Specifiying tPD

You can specify the maximum tPD (propagation delay) for all combinatorial paths in your design using a pad-to-pad timespec. Combinatorial pad-to-pad paths begin at input pads, propagate through one or more levels of combinatorial logic and end at output pads. Combinatorial paths also trace through the enable inputs of 3-state controlled pads.

Combinatorial paths are not traced through clock, and asynchronous set and reset inputs of registers. These paths are also broken at bidirectional pins.

The format of the pad-to-pad timespec is:

TSnn=FROM:input_pad:TO:output_pad:delay_value

The following timing constraint specifies that the maximum propagation delay from all input pins to all output pins is no more than 10 ns.

TS01=FROM:PADS:TO:PADS:10

The next timing constraint specifies that in1 enables the output buffer for out2 in no more than 15 ns:

TS02=FROM:PADS(in1):TO:PADS(out2):15

Entering Timespecs in a UCF File

Grouping Signals