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Altera_Forum's avatar
Altera_Forum
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16 years ago

PLL usage

Hello all,

I've noticed that many use PLL on the reference clock input even if they do not need to divide/multiply/shift this clock or do not have external clock to synchronize to the reference frequency.

For example 50 MHz input clk goes to PLL and then 50MHz c0 is used everythere in the design.

What is the purpose of this type of PLL usage?

Thank you in advance.

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

    --- Quote Start ---

    How to set the PLL in free run mode?

    --- Quote End ---

    As far as I know, the PLL behaviour during and after loss of lock is unspecified. I observed, that a PLL without input clock is often working at a low frequency (below specified range), if you don't enable automatic reset. But I fear, you can't rely on it. Particularly, you can't rule out a temporary move to high frequencies, that can cause erratic design behaviour due to timing violations.