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Contributor
Contributor
925 Views
Registered: ‎07-09-2012

Clock capable pin pair as input and output

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In my application I need to bring a single-ended clock to my Zynq device and also generate an 80MHz clock out of the device. according to the clocking UG (UG472) the single-ended input should be connected to the positive port of a CC I/O pair. I understand that the negative port cannot be used as input for another clock, but will there be an issue if I used that port as an output for the clock that I am generating? So one port of the CC pair will be used as input and the other one as output for another clock with a different frequency.

 

 

Thanks,
Rouben

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Moderator
Moderator
905 Views
Registered: ‎04-18-2011

Re: Clock capable pin pair as input and output

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Hi Rouben

As long as the incoming clock is on the P side then it will work. The positive side of the pair will put the input clock onto the clock routing resources and then you are free to use the n side for whatever you wish. This includes outputting a clock
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Moderator
Moderator
906 Views
Registered: ‎04-18-2011

Re: Clock capable pin pair as input and output

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Hi Rouben

As long as the incoming clock is on the P side then it will work. The positive side of the pair will put the input clock onto the clock routing resources and then you are free to use the n side for whatever you wish. This includes outputting a clock
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Historian
Historian
900 Views
Registered: ‎01-23-2009

Re: Clock capable pin pair as input and output

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There is nothing special about outputting a clock - a clock output can go out on any pin. So the N side of an SRCC is just as usable as a clock output as any other pin.

 

Restrictions only exist on clock inputs - SRCC and MRCC pins are normal pins with an extra set of routing connections from the IBUF on the P side to some of the dedicated clocking resources (BUFG, MMCM, PLL, BUFR, BUFIO).

 

It is worth noting that the "best" way to output a clock is to use an ODDR to "mirror" the clock out (assuming the clock you are trying to output is a clock that exists on a dedicated clock network). Clock the C pin of the ODDR with the clock you want to mirror, and tie the D1 to 1'b1 and the D2 to 1'b0.  The output of the ODDR can then be connected to the OBUF.

 

When done this way the internal clock never needs to enter the general routing fabric - it stays on the dedicated clock network all the way until the ODDR.

 

Bringing a clock directly to an OBUF is not recommended since this requires the internal clock to leave the dedicated clock network to be routed to the OBUF through general fabric routing; there is no dedicated connection from the clock network to the OBUF (only to the ODDR).

 

Avrum

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Contributor
Contributor
831 Views
Registered: ‎07-09-2012

Re: Clock capable pin pair as input and output

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Thank you for your response.
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