Constrain OFFSET IN for only one clock through a BUFGMUX
Can someone tell me how to constrain only one path through a BUFGMUX for an OFFSET IN constraint?
What I have is a BUFGMUX that selects between in input pin and the ClkFx180 output of a DCM. I need the input pin clock before the DCM locks, but I do not need to acquire the external pins until after the DCM locks. So, I need an OFFSET IN constraint that only applies to the ClkFx180 path through the BUFGMUX.
What I have looks like this:
NET ClkFx180 TNM = TNM_Setup; TIMEGRP TG_SetupRising = RISING TNM_Setup; OFFSET = IN 2.4 ns VALID 2.4 ns BEFORE DPin_P TIMEGRP TG_SetupRising;
ClkFx180 is the clock I care about for the OFFSET constraint. DPin_P is the external clock source pin, and also the alternative source through the BUFGMUX for use when the DCM is not yet locked. However, the DCM is skewed, so it is not lined up with the DPin_P signal pair (DPin_P/DPin_N of a differential pair, but I do not think that matters); it does not tell it that I only care about that path, but do not care about the DPin_P path through the BUFGMUX.
So, TRCE correctly analyzes the DPin_P path for the clock and so reports a failure, but there is no real failure along that path because I never look at the input pins I am constraining except when clocking from ClkFx180.
What I want to do is tell the timing analysis that this OFFSET IN constraint only applies to the ClkFx180 path through the BUFGMUX, but not to the DPin_P path through the BUFGMUX. I have found no way to say this.