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Altera_Forum's avatar
Altera_Forum
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14 years ago

sta reporting feedback path

Dear All,

I am running the timing analyzer on my design and it reports slack on a path which looks like a "unintentionally" feedback path. And there are more like these.

Device is a cycloneII EP2C20. SW used is Quartus II version 11.

; -15.651 ns ; reg_cen_rstn:u_alu_sign|q ; reg_cen_rstn:u_alu_sign|q ; clk ; clk ;

best regards

Simon

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    Launch TimeQuest and run report_timing on the path to get details. Once you know what the path goes through, you can either change the code so it doesn't exist or do a set_false_path assignment to tell TimeQuest not to analyze it.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    Once you know what the path goes through, you can either change the code so it doesn't exist or do a set_false_path assignment to tell TimeQuest not to analyze it.

    --- Quote End ---

    I have tried :

    1. analyze the path and found a large ripple carry path and ignored it for the wrong reason

    2. to set a max delay for the path. This did not help.

    3. to change the code by adding additional pipelining registers to the control path. It did not help.

    Coming back to point 1. I ignored this one cause I tought that the used Carry look ahead adder (with 196 LE's)

    was not so dramatic in size as the LPM_ADD_SUB (96 LE's)

    Well in timing it does matter. The LE's for the LPM_ADD_SUB are put in arithmetic mode and implements a Full Adder.

    I have gone from -15 ns slack to +2.5 ns.

    best regards

    Simon