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Altera_Forum's avatar
Altera_Forum
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15 years ago

cyclone 3 restricted fmax

Hi,

at some point during my work I needed to use a higher frequency clock in Cyclone 3 (EP3C16) speedgrade -8 and thought of a 400 MHz clock. In the Cyclone 3 Handbook volume 2 table 1-19 it is specified 402 MHz for this core. However after running implementation TimeQuest reports an fmax of ~250 MHz with the mention that it is restricted to 238 MHz due to minimum period restriction (tmin).

Where did this tmin come from? I haven't seen it in the datasheet. Any hints?

6 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    The fmax will come from your design.

    The datasheet fmax of 402MHz is the theoretical fmax. The actual fmax will depend on your design.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Tricky,

    you haven't read my post carefully... I know about fmax and design but i have no idea why the 238 restricted fmax (second column in timequest->report fmax) is there... my understanding it should be 402 instead of 238... right?
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    I think, I have seen similar things. My explanation for it was that some global clock networks have a lower fmax then others, but this is just a guess. Have you constrained your clock, so that the fitter knows that you want to achieve 400MHz?

    Thomas

    P.S.: I suppose you have not specified a duty-cycle for your clock different to 50%?
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    P.P.S.: Or do you have a M9K-block connected to this clock? That has a fmax of 238MHz with -8 speedgrade.

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    --- Quote Start ---

    P.P.S.: Or do you have a M9K-block connected to this clock? That has a fmax of 238MHz with -8 speedgrade.

    --- Quote End ---

    Yes, Quartus knows about the frequency, i constrained the PLL input and the 400 is inclk x 10. Duty cycle is 50%.

    There is a small altera fifo generated here (built around an M9K), i guess you're right. I should generate it in logic (size only 4x16). Hope it works!