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Registered: ‎06-21-2017

MMCM Clock Switching

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I'm looking at UG572-UltraScale Architecture Clocking Resources.  Chapter 3, describing the CLKINSEL pin (the clock input select) states "The MMCM must be held in RESET during clock switchover."  To me, this indicates that the RESET should be asserted, then the CLKINSEL pin should be switched to choose the other clock, then RESET should be deasserted.

Chapter 3 in the Reference Clock Switching section states "The MMCM reference clock can be dynamically switched by using the CLKINSEL pin. The switching is done asynchronously. After the clock switches, the MMCM is likely to lose LOCKED and automatically lock onto the new clock. Therefore, after the clock switches, the MMCM must be reset."  This would indicate that the MMCM should be reset after changing the state of CLKINSEL.

My gut tells me that either sequence should work but some confirmation would be nice.  I expect to have a board to test Monday and getting the clocks running is high on the list of things to do.

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Registered: ‎02-09-2017

Hi @bruce_karaffa,

 

You are correct. Both of these methods should work. 

In a real application, it makes more sense to first reset (and hold) the MMCM, in order to stop all the other logic dependent of such clock and avoid the possibility of any glitches.

But if you do the other way, what will happen is that the MMCM will loose Lock (the Locked pin will go down, indicating that the output clock is not safe to use) at which point a Reset is mandatory to recover the MMCM and have it work again.

Thanks,

Andre Guerrero

Product Applications Engineer

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Moderator
Moderator
658 Views
Registered: ‎02-09-2017

Hi @bruce_karaffa,

 

You are correct. Both of these methods should work. 

In a real application, it makes more sense to first reset (and hold) the MMCM, in order to stop all the other logic dependent of such clock and avoid the possibility of any glitches.

But if you do the other way, what will happen is that the MMCM will loose Lock (the Locked pin will go down, indicating that the output clock is not safe to use) at which point a Reset is mandatory to recover the MMCM and have it work again.

Thanks,

Andre Guerrero

Product Applications Engineer

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Don’t forget to reply, kudo, and accept as solution.
-------------------------------------------------------------------------

View solution in original post