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Altera_Forum's avatar
Altera_Forum
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9 years ago

ALTCLKCTRL vs PLL with 2 reference clocks

Hello,

My Cyclone V design has 2 reference clock inputs.

I want to be able to manually switch between them during runtime.

I know that the ALTERA_PLL megafunction can be configured to receive 2 input clocks. I understand that it also provides a manual switchover signal to select between the 2.

I also know that Altera provides the ALTCLKCTRL megafunction - which if I understand correctly can serve as a clock MUX.

Which one should I use ? ALTERA_PLL with 2 inputs or ALTCLKCTRL ?

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    9 years ago

    Which one? It's up to you. Based on what you've described of your requirements, either will do.

    ALTCLKCTRL is arguably better as it can be configure to guarantee glitch free switchover. ALTPLL doesn't. Depending on your design this may be desirable.

    Cheers,

    Alex
  • Altera_Forum's avatar
    Altera_Forum
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    9 years ago

    Thanks.

    Another question about the ALTERA PLL when configured to use 2 reference clocks.

    Is this achieved buy cascading an upstream ALTCLKCTRL "behind the scene" and connecting its output to the PLL?

    Or is the dual clock input is a native feature of the PLL itself ?
  • Altera_Forum's avatar
    Altera_Forum
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    9 years ago

    Why not review the Cyclone V handbook? It describes clock switchover as a dedicated feature of Cyclone V PLLs. It comes with automatic switchover and other details that can't be provided by a CLKCTRL block.