Setup and hold time for D flip-flop

For those interested in nature of setup and hold time for D flip-flop there is a good explanation at www.edn.com,  just follow the link Understanding the basics of setup and hold time

I borrowed two illustrations that show the reason for setup and hold time

Reason for SETUP Time:

The time it takes data D to reach node Z is called the setup time.

Reason for HOLD Time:

The darkened line shows the conducting path for hold time.

For the reference the definition for propagation and contamination delays borrowed from here ( (c) 6004 MIT )

  Altogether,  the illustrations from here ( (c) Nilesh Goel )

Setup and Hold Time for D flip-flop

Combinational Propagation and Contamination Delay