SN74LVC74A: Can't correctly trigger the flip-flop

Yue TANG

Part Number: SN74LVC74A
Other Parts Discussed in Thread: SN74HCS74-Q1, SN74LVC1G97

Hi expert,

Below is the schematic of my customer. positive edge can't correctly trigger the flip-flops, but used a signal generator as a input to pin3 '1CLK', can work. They also tried to reduce the R791 from 100Ω to 0Ω， it also works, but don't know why? please help to give your comments. thanks.

1. Schematic

2. Directly inject the signal to 1CLK(Pin 3) with SG, work normally.

Blue: CLK signal

purple and Yellow: Q and inverted Q

Actual circuit test. the same signal as above SG , but can't trigger the flip flop

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 267780 points

The schematic shows a different device.

LVC devices require fast edges at all inputs (except when they are explicitly designed to have Schmitt-trigger inputs). Remove C936. (What is its purpose?)

0 Yue TANG over 4 years ago in reply to Clemens Ladisch

TI__Expert 8565 points

Hi,

1. I‘m sure it's SN74LVC74A. Just used the symbol of LS74

2.according to customer said, RC is for filter purpose.

3. From above my screenshot 2,3, looks like the edge is same. but one worked[signal generator], one failed. Could you please comment on this? thanks.

0 ShreyasRao over 4 years ago in reply to Yue TANG

TI__Guru 57360 points

Yue,

Please consider using SN74HCS74-Q1 device ()

This has Schmitt trigger inputs to tolerate slow rising edge signals generated by the RC filter.

The transition rate spec for LVC is 10ns/V. The scope shots have 20ms time scale which masks the slow edges at the CLK pin. Also remove the C2682 100nF cap on the CLR pin.

Refer to the white paper on solving slow rising edges using schmitt trigger inputs:

www.ti.com/.../slla364a.pdf

0 Yue TANG over 4 years ago in reply to ShreyasRao

TI__Expert 8565 points

Thanks for your reply. It's helpful.

One more question, does TI have a P2P device with 74AHC1G08 which can support the schmitt input?

0 Emrys Maier over 4 years ago in reply to Yue TANG

TI__Guru* 96681 points

Hello,

Using the parametric search on our website (http://www.ti.com/logic-circuit/gate/and-gate/products.html#p776=Schmitt-Trigger), I found that there are only 4 AND gates available with Schmitt-trigger inputs. None are single gates.

0 ShreyasRao over 4 years ago in reply to Yue TANG

TI__Guru 57360 points

Yue Tang,

I do not believe that the AHC1G08 needs to have Schmitt trigger inputs capability since there are no RC on the input pins.

In case the schmitt trigger input AND gate is required, then consider using a configurable gate like the SN74LVC1G97 which have schmitt trigger inputs and configured as AND gate.

It has 6 pins, however, not P2P as the gnd / Vcc will be different.

More options below filtered to SOT/ SC70 packages.

Logic

Logic forum

SN74LVC74A: Can't correctly trigger the flip-flop