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SN74LVC1G08DCKR Issue About Ringing Sgnal From PCIe Reset

Kuo Nowal
Expert 2315 points
Other Parts Discussed in Thread: SN74LVC1G97, SN74AHC1G08, SN74LVC1G57, SN74LVC1G58

Hi Sir,

We found the following waveform on our board. Do you have any explanation ? M/B should not have such ringing signal from PCIe reset.

We couldn’t find same wave form on NXP 74LVC1G08GW.

Signal Measurement :

Yellow is PCIe reset signal from MB.

Blue is PERSTB_BUF, the output of AND gate.

Condition1: MB: BIOSTAR B150S1 D4, U4513: TI SN74LVC1G08DCKR, C5060:0.1uF

Condition2: MB: BIOSTAR B150S1 D4, U4513: NXP 74LVC1G08GW, C5060:0.1uF

  • 0
    Guru 317060 points

    Most devices in the LVC family do not have Schmitt trigger inputs. With that fat capacitor on the PERSTB line, the input signal is much too slow.

    For some reason, NXP decided to add Schmitt triggers to their 74LVC1G08. This means that these two devices are not compatible.

    If you can redesign the board, use the SN74LVC1G97, where the clones of all manufacturers have Schmitt triggers.

    Alternatively, try the SN74AHC1G08; the AHC family has a little bit of Schmitt trigger action in its inputs.

  • 0 in reply to Clemens Ladisch
    TI__Guru 62180 points
    Thank you , Clemens for your contribution .
    Kuo ,
    I am not qualified to comment on the NXPs part , however,the NXP has schmitt trigger action on the inputs according to the datasheet which is not the same as schmitt trigger inputs . The input transition rate on the ROC table is the same for both devices.
    With that said , you could try the SN74LVC1G57,SN74LVC1g58,or SN74LVC1G97 which have schmitt trigger inputs and are tolerant of slow rising edge signals.
  • 0 in reply to ShreyasRao
    Expert 2315 points

    Clemens & Shreyas Thank you for your explanation.