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Ruturaj_D_Intel's avatar
Ruturaj_D_Intel
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6 years ago

How TimeQuest analyzes path though the latch?

I want to know if TimeQuest analyzes path through the latch. If there is latch in the design how do I make sure that timing gets analyzed by static timing analyzer. In my design I see that tool gives warning for latch as computational loop and does not analyze path through latch. I want to know how do I make sure path gets analyzed and what are recommendations from the team. Is there any document which describes how timing nalysis is done when tool encounters latch in the path and how timing constraints are to be given.

Thank you.

-Ruturaj.

3 Replies

  • KhaiChein_Y_Intel's avatar
    KhaiChein_Y_Intel
    Icon for Regular Contributor rankRegular Contributor
    6 years ago

    Hi,

    I am sorry for the late response.

    Timing Analyzer found latches across multiple look-up tables (LUTs), or latches that do not include an enable (e.g. SR latches). The Timing Analyzer does not support analyzing this latch implementation as a synchronous element, but treats these latches as a combinational loop.

    It is recommended to design without the use of latches whenever possible. You have to implement these latches with registers using asynchronous load and data signals, or remove these latches from your design.

    You may refer to Intel Quartus Prime Pro Deisgn Recommendation User Guide https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/ug-qpp-design-recommendations.pdf (Chapter 1.5.3 Latches)

    Please let me know if you have any questions.

    Thanks.

    • Ruturaj_D_Intel's avatar
      Ruturaj_D_Intel
      Icon for New Contributor rankNew Contributor
      6 years ago
      Hi, Thank you for clarification. I do have one more follow up question; my understanding is that for timing analysis of latch as synchronous element, enable signal needs to be defined as latch. I could not find document describing this requirement and how to define clock on enable signal. Is there a doc which talks about how to define clock on enable signal for STA? Other question is regarding flop with asynchronous load; does Startix10 FPGA support implementation of flop with asynchronous load. Thanks, Ruturaj.
  • KhaiChein_Y_Intel's avatar
    KhaiChein_Y_Intel
    Icon for Regular Contributor rankRegular Contributor
    6 years ago

    Hi,

    If you use asynchronous load and data signals to implement a latch. These nodes will be treated as latches during timing analysis. The Timing Analyzer may not correctly analyze designs containing latches. In some cases, timing analysis does not completely model latch timing . As a best practice, avoid latches unless required by the design and you fully understand the impact. It is recommended to change your design to remove the latches whenever possible.

    https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ug/ug-qps-design-recommendations.pdf (Chapter 2.5 Register and Latch Coding Guidelines)

    Thanks.