[SI-LIST] Re: Undershoot issues

  • From: "PRAJIT S NAIR" <prajitsnair@xxxxxxxxx>
  • To: vani.chandrasekharan@xxxxxxxxx
  • Date: Thu, 23 Aug 2007 15:16:13 +0900

Hello Vani
Let's see it this way...

We all know the relationship V=I  Z

Now in your case you have an undershoot and that means V is going below a
specified level i.e
Change in V means either current (I)  has changed or impedance (Z)  has
changed i.e impedance mismatch.

Change in current (I)  means less current is available i.e
Total Current=Instantaneous Current - Current lost

Current is not lost actually but what is happening is that V is changing
very fast and the system is not able to supply current at that rate. It
means something is resisting the flow of current. Therefore,you have an
inductive effect on your trace. So check your trace length, pin leads, and
also see whether any decoupling capacitors are required or not.

Also can you tell us what kind of termination are  you using on the line?


With Best Regards
Prajit  S Nair










On 8/23/07, vani.chandrasekharan@xxxxxxxxx <vani.chandrasekharan@xxxxxxxxx >
wrote:
>
>
> I have observed this issue in some interfaces at 133MHz rate too,  I
> observe undershoots of 40% compared to the allowed percentage of 27%.
>
> Would you consider this going a little overboard?
>
> Either way thanks a lot for all your valuable insights in this issue.
> Will try and analyze!
>
> Thanks and warm regards,
> Vani
>
> ________________________________
>
> From: art_porter@xxxxxxxxxxx [mailto:art_porter@xxxxxxxxxxx]
> Sent: Wednesday, August 22, 2007 10:24 PM
> To: jory_mckinley@xxxxxxxxx; Vani Chandrasekharan (WT01 - Embedded &
> Product Engineering); art_porter@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Undershoot issues
>
>
>
> Yeah, I missed the part about 50 MHz. ISI should never be a problem at
> that slow data rate (unless the undershoot is really extreme).
>
>
>
> Art
>
>
>
> ________________________________
>
> From: Jory McKinley [mailto:jory_mckinley@xxxxxxxxx]
> Sent: Wednesday, August 22, 2007 10:47 AM
> To: vani.chandrasekharan@xxxxxxxxx; art_porter@xxxxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Undershoot issues
>
>
>
> I have not seen ISI cause over/undershoot at this slow toggle rate but
> another area you may want to check is potential crosstalk pull-in (same
> edges toggling together given a slightly larger forward capacitive
> coupling coefficient between traces).
> -Jory
>
>
>
> ----- Original Message ----
> From: "vani.chandrasekharan@xxxxxxxxx" < vani.chandrasekharan@xxxxxxxxx>
> To: art_porter@xxxxxxxxxxx; jory_mckinley@xxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> Sent: Wednesday, August 22, 2007 9:10:18 AM
> Subject: [SI-LIST] Re: Undershoot issues
>
>
> =20
> Hi Art Porter,
>
> I am curious how ISI is related to undershoot, could you please
> enlighten me on that?
>
> Thanks and warm regards,
> Vani
> -----Original Message-----
> From: art_porter@xxxxxxxxxxx [mailto:art_porter@xxxxxxxxxxx ]=20
> Sent: Wednesday, August 22, 2007 7:18 PM
> To: jory_mckinley@xxxxxxxxx; Vani Chandrasekharan (WT01 - Embedded &
> Product Engineering); si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Undershoot issues
>
> Undershoot could be an indicator of ISI, which could increase the jitter
> and lead to poor bit error rates, so you shouldn't ignore it.
>
> Art Porter=20
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx ]
> On Behalf Of Jory McKinley
> Sent: Tuesday, August 21, 2007 11:08 PM
> To: vani.chandrasekharan@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Undershoot issues
>
> Undershoot may not be a problem in your case(s) depending on the
> amplitude and duration.  Do you have the input specs for the devices
> affected?  Most technologies have a 300mV rating or so DC level above
> and below the supply and some have an AC level spec which allows for
> more of an overshoot/undershoot but only for some period of time. If you
> have a real problem look to better match the trace terminations with
> your trace impedance and/or if budget allows slow down the drivers
> somewhat.  You should be able to do this in simulation since it appears
> you have reasonable lab correlation.
> -Jory
>
> ----- Original Message ----
> From: "vani.chandrasekharan@xxxxxxxxx" <vani.chandrasekharan@xxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx
> Sent: Tuesday, August 21, 2007 11:10:22 PM
> Subject: [SI-LIST] Undershoot issues
>
>
> =3D20
> Hi,
>
> I am observing undershoot alone in my simulation and also in some of the
> on-borad captures of some signals(single ended, less than 50MHz). I am
> not sure how to mitigate this issue and how to rootcause this problem.
> Kindly give me some perspective on this issue and also some options I
> can try to mitigate the same.
>
> Thanks and warm regards,
> Vani
>
>
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