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Altera_Forum
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12 years ago

VCO post-scale problem

I’m using the alt pll reconfig IP to generate different output frequency and confonting a problem which puzzles me long. The output frequency of PLL can be calculated as follows: Fout=F_in× (M/(...
Altera_Forum's avatar
Altera_Forum
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12 years ago

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So,would anybody could tell me how to calculate this VCO post scale parameter?

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IIRC, Cyclone IV has a nominal VCO range of 650 to 1300 MHz. When the VCO post scale counter is set the VCO range is divided by 2, i.e. 325 to 650 MHz. So if your calculated VCO frequency is >= 650MHz, use 1, if it is smaller, use 2. The interesting thing is that this has no influence on the calculation of the other counters. This is all from top of my head, so I hope I got things right, but I am quite sure that 650MHz was the "magic" limit.

Regards,

Thomas

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