DC综合的时候,时序slack为负,综合的信息会出现很多这种warning,
Warning: The following cells only drive
asynchronous pins of sequential cells which have no
timing constraint. Therefore retiming will not optimize
delay through them:
digital2/mult13/*cell*63670 (CLKBUFX20TF)
digital2/mult13/*cell*63667 (CLKBUFX20TF)
digital2/mult13/*cell*63668 (CLKBUFX20TF)