SN74LVCH244A: We've encountered CLK and LD signal coupling issues with SN74LVCH244APWR.

Lawrence He

Genius 3115 points

Part Number: SN74LVCH244A
Other Parts Discussed in Thread: SN74LV244A

Hi, Support Team

We've encountered CLK and LD signal coupling issues with SN74LVCH244APWR.

schematic:

Issue:
Shifty_SGPIO_CLK_R and Shifty_SGPIO_LD_R have coupling as below chart:

And when we cut signal from SHIFT_CLK_R, we can see coupling signal on Shify_SGPIO_CLK_R.

if any suggestion, Please advise me.

Thanks,

Best regards,

Lawrence

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 316690 points

What is the placement of the 33 Ω source termination resistors? Can you show the board?

0 Lawrence He over 2 years ago in reply to Clemens Ladisch

Genius 3115 points

Hi, Clemens

sorry. we cannot publish the pcb board picture, that is confidential.

Could share your mail addr?

if any, Please advise me.

Thanks,

Best regards,

Lawrence

0 Clemens Ladisch over 2 years ago in reply to Lawrence He

Guru 316690 points

The source termination resistor should be as near the output pin as possible.

0 Lawrence He over 2 years ago in reply to Clemens Ladisch

Genius 3115 points

Hi, Clemens

The SHIFTY_CLK is from edib board dcpld to fan board.

edib board:

fan board:

On the broad file, R202 is close to connetor J_SIG_1. And r29, r512, r31, r40, r34, r513 ,r36 is close to U1

if any suggestion, Please advise me.

Thanks,

Best regards,

Lawrence

0 Clemens Ladisch over 2 years ago in reply to Lawrence He

Guru 316690 points

SHIFTY_CLK does not matter; the buffer isolates the inputs from the outputs.

These spikes are caused either by coupling between the traces, or by noise on the power supply. The termination resistors look OK, so it's probably not the first.

Power supply noise is a result of the switching noise inside the buffer. Ensure that the decoupling capacitor (C16) is near the supply pin. There is nothing else you can do, except maybe replacing the buffer with another device with less switching noise (e.g., SN74LV244A, without bus-hold inputs).

Logic

Logic forum

SN74LVCH244A: We've encountered CLK and LD signal coupling issues with SN74LVCH244APWR.