TimeQuest Timing Analyzer[Setup slack analysis]

Setup time and hold time of a register are defined as positive values with respect to clock edge. if the window shifts when viewed somewhere in advance e.g. pins then it could shift relative to clock edge due to delay variation of data and clock between pin and register port. if (at the pin) timing window shifts past behind clock edge then tSU becomes negative and tH stays positive. If timing window shifts to front of clock edge then tSU stays positive, tH becomes negative.