[SI-LIST] Re: NPU / DDR interface bug issue
- From: Chris Johnson <crj@xxxxxxxxxxxxx>
- To: Jean_Pierre.Bouthemy@xxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 10 Apr 2007 10:36:15 -0400
When you say that the DDR is blocked, what do you mean? The interface
is no longer working at all?
It is not unusual for a marginal problem to go away when you load the
lines in question by probing them.
It could be that there is a meta-stable condition that is affected by
the slight change (usually slowing down) of the bus signals caused by
probing.
You can see if probing one particular line causes the problem to go away
by removing probes until it stops working again.
Even with 3" wires, I'm not sure that your interface signals should look
all that bad, so it could be a signal integrity problem as well.
You can try using a resistive probe so that the ground issues are not as
much of a concern.
See http://www.sigcon.com/Pubs/news/5_4.htm .
You could also try spraying any suspect chips with "freeze spray" to see
if the change in temperature also affects the problem.
If a cold chip runs better, that also implies that it is a timing issue.
Good Luck.
Chris Johnson
BOUTHEMY JEAN PIERRE wrote:
> =20
> Hello SI experts,
>
> I have to solve for a DDR interface issue when traffic is full bandwith
> (1Gb/s): DDR is then blocked and we can not access to it anymore.=20
> Note that this "should not be" a card bug because it happens only on a
> few cards, about 2 or 3 protos among more than 100 cards.=20
> FYI, the Controller is an Intel Network Processor IXP2400 and the DDR
> interface is running at 150MHz.=20
>
> On one of the fail cards, I have soldered some wires (about 3" long) on
> a few signals (CS, WE, RAS, CAS, CK, DQS0 and DQ0) and attached 4 probes
> on CK, WE, DQS0 and DQ0. I have launched the full bandwith test on this
> card and observed that it is now running OK for more than 118 hours
> without failing!!=20
>
> The scope signals are very poor on the scope (scope + probe bandwith >
> 6GHz) because of the too long wires and the ground connection which is
> very poor.=20
> But I can not understand why the interface is running OK for more than 5
> days, whereas it normally fails before 2 minutes of full bandwith test!!
>
>
> Has someone already experienced such a case?=20
> Could you please explain if some stubs on signals could make the signal
> integrity better, or ameliorates a timing, or so ...=20
>
> Please remember that only 2 or 3 cards among more than 100 have this
> issue, which means that it "should not" be a design issue.=20
>
> Thank you for your help,
> Regards,=20
>
> Jean-Pierre.=20
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