SN74LVC74A: Input transition rise and fall rate questions

John Liu59

Part Number: SN74LVC74A
Other Parts Discussed in Thread: SN74HCS74, SN74HCS74-Q1

Dear all,

My customer has below question for SN74LVC74A, for the delta t/ V 10ns/V（input transition rise and fall rate), is it for all input pins including CLR,CLK, PRE,D?

Thanks,

over 5 years ago

0 Karan Kotadia over 5 years ago

TI__Genius 16725 points

Hello,

This spec is listed to avoid slow rise or fall times of any signals at any of the CMOS inputs. The datasheet is saying that any input should not be slower than 10ns/V or it will have some side effects. So yes, if any of the inputs you have mentioned (CLR,CLK, PRE,D) are slow or floating, there will be some side effects. I can recommend the SN74HCS74 which allows for slower inputs since it has schmitt triggers on all of its inputs.

Here is an FAQ on side effects of slow or floating inputs: https://e2e.ti.com/support/logic/f/151/t/737694?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

Thanks!

-Karan

0 John Liu59 over 5 years ago in reply to Karan Kotadia

TI__Intellectual 2735 points

Hi Karan,

Thanks for your feedback, I searched on TI website, only find the SN74HCS74-Q1, is there any commercial version for this part, my customer application is sever, industrial application.

Besides, is there any one channel available?

In the application, only one channel is used, for the unused channel, will the pins should be connected to the GND?

Thanks.

0 Karan Kotadia over 5 years ago in reply to John Liu59

TI__Genius 16725 points

Hi,

The SN74HCS74-Q1 is a newly released part. I cannot tell you the timeline for the commercial version so I think your best option is to use the Q1 version. We do not have any other flip flops with schmitt trigger inputs.

If you don't want to use the Q1 version then you can use schmitt trigger buffers and place them at the inputs of any of the channels you plan to drive slowly.

As for unused inputs: https://e2e.ti.com/support/logic/f/151/t/741195?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

You can connect the input directly to Vcc to Gnd, while you can leave the outputs floating.

Thanks!
-Karan

0 John Liu59 over 5 years ago in reply to Karan Kotadia

TI__Intellectual 2735 points

Thanks for your suggestions, Karan.

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SN74LVC74A: Input transition rise and fall rate questions