Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
18 years ago

Cyclone III - PLL VCO range ?

Hello,

What is the frequency range of CycloneIII PLL VCO ?

PDF writes 600-1300MHz, but altpll withard creates PLLs with frequencies less than 350MHz. For PLL reconfiguration feature it's important to know VCO frequency, where I have to calculate correct VCO value without a withard.

7 Replies

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

    You can try with new Quartus II v7.2 that just has been released by Altera. I have tried using ALTPLL megawizard, which allows for generating frequency that can go upto 1300 MHz for input clock frequency of 48 MHz. I don't know how that worked with prior releases since I don't have it installed on my machine. The output frequency will be limited if you are using multiple clock outputs with various settings. If you are using only one input and one output, you should be able to achieve the maximum as is advertised. But if you are using various combinations, then the output options will be limited.

    -BD
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    18 years ago

    --- Quote Start ---

    I have tried using ALTPLL megawizard, which allows for generating frequency that can go upto 1300 MHz for input clock frequency of 48 MHz.

    --- Quote End ---

    Output frequency - that's clear. But VCO frequency is not the output frequency. After VCO, additional counter divisors exists, which creates final output. VCO should be upto 1600MHz. And the most interesting parameter - the lowest VCO frequency. Is it 600MHz? or 350MHz (like in altpll withard).
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    18 years ago

    It's surprising that megawizard allows for a value that goes below specified minimum value. May be the document specifies preliminary values subject to characterization of the device - Just a guess. I guess, it would be better to go with the value that is referred to by the megawizard.

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

    Hi,

    I am betting there is an error in the MegaWizard when it comes to showing the VCO frequency. The Cyclone III device family's PLL has a new counter called the "VCO post scale counter" which divides the VCO frequency by 2. It can also be bypassed.

    The MegaWizard seems to report the frequency _after_ the VCO post scale counter as the actual VCO frequency. So, if your details show the VCO post scale counter a value of 2, you would have to multiply the reported VCO frequency by 2. In your screendump the actual VCO frequency would then be 720 MHz..

    Rgrds,

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

    If it shows after post-scale counter, so upper frequency "after post-scale" should be 1300/2=650. But withard makes VCO upto 1300MHz. So I believe, a withard shows real VCO frequency. Not after post-scale.

    I also asked for Altera at MySupport. Will see what is the truth.