The Period Constraint
Period : This constraint covers all timing paths which start and end at a FF, Latch or synchronous RAM which is clocked by the referenced net. (Every synchronous element is effectively identified by forward propagation.)
It does not cover paths to output pads, but does cover input pads.
UCF Example:
- Using “simple” method:
NET A_CLK PERIOD=40 : LOW 15; ……(LOW is optional; Specifies duty cycle)
- Using TIMEGROUP/TNMs
NET A_CLK TNM = A_CLK_GRP; …...(make a group)
TIMESPEC TS_CLK = PERIOD A_CLK_GRP 40; …..(apply PERIOD to the group)
Path controlled by PERIOD