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Constraint set up for delta and tolerance

jemarods over 12 years ago

Hi all,

What is the difference of setting up a delta of 0 , tolerance of 50.... (so 0:50)

with

no delta, tolerance of 100...... (so :100)

since in my case, a target appears if a delta is used.

follow up question:, how does constraint manager identifies a target net....?

Thanks in advance!!

oldmouldy over 12 years ago

OK, let's say you have a 16-bit bus, a tolerance means that the length, in this case, of all 16 must be within the specified tolerance +/- 100 (:100) with no particular preference to any of the nets, called a "matched delay". As you found, a delta means that ONE of the nets is picked as a target, typically the longest unrouted but PCB Editor takes a "guess" at this and you can use right-click>Set as Target in CM if you want another target, you then get the delta and tolerance, delta could be 0, applied to the other 15 nets, with respect to the target, called a "relative delay". (See cmug.pdf, doc\cmug in the installation, page 56 for 16.6 for the description)
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jemarods over 12 years ago

Thanks again sir...
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mcatramb91 over 12 years ago

I wanted to clarify a couple things:
Setting the Delta:Tolerance field to 0.000mil:100.000mil means the longest net in the group could be 100mils longer than the target net and the shortest net in the group could be 100 mils shorter than the target. So across all the nets you could have a variance in length as much as 200mils before seeing a DRC. (+/-100mil)
Setting the Delta:Tolerance field to :100.000mil (blank Delta field) means that all the nets could have a variance in length as much as 100mils with no specific Target. This is sometimes harder to match the members and could cause you to overly lengthen the nets.
Hope this helps,
Mike Catrambone
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Dennis Nagle over 12 years ago

You might want to grab a copy of the following presentation from CDNLive in 2008:
8ICP12 : Demystifying Relative Propagation Delay Constraints
https://www.cadence.com/cdnlive/library/documents/2008/Silicon%20Valley/8ICP12_Dennis%20Nagle.pdf
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jemarods over 12 years ago

Thanks Mike and Dennis..
Finally figuring this out. Though sometimes I encounter this problem:
When I set delta:tolerance, let's say, 0:50 which also means :100 (pls correct me if i'm wrong), sometimes at 0:50 setting, constraint manager reflects red colors. I think, the problem with this is the random picking of the target.
BR
jes
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