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Observer rty
Observer
5,619 Views
Registered: ‎10-08-2010

Spartan-6: How to make a differential output for a clock?

Hello.

 

I am trying to output clock that is recovered by the GTP (RecClk) to the output pins.

RecClk is connected to BUFG, so I am trying to output global clock signal.

I want to make a differential output for this clock, but when I tryed to use OBUFDS for it, Mapper said me:

 

ERROR:LIT:544 - BUFDS symbol
   "physical_group_GTP_RefClks<1>/IBUFDSBlock[1].GTP_REFCLKBUFDSs_ML_BUFDS"
   (output signal=GTP_RefClks<1>) can only drive a GTPA1_DUAL's CLK00, CLK01,
   CLK10, or CLK11 pins. Please modify the design to avoid an unroutable
   situation.

 

So, is it possible to output global clock as differential signal?

 

Thank you in advance.

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3 Replies
Advisor eilert
Advisor
5,614 Views
Registered: ‎08-14-2007

Re: Spartan-6: How to make a differential output for a clock?

Hi,

you said that ref_clk is connected to a global Clock buffer (BUFG) . Normaly these routes are special and can not be accessed for other purposes.

So, you need to take the signal from the clock net. It may be difficult to connect a OBUFDS directly to a Clock net,but you can use one or two ODDR2 modules to "reconstruct" the clock signal.

Instantiate a  ODDR2 with a constant 1 on the data input that is affiliated with the normal clock and a constant '0' to the data input thats affiliated with the inverted clock. drive the clocks from the clock net that has ref_clk on it.

 

If necessary use a second ODDR2 with inverted data inputs to create the inverted input for the OBUFDS. (I'm not sure about the necessary inputs of the OBUFDS).

 

But maybe there is a simpler solution to it.

Have a nice synthesis

  Eilert

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Observer dmitry_
Observer
5,607 Views
Registered: ‎09-20-2010

Re: Spartan-6: How to make a differential output for a clock?

To what pins you are trying to connect those differential signals ? user sma A3 B3 ?

Try using bufio2fb insted of bufg and check attributes of obufds. Or try place two bufgs after obufds ))

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Visitor jatink007
Visitor
4,842 Views
Registered: ‎02-01-2012

Re: Spartan-6: How to make a differential output for a clock?

Thanks,

 

it works when i used bufio2fb insted of bufg. Now i can connect clock directly to a OBUFDS pin.

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