SN74AVC4T774: SN74AVC4T774 long propagation delays

Nir Flysher

Expert 3367 points

Part Number: SN74AVC4T774
Other Parts Discussed in Thread: SN74LV14A, SN74LVC3G17

Dear Team,

I have a design at a customer who is using our SN74AVC4T774RSVR.

They are reporting very long propagation delays (of a few seconds).

If they take it out and use short the circuit works perfectly.

Can you please advise what might have gone wrong ?

Regards,

Nir.

over 7 years ago

0 Emrys Maier over 7 years ago

TI__Guru* 97046 points

Hi Nir,
Can you tell me what their intent is with the voltage divider at the DIR4 pin? That will result in at least 0.3V at the DIR4 pin input, which is not advisable.

Additionally, the inputs are not tolerant to extremely slow signals -- note the recommended operating conditions table with the edge rate of 5 ns/V. Based on the RC values above, they are likely seeing edge rates thousands of times slower than what is allowed, which will cause oscillation, overcurrent, and possibly damage to the device.

0 Nir Flysher over 7 years ago in reply to Emrys Maier

TI__Expert 3367 points

Dear Emrys,

Please Note R3 is not assembled - so there is no voltage divider here.
The purpose of this circuit is to buffer a press button between the external world and the CPU.
Each button press is an open-drain.
Do you believe this signal is too slow for this device ?
Which device would you recommend to use ? maybe a device that have internal pull ups ? (using the same footprint/package)

Regards,
Nir.

0 Emrys Maier over 7 years ago in reply to Nir Flysher

TI__Guru* 97046 points

Typically, a button debounce circuit would be made with a Schmitt-trigger inverter such as SN74LV14A. The logic on the button just has to be inverted.

0 Nir Flysher over 7 years ago in reply to Emrys Maier

TI__Expert 3367 points

Dear Emrys,

Will the SN74LVC3G17 be suitable for button press (open drain) buffering ?

Regards,

Nir.

0 Emrys Maier over 7 years ago in reply to Nir Flysher

TI__Guru* 97046 points

Hi Nir,
Yes the SN74LVC3G17 is a Schmitt-trigger buffer that would work perfectly.

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SN74AVC4T774: SN74AVC4T774 long propagation delays