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Altera_Forum's avatar
Altera_Forum
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8 years ago

Cyclone V Fractional PLLs don't output expected clock frequency

I'm using Quartus 17.0.2. I have trying to generate a 148.5MHz clock with a 50MHz input reference clock. When I put the PLL in fractional mode and enter 148.5M as my target frequency the megawizard s...
Altera_Forum's avatar
Altera_Forum
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8 years ago

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One of the issues is you don't check the 'Enable Physical Output Clock Parameters' check box in the wizard.

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I'm not sure what that has to do with it generating the incorrect frequency. There are two options to generate an output clock frequency:

1) Enter the frequency you want and the megawizard will calculate the proper M/N/etc parameters for you. This is when the 'Enable Physical Output Clock Parameters' box is UNCHECKED.

2) Enter the M/N/etc parameters yourself and the megawizard will tell you the resulting frequency. This is when the 'Enable Physical Output Clock Parameters' box is CHECKED.

The document that you linked to simply shows the option# 2 approach to configuring the PLL.