[SI-LIST] Re: Undershoot issues

  • From: <vani.chandrasekharan@xxxxxxxxx>
  • To: <prajitsnair@xxxxxxxxx>
  • Date: Thu, 23 Aug 2007 12:05:25 +0530

 
Hi Prajit,
 
We are planning for the series termination to mitigate issues of
impedance mismatch between source and the track. Since it is a bi
directianal bus I think I would place the termination near the driver
which is causing this issue.
 
Your argument makes lot of sense. But could nt relate decoupling, is it
for filtering the problem caused by fast rising voltage(high frequency
component??)
 
Please do enlighten.
 
Thanks and warm regards,
Vani

________________________________

From: PRAJIT S NAIR [mailto:prajitsnair@xxxxxxxxx] 
Sent: Thursday, August 23, 2007 11:46 AM
To: Vani Chandrasekharan (WT01 - Embedded & Product Engineering)
Cc: art_porter@xxxxxxxxxxx; jory_mckinley@xxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Undershoot issues


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
<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
<mailto:vani.chandrasekharan@xxxxxxxxx> " <
vani.chandrasekharan@xxxxxxxxx <mailto: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 <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
<mailto: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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