UCF constraints for DCM outputs?

[Kelvin @ SG]

Hi, all:

I am using a DCM to multiply my clock by 2.5...In the UCF file, do I define
a Period constraint for
the 2.5X clock or the original input clock?

Thanks for your comments.

Best Regards,
Kelvin

 

[Robert Sefton]

I am using a DCM to multiply my clock by 2.5...In the UCF file, do I
define
a Period constraint for
the 2.5X clock or the original input clock?

Kelvin -

You specify the input clock period in the DCM attributes. You can/should
also specify the output clock period in a timespec to constrain your
logic. Like this:

# for the DCM
INST "dcm_inst" CLKIN_PERIOD=25;

# for your logic
NET "clk_out" TNM_NET = "clk_out";
TIMESPEC "TS_CLKOUT" = PERIOD "clk_out" 10 ns HIGH 50 %;

Robert