Why do output pins in timing libraries have max_transition constraint? Isn't this only for input pins?

[tro2212]

Im a little confused. If you have a driver/buffer cell driving another cell, it will be something like this:

buffer cell: inputA > buffer > outputX
load cell: inputC > cell > outputD

the load cell, inputC usually has max_transition constraint. If this is not met, we must use a stronger buffer with sharper transition.

However, I am also seeing that the buffer outputX also has max_transition constraint. What is this used for?

 

[ThisIsNotSam]

interesting question. might be a proxy for max_fanout or max_load that the cell's output can drive.

it's also possible that the cell may drive a primary output or a macro for which there are no max_tran constraints defined for. so it would make sense to define them at the output of the cell.