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Altera_Forum's avatar
Altera_Forum
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16 years ago

Usage of PLL output pins

In Cyclone-III EP3C16, there are 4 PLLs. I only need to use 2 of them. Each PLL has one dedicated physical output pin. In my application, I need 2 or 3 physical outputs from one PLL. I think that I c...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

There is only one dedicated output pin that is directly driven by the PLL. A PLL can drive other I/O, but most drive a global to get to these I/O. The global adds a little jitter/noise to the clock output.

That being said, I've never seen using regular I/O as a problem. I've seen people drive clocks off regular I/O and run 200-300MHz interfaces quite easily. What are you doing with your clocks? Personally, I think using regular I/O is almost always good enough(I've never seen a case where it wasn't) and that the dedicated outptus tend to just throw users off thinking they have to use it.