recommented timing constraint for differential clock signal / coding style of manually inserted global clock buffer

[cds_team]

Hello!

what is the recommended way to apply a timing constraint on a differential clock signal?

example module:

module designTop (
  input  reset_N,
  input  fpgaClk200MHzP,
  input  fpgaClk200MHzN,
);
    // 200 MHz clock
    wire fpgaClk200MHz_BUFG;
    (* clock_signal = "yes"*) wire fpgaClk200MHz;
    
    IBUFGDS_LVDS_25 CLKLVDS (.I(fpgaClk200MHzP), .IB(fpgaClk200MHzN), .O(fpgaClk200MHz_BUFG));
    BUFG CLK200MHZ_BUFG (.I(fpgaClk200MHz_BUFG), .O(fpgaClk200MHz));
    
    always@(posedge fpgaClk200MHz, negedge reset_N) begin
        if (!reset_N) begin
            ...
        end
    end
endmodule

1.) how would you recommend to apply the timing constraint in the UCF file:

     a.) timing constraint like this:

NET "fpgaClk200MHzN" TNM_NET = "fpgaClk200MHzN";
NET "fpgaClk200MHzP" TNM_NET = "fpgaClk200MHzP";
TIMESPEC "TS_fpgaClk200MHzN" = PERIOD "fpgaClk200MHzN" 5 ns LOW 50%;
TIMESPEC "TS_fpgaClk200MHzP" = PERIOD "fpgaClk200MHzP" "TS_fpgaClk200MHzN" HIGH 50%;

     b.)

NET "fpgaClk200MHzN" TNM_NET = "fpgaClk200MHzN";
NET "fpgaClk200MHzP" TNM_NET = "fpgaClk200MHzP";
TIMESPEC "TS_fpgaClk200MHzN" = PERIOD "fpgaClk200MHzN" 5 ns LOW 50%;
TIMESPEC "TS_fpgaClk200MHzP" = PERIOD "fpgaClk200MHzP" 5 ns HIGH 50%;

 or c.)

NET "fpgaClk200MHz" TNM_NET = tnm_fpgaClk200MHz ;
TIMESPEC TS_fpgaClk200MHz = PERIOD tnm_fpgaClk200MHz 5ns HIGH 50%;

2.) is it necessary to define the clock_signal attribute of fpgaClk200Mhz if the global clock buffer is inserted manually?

Thank you very much,

                                       cds.

Message Edited by cds_team on 03-16-2010 07:34 AM

[htsvn]

Hi,

This is the recommended way to constrain Differential pair.

http://www.xilinx.com/support/answers/15833.htm

Thnx

[cds_team]

Hello!

Thx for your reply!

Just to share my experiences...

The following code resulted to work best in my case (especially, in this way I didn't get any problems with the subsequent DCMs, which are included in the real code..)

I decided not to instantiate the global clock buffer (resulting in the following  code:)

module designTop (
  input  reset_N,
  input  fpgaClk200MHzP,
  input  fpgaClk200MHzN
);
    // 200 MHz clock
    (* clock_signal = "yes"*) wire fpgaClk200MHz;
    
    IBUFGDS_LVDS_25 CLKLVDS (.I(fpgaClk200MHzP), .IB(fpgaClk200MHzN), .O(fpgaClk200MHz));
    
    always@(posedge fpgaClk200MHz, negedge reset_N) begin
        if (!reset_N) begin
            ...
        end
    end
endmodule

and to apply following timing constraint:

NET "fpgaClk200MHzP" TNM_NET = "fpgaClk200MHzP";
TIMESPEC "TS_fpgaClk200MHzP" = PERIOD "fpgaClk200MHzP" 5 ns HIGH 50%; 

with following pin location constraints

NET "fpgaClk200MHzP" 	LOC = H19 | IOSTANDARD = LVDS_25 | DIFF_TERM = TRUE ;
NET "fpgaClk200MHzN" 	LOC = H20 | IOSTANDARD = LVDS_25 | DIFF_TERM = TRUE ;

I just want to mention, that there is no example of a PERIOD constraint of a differential pair input in ug612.pdf (nor in the constraints guide)! (At least I didn't find one..)

In AR#15833, the solution is to place a TNM_NET PERIOD on the DIFF clock input... How would such a constraint look like, or is this exactly what the posted code does? (Please forgive me this kind of questions, but I'm a newbie to constraints... (I've read the user guides about constraints and timing constraints, but as already mentioned, I didn't find any detailed information on differential inputs...))

Thx again and best Regards,

                      cds.

Message Edited by cds_team on 03-16-2010 10:01 AM

Message Edited by cds_team on 03-16-2010 10:02 AM

Message Edited by cds_team on 03-16-2010 10:06 AM

[dbarrowman]

I'd like a better answer to this question as well. The answer record referred to has less detail than the original question that was asked by the poster. The question was very clearly spelled out with 3 different examples given. Please, just tell us which one is correct.

The Constraints User Guide should also be updated with specific examples.

Regards,

Dan

[mcgett · Accepted solution]

The answer record said to apply the period constraint to the P side of the differential input buffer which is exactly what the previous poster did.

NET "fpgaClk200MHzP" TNM_NET = "fpgaClk200MHzP";
TIMESPEC "TS_fpgaClk200MHzP" = PERIOD "fpgaClk200MHzP" 5 ns HIGH 50%;

or more simply

 NET "fpgaClk200MHzP" PERIOD = 5 nS;

[dbarrowman]

Thanks much for the clarification.

Regards,

Dan

[endab]

I would recommend using the TNM_NET syntax as there is a known issue with the NET syntax. PLease see the following:
http://www.xilinx.com/support/answers/17063.htm

I'll also mark this tread as solved from your response on 23 Nov.

[jwilkinson7]

---

mcgett wrote:

The answer record said to apply the period constraint to the P side of the differential input buffer which is exactly what the previous poster did.

NET "fpgaClk200MHzP" TNM_NET = "fpgaClk200MHzP";
TIMESPEC "TS_fpgaClk200MHzP" = PERIOD "fpgaClk200MHzP" 5 ns HIGH 50%;

or more simply

 NET "fpgaClk200MHzP" PERIOD = 5 nS;

---

Perhaps the referenced AR # 15833 wasn't what you meant, or perhaps it changed since this original conversation (2010), but now it doesn't say anything of the sort.

Is there some other AR you meant? 

Also, AR 15833 is answering a specific question from someone who wants the N input to drive the clk analysis, but what about normal uses? (where P is the main phase you want)

Do you add PERIOD constraints to both P and N inputs?  Or just P?

If so, how?  Syntax examples would be useful, especially if you are constraining both P and N. .

As others here pointed out, the xilinx docs have very little on constraining differential inputs.

thanks,

jeff

[graces]

You can add PERIOD constraint to just the _P side of diff pair.

e.g.

NET "clk_p" TNM_NET = clk;

TIMESPEC TS_clk = PERIOD "clk" 10 ns HIGH 50%;