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clock tree quality

Rajesh Vembu

What are the metrics (physical and timing-related) that determine if the clock tree built is robust and of good quality?

If i want to compare the clock tree quality built using 2 different clock tree spec files, what would be the steps to follow?

It should be noted that the AutoCTSRootPin definitions in the 2 spec files could be different, hence a one-to-one comparison of the clock tree report (generated using reportClockTree) won't be helpful.

 - Rajesh

Parents

  • Hi, Rajesh,

    If only judge from the clock tree itself, I will take one with smaller skew.  Before a clock tree exists, idea (zero skew) tree is used during the synthesis and pre-CTS opt. If the clock tree has small skew, the chance is higher that we will not see any big surprise when we check timing after CTS since the actual clock tree is very close to idea clock. 

    The other parameter we need to check is the insertion delay. This should satisfy the front-end requirement. If the insertion delay is too huge, it may make the timing outside of this block hard to meet.

    You mentioned that you have different ctstch files. If you like to compare "apple" to "apple", you could clean up a ctstch file after you have the clock tree built according to it using cleanupSpecifyClockTree command and then, reload a common one by using "specifyClockTree -clockfile the_common_ctstch_file" and report your clock tree (reportClockTree).

    Gele

     

Reply

  • Hi, Rajesh,

    If only judge from the clock tree itself, I will take one with smaller skew.  Before a clock tree exists, idea (zero skew) tree is used during the synthesis and pre-CTS opt. If the clock tree has small skew, the chance is higher that we will not see any big surprise when we check timing after CTS since the actual clock tree is very close to idea clock. 

    The other parameter we need to check is the insertion delay. This should satisfy the front-end requirement. If the insertion delay is too huge, it may make the timing outside of this block hard to meet.

    You mentioned that you have different ctstch files. If you like to compare "apple" to "apple", you could clean up a ctstch file after you have the clock tree built according to it using cleanupSpecifyClockTree command and then, reload a common one by using "specifyClockTree -clockfile the_common_ctstch_file" and report your clock tree (reportClockTree).

    Gele

     

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