CD74HC4094: Qx outputs when STR is High.

Atsushi Takahashi

Part Number: CD74HC4094

Tool/software:

Hi Team,

I understand that Qx outputs when STR is High in this device.
However, Qx also outputs when STR is Low.
Please see the attached waveform.

This behavior occurs randomly.
It has been observed in two products applied PCN#20220628000.1.
It has not been observed in products prior to PCN.

The customer do ABA swap, this behavior moved to other board.

I suspect it may be failure, is there anything that you think might be the cause in how they are using it?

Best Regards,

0 Clemens Ladisch over 1 year ago

Guru 316920 points

Please show the schematic.

The rising edges on QX1 and QX2 are very slow. This looks as if the output is disabled, and is pulled high only with a pull-up resistor.

Please zoom in on the time when STR goes low. How long is the interval between STR going low and CP starting again?

0 Atsushi Takahashi over 1 year ago in reply to Clemens Ladisch

Expert 1415 points

Hi Clemens-san,

Thank you for your reply.

I send you the schematic.

The six ICs on the left are CD74HC4094.

They don't use external pull-ups.

I'll contact you with additional waveform information as soon as it is available.

Could you please give me your opinion on the schematic in advance?

0 Clemens Ladisch over 1 year ago in reply to Atsushi Takahashi

Guru 316920 points

There are no obvious errors in the part of the schematic you've shown.

I suspect that there is a problem with the input signals; a higher-resolution oscilloscope trace might help.

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