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Observer niu_zun
Observer
283 Views
Registered: ‎12-19-2018

question about using ODDR to forward a clock as an output in XCVU13P

Hello.I want to use ODDR to forward a clock as an output, which will drive another XCVU13P. I wonder  if the driving capacity is enough for anthor FPGA? I also want to konw is there a delay betweent the outputing clock and the source  clock.?  Thank you .

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Scholar drjohnsmith
Scholar
267 Views
Registered: ‎07-09-2009

Re: question about using ODDR to forward a clock as an output in XCVU13P

using the ODDR is  the normal way to do this sort of thing.

Use a fast differnetial signal like LVDS between the FPGAs is a good tip as well.

 

As for the more general question of phase / clocking.

   thats  VERY big one.

 

The optoins are

a) ignor and hope,

b) send clock with data between the fpgas, and re syncronise on the fpga

c) use the PLL's in the FPGAs to cancel out the delay

 

this is an old note, fig 11 might be of help

https://www.xilinx.com/support/documentation/application_notes/xapp174.pdf

Note to adminstrators if they are on line:

    These old app notes and techniques are in danger of being missed, as people looking at modern chips would not look at or for these old docs.

 

 

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Observer niu_zun
Observer
238 Views
Registered: ‎12-19-2018

Re: question about using ODDR to forward a clock as an output in XCVU13P

@drjohnsmith  sorry. but i dont understand the second way using LVDS between FPGAs.In my opinon, every pin can be used as clock-outputing pin and I must use ODDR to keep the duty cycle and provide the best possible path.Does your second way mean that we can give a lvds clock directly to anthor FPGA without using ODDR? 

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Scholar drjohnsmith
Scholar
230 Views
Registered: ‎07-09-2009

Re: question about using ODDR to forward a clock as an output in XCVU13P

Quick recap.

LVDS. Is a normaly differential logic standard,

ODDR. is a Xilinx primative in the chip , output Double Datarate register

What format you select to send clock out / in on is up to you, I'd normaly select differential LVDS, is very reliable.

same goes for data

 

have a look here,

https://www.xilinx.com/support/documentation/application_notes/xapp585-lvds-source-synch-serdes-clock-multiplication.pdf

 

 

 

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Observer niu_zun
Observer
217 Views
Registered: ‎12-19-2018

Re: question about using ODDR to forward a clock as an output in XCVU13P

@drjohnsmith hello,thanks for reply. What confused me is that If I generate an outputing differential clock through ODDR and OBUFDS. what's the signal type of the outputing clock? Is it LVDS?  

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Scholar drjohnsmith
Scholar
210 Views
Registered: ‎07-09-2009

Re: question about using ODDR to forward a clock as an output in XCVU13P

LVDS is the IO standard of the pins.

 

ODDR is a primative inside the fpga used to register

 

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