Delay Parameters (continued)
For example, Fig. 6.10 illustrates the timing behavior of the SR latch of Fig. 6.7c.
Following a change in S from 0 to 1, note that output Q changes from 1 to 0 after propagation delay tPHL through N1.
Then the feedback signal causes the Q output to change from 0 to 1 after propagation time tPLH through N2.
Thus, output Q always changes before output Q when setting an SR latch built from cross-coupled NOR gates.
Therefore, tPHL from input S to output Q involves a single gate delay, whereas tPLH from input S to output Q includes two gate delays.