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CDC functional checks are not validated for design.

vaizguy

Hello everyone, I was recently required to perform both structural CDC checks and functional CDC checks on a design but when I go through the reports I noticed that the structural checks are performed fine but the functional reports simply say that the paths are not validated. I would like to know why these functional checks aren't being performed as there are no warnings in the log indicating why. Even the GUI has a question mark next to the functional checks tab. while the structural checks has a green/red dot next to it indicating pass/fail resp.

The tool is unable to detect direct bus synchronization instances like the following example and I was hoping that performing functional checks especially the single_bit_change check would highlight all such paths.

Eg.
input [3:0] A; // In clkA domain.
reg [3:0] B_r;
output reg [3:0] B_rr; // In clkB domain.

always @(posedge clkB) begin
  B_r <= A;
  B_rr <= B_r; // This is fine when A is a single bit.
end

Thanks in advance.

  • Hi Did you have functional checks enabled? They're off by default since it's recommended structural checks be cleaned up first. Chrystian

  • Hi Chrystian, Thanks for the quick reply,
    When you say "enabled" do you mean added , as I added the CDC structural checks, validated them and only then did I add the functional checks with the following command;
    add cdc check -functional... (and then reported them)
    If this is not enough to enable functional checks would you please tell me about the functional checks flow. Also when you mean structural checks cleaned up, does it mean that until I get zero path failures on structural checks I cannot perform functional checks or will functional checks be performed on the CDC paths that passed the structural checks and failed CDC paths ignored. Thanks,

  • I assume you are using the cdc checks in conformal lec, if this is the case then structural checks must be performed (and pass) first before functional checks.  This said, if you run functional checks before the structural checks have passed (but have been run) I believe the tool will ignore any crossings that are failing structural checks.

  • Okay, I understand that the functional checks will not be performed for CDC paths that have failed structural checks but in this case there seems to be functionally unvalidated CDC paths which cleared structural checks, shouldn't these paths be validated for functional checks?
    The problem I'm facing is that even though there are a lot of CDC paths that cleared structural checks there seem to be no paths that are Put through functional checks, when I get the functional checks reports they contain all the paths, under which the different functional checks are reported with the phrase "Not validated" next to them, and the final validated data summary just mentions the path totals which are fine, and the path fails/passes which are both 0.

    Thanks, PS: yes I am using Conformal lec
  •  Your dofile should look something like this:

    set log file cdc.log -replace                                      

    set flatten model -noncontrolling_remodel

    add search path rtl

    read design rtl/fsm.v s_d_data.v sync.v top.v

    set clock_domain rule -extract -noderived

    add pin constraint 1 rstan

    add pin constraint 1 rstbn

    add clock 0 -waveform 1 1 2 clka

    add clock 0 -waveform 3 3 6 clkb

    read initial state init.seq

    set system mode verify

    add sync rule dff -dff 2 -sync_chain buffer single_chain -cdc_path buffer single_destination

    add sync rule mux -mux 0 0 -hold -cdc_path buffer single_destination

    add cdc check -structural -source -all -destination -all -from -all -to -all

    validate

    add cdc check -functional -source -all -destination -all -from -all -to -all

    validate

     

    Is this what you basically have?

    Let me know.

    Regards,  Clay

  •  Thanks for the example Clay, I was missing the validate statement after adding the functional checks, the problem was solved on adding the statement. I had removed it due to an error i was getting ealier with Clock analysis failure but now the flow seems to work just fine.

     

    Thanks again.

    Regards,

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