The Setup and hold times for gates or logic should always be positive.
Positive setup and hold times indicate that there is enough time for the data to be stable before the arrival of the clock and also that the data will be stable for long enough.
If you get negative setup/hold times (slack) it means that ur design has a high probability of setup and hold violations. Check the failing paths (paths with -ve slack) and see if you can somehow reduce this -ve slack and make it +ve.
The Setup and hold times for gates or logic should always be positive.
Positive setup and hold times indicate that there is enough time for the data to be stable before the arrival of the clock and also that the data will be stable for long enough.
If you get negative setup/hold times (slack) it means that ur design has a high probability of setup and hold violations. Check the failing paths (paths with -ve slack) and see if you can somehow reduce this -ve slack and make it +ve.
The concept about Holding time, setup time, rising time, delay time and propagation time all are defined very well in Digital Design Fundamental of Tocci.