When I run a design that contains OFFSET constraints, the constraints are renamed and rewritten in the form of FROM:TO constraints.
Given the following UCF file:
NET "clk" TNM_NET = "clk";
TIMESPEC "TS_clk" = PERIOD "clk" 10 ns HIGH 50 %;
OFFSET = IN 10 ns BEFORE "clk";
OFFSET = OUT 10 ns AFTER "clk";
The timing report (.tim) shows:
OFFSET=IN : 10.00: N/A
OFFSET=OUT : 10.00: N/A
The OFFSET IN and OFFSET OUT constraints have been renamed: TS2001 is the OFFSET OUT and TS2002 is the OFFSET IN. In both cases, the constraints are renamed and rewritten as FROM:TO constraints. The new constraints are correct.
- The OFFSET OUT constraint, written as a FROM:TO constraint, should subtract the clock propagation delay (Tgck). This is corrected in 6.1i Service Pack 2.
- The OFFSET IN constraint, written as a FROM:TO constraint, might be confusing because the offset value has changed from 10 ns to 11.1 ns. The extra delay added to the FROM:TO constraint is due to the addition of clock propagation delay (Tgck). When this is taken into consideration, the new FROM:TO constraint is equivalent to the original constraint.
This issue is fixed in the 6.1i software release.