[SI-LIST] Re: SQ Question.
- From: "Todd Westerhoff" <twester@xxxxxxxxxxx>
- To: <signalguy@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 14 Mar 2002 12:59:51 -0500
Josna,
Actually a very good question, and one for which the answer is not at all
obvious. This threw me for a loop the first time I tried it, too.
Forget about source-sync for a moment, and imagine that you were writing
custom measurements for a topology with a custom stimulus. For sake of
argument, let's assume it's a 200 MHz DDR signal that used the stimulus
pattern:
001101000010001111000
where each data bit represents a 2.5 ns time slice (the data width for 200
MHz DDR). You set up the measurement, tell SigXp to measure the 6th cycle,
run the simulation, get an error message, and wonder why -- after all, there
are more than 6 cycles, right?
Well, no. It turns out the SQ measurement system delinates the beginning
and ending of each cycle by looking at when the receiver crosses through the
thresholds. So, in this example, the measurement system sees the following
transitions:
0 - 1 - 0 - 1 - 0 - 1 - 0 - 1 - 0
... or 4 cycles, not 6, or whatever. The duty cycle, of course, is all over
the place, but the system only sees 4 measurement cycles. Hence, the
measurement fails, because the system doesn't see enough complete
transitions at the receiver. Although it seems unusual at first, the system
works this way primarily because it provides the flexibility of having the
flight time be longer than the data period.
Bottom line, with a custom stimulus, the number of measurement cycles at the
receiver are dependent on the stimulus pattern you use, not how many "clock
cycles" the simulation runs for.
As others have observed, you can get around this by also simulating a clock
or strobe signal as part of the same topology, and using that signal as the
basis for counting the cycles at the receiver. The technique works because
the clock (strobe) is a regular signal, and provides the cycle boundaries
you expect.
Hope that helps (and makes sense!)
Todd.
Todd Westerhoff
Signal Integrity Engineer
Hammerhead Networks
5 Federal Street - Billerica, MA - 01821
email:twester@xxxxxxxxxxx - ph: 978-671-5084
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of fname lname
Sent: Thursday, March 14, 2002 11:15 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SQ Question.
Hi folks,
I am doing a source synchronous simulation. I am using custom measurements
feature in SQ. I have written custom measurements for the setup and hold
time measurements. when I run the simulation I get the warning " wave form
has less than the required 10 cycles" since I am measuring the setup and
hold for 10 cycles at the reciever. when I look at the wave form the
simulation is running for more than 10 cycles. Also I am not getting any
values for the custom measurements I have written. All I get is NA for all
the columns. I would really appreciate if some one can help me find the
source of the problem.
Thanks,
Josna.
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