Constraints interaction report

[Francisco Rodriguez]

Hello group,

I'm working on a design with two external clocks in a xc2v3000.
Clocks are dspclk and sysclk, and signals crossing clock domains
come from dspclk and go to sysclk only.

Apart from using blockrams, some signals from dspclk cross to sysclk
using mults and dffs-based synchronizers.
I've used the following UCF constraints to cover those signals crossing clocks:

TIMEGRP "TRG_busnoffs" = TNM_dspclk EXCEPT FFS;
TIMEGRP "TRG_busffs"= TNM_dspclk EXCEPT MULTS;
# When crossing from dspclk to sysclk using mults delay is not important:
TIMESPEC "TS_mults2fast" = FROM "TRG_busnoffs" TO "TNM_sysclk" TS_dspclk;
# Crossing from dspclk to sysclk using dffs require maximum setup to minimize
metastability:
TIMESPEC "TS_bus2fast" = FROM TRG_busffs TO TNM_sysclk 3 ns;


After P&R, trace generates a constraints interaction report which states that
some constraint intersection issues exist between sysclk period and TS_mults2fast
(but not TS_bus2fast).

However, from the constraints guide
"PERIOD is a basic timing constraint and synthesis constraint. A clock period
specification
checks timing between all synchronous elements within the clock domain as defined in the
destination element group. The group may contain paths that pass between clock domains
if the clocks are defined as a function of one or the other."

So i think paths coming from mults (clocked by dspclk) should not be included into the
sysclk
period constraint.

Can you help me to understand whats wrong with the constraints?
Or should I simply ignore this section of the constraint report?

A similar constraint intersection exists between the dspclk period and OFFSET IN
constraint,
and I've always thought the period constraint does not cover paths starting in pads.
Am i right?

Regards

Francisco Rodriguez


BTW, I'm using ise 6.2.02i