SDF Back-Annotation issue

[shopping90]

I.m dealing with post sim issue and confuse about the description in SDF 3.0 that setup and hold time could be negative and the sum of setup and hold must be greater than 0?
1. negative setup??
2.why the sum of setup and hold must be greater than 0? what happen if not?

 

[ThisIsNotSam]

I.m dealing with post sim issue and confuse about the description in SDF 3.0 that setup and hold time could be negative and the sum of setup and hold must be greater than 0?
1. negative setup??
2.why the sum of setup and hold must be greater than 0? what happen if not?

negative setup is pretty common in complex IP, not in flip-flops. I have seen it in memories a bunch.

I will let you think about the sum. draw a timing diagram and you will see...