at CLR?

Regards,

Kotaro Yamashita

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 316690 points

If the flip-flop is not initialized, then the output state is stable, but unknown; see [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register)

You can use a delay circuit to pull CLR high after a certain amount of time. However, you must take care to not exceed the Δt/Δv specification; if you're using a RC delay, you might need to add a buffer with a Schmitt-trigger input.

0 Karan Kotadia over 5 years ago in reply to Clemens Ladisch

TI__Genius 16735 points

Hi,

As Clemens has mentioned, the output is unknown, not unstable.

Here is a logic minute video on a flip flop, it has an example circuit on the CLR pin of a similar form that you are looking at. Can I also suggest using the SN74HCS74? With this device having schmitt trigger inputs, you will not need this external buffer but will be able to solve this issue with just the RC passive components.

https://training.ti.com/convert-momentary-switch-toggle-switch?context=1134826-1139263-1136772

Thanks!

-Karan

0 Kotaro Yamashita over 5 years ago in reply to Karan Kotadia

TI__Intellectual 2530 points

Hi Team,

Thank you for your help. Is there any specific timing recommendation for PRE(or VCC) to CLR?

Regards,

Kotaro Yamashita

0 Karan Kotadia over 5 years ago in reply to Kotaro Yamashita

TI__Genius 16735 points

Hi Kotaro,

Sorry for the delayed response. The recommendations for the PRE and CLR and mentioned in the Video. You can tie PRE to Vcc directly and the other, you will typically want to have a schmitt trigger buffer driving the CLR with the delay to avoid having a slow signal on your input. This is why I recommend the HCS family part as it has built in schmitt trigger buffers on the input.

Thanks!

-Karan

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SN74LV74A: How to set Tdelay for ^CLR?