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Clock tree synthesis (ccopt): where set_clock_latency has been defined?

Clidre

Hello,

for my clock tree synthesis, I use the ccopt flow, and I source the following commands:

setCTSMode -engine ccopt

create_ccopt_clock_tree_spec -file ccopt.spec

The .spec file should be based on my .sdc. However, in the generated .spec file I can read something like the following:

# Clock tree offset inferred from set_clock_latency -source assertions:
set_ccopt_property source_latency -delay_corner corner_max_125c -late -rise -clock_tree CK_4 -0.273
set_ccopt_property source_latency -delay_corner corner_max_125c -late -fall -clock_tree CK_4 -0.276
set_ccopt_property source_latency -delay_corner corner_min_m55c -early -rise -clock_tree CK_4 -0.125
set_ccopt_property source_latency -delay_corner corner_min_m55c -early -fall -clock_tree CK_4 -0.126

However I don't have any set_clock_latency command in my .sdc!!!

Where do these numbers come from? And why they're negative?

I hope you can help me understand. Thanks!!!

  • TungVo

    Hi you,

    Can you take carefully a look your sdc if there is a set_input_delay for clock ports. If yes, tool will convert this setting to set_clock_latency for those clock ports.

    " why they're negative?"

    I hope the below will help you.

    set_clock_latency -0.2 CK (-0.2 is pull up clock)

    when create_ccopt_clock_tree_spec then It will be

    set_ccopt_property insertion_delay 0.2 -pin CK

    Best,

    TungVo

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