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JMedi's avatar
JMedi
Icon for New Contributor rankNew Contributor
5 years ago

Clock generated by Fractional PLL Megawizard vs Timing analyzer

Hello,

We have seen that it is different the clock configured/generated by Fractional PLL and the generated by the timing analyzer.

They use different factor to generate VCO frequency:

refCLK x (M+f/2^32) / N
For the example in the figure: (9+2164663431/4294967296) / 1 = 9.50399997993372380733

refCLK x M' / N'
For the attached in the figure: (2433 / 256) = 9.50390625

Those figures are: timing report (clocks) and fitter (pll usage summary)

How can we manage it?
What is the correct frequency?
What frequency use the timing analyzer?

Quartus Prime 18.1.1 + Cyclone V

Regards,

JMedi

2 Replies

  • Ash_R_Altera's avatar
    Ash_R_Altera
    Icon for Regular Contributor rankRegular Contributor
    5 years ago

    Hi,


    I got some information from our team:

    The Timing Analyzer numbers do have some inaccuracies, though they are consistent with each other & don't explain the PLL report's numbers:

    • Timing Analyzer stores clock periods, not their frequencies
    • Periods are only stored to three decimal places
    • The period of descendant clocks uses the mult & div factors of all of its ancestors, instead of using the mult & div factors of the truncated period of a parent clock


    Regards.