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overshoot/undershoot on reset signal of AM5728

Michi Yama
Guru 10030 points
Other Parts Discussed in Thread: AM5728

Hi,

I have one question regarding overshoot/undershoot on reset signal of AM5728.

Please see the below pictures.

   

This reset signal is input to POR(F22 pin) and RTC_POR(AB17 pin) of AM5728.

As you see, undershoot is -1V/ 6ns approximately. And overshoot is 1.2V/6ns approximately.

According to the table 5-1 on data manual of AM5728, Vio(Transient Overshoot/Undershoot) is 0.2 *VDD (max).

The above overshoot is over the spec(Absolute Maximum Rating). But transient time is short. Is this problem?

Also how about undershoot?  The undershoot is not written in data manual of AM5728.

Please advise me.

Best regards,

Michi

  • 0
    TI__Guru**** 393215 points
    0.2 *VDD (max) is absolute value, valid for both overshoot and undershoot. There is also a note that says that this is valid for up to 20% of the signal period.
  • 0 in reply to Biser Gatchev-XID
    Guru 10030 points
    Dear Biser-san,

    Thank you for your quick reply.

    I don't know the meaning "this is valid for up to 20% of the signal period."
    What is 20% signal period for reset signal? I think "period" is time, not voltage. What is the 20% of "reset" signal?

    Please advise me.

    Best regards,
    Michi
  • 0 in reply to Michi Yama
    TI__Guru**** 393215 points
    I have asked the factory team for clarification.
  • 0 in reply to Biser Gatchev-XID
    TI__Guru 62875 points

    Michi,

    I have seen similar overshoot and undershoot measurements that do not represent what is actually happening on the signal. The reset signal may have a lot of inductance which causes real overshoot and undershoot, but it could also be caused by the scope probe. The inductance of the scope probe may falsely represent the signal waveform if you used an old passive scope probe with a long ground wire or no ground wire. I can see you are using a scope with a lot of bandwidth, but it is not clear how you were probing the signal. I recommend using a high-bandwidth, low-capacitance, single-ended FET probe with a very short ground or a high-bandwidth, low-capacitance, differential probe connected across the signal and signal ground.

    Regards,
    Paul

  • 0 in reply to peaves
    Guru 10030 points
    Dear Paul-san,

    Thank you for your advice.

    This overshoot/undershoot issue is for my customer with AM5728. And they use FET proble with short ground. So I think probe issue is very few.
    My customer knows device spec. But they would like to know the ability level of AM5728. Of course I know, TI can't gurantee device reliability when using with out of specification. But customer would like to know AM5728 real performance as a reference.

    Please adivse me.

    Best regards,
    Michi