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Inquiry regarding suggested pull-up resistance on SN74HVD08 receiver side

Atsushi Okui
Genius 4015 points
Other Parts Discussed in Thread: SN75HVD08

Hello, all

Now we have one inquiry regarding suggested pull-up resistance on SN74HVD08 receiver side from our customer.

Please refer to the item below, and feedback us with your comment.

They have one issue when using 48kohm pull-up resistance on "R" signal, which results in forcing "R" signal into LOW even though RE/=HIGH.

Actually this 48kohm pull-up resistance is configured into FPGA which receives the "R" signal from SN74HVD08.

Please let us clarify which pull-up resistance would be suggested for "R" signal line.

We thank you in advance for your information.

Best regards,

  • 0
    TI__Expert 5400 points

    Atsushi-san,

    If possible, please share a schematic and scope shots showing customer's issue.  As described, with 48kohm from R to VCC, the circuit should operate normally when the receiver is enabled, and R should pull high to VCC when the receiver is disabled.  And indeed I checked this on the bench for confirmation.

    In general, 48kohm sounds a bit high (perhaps replace with 5k or 10kohm), but should still work from the perspective of the SN75HVD08.  At this point, my guess is that there is some characteristic of the FPGA input that is not understood which is causing the unexpected behavior.

  • 0 in reply to Bart Stiller
    Genius 4015 points

    Bart-san,

    Thank you for your reply.

    Our customer could find the root cause on this issue, as the pull-up setting inside of FPGA was not correct which results in causing above issue.

    After setting correctly, this issue was resolved.

    Best regards,