Example 8-4
Figure 8-13 shows the input waveforms R, S, and EN applied to a clocked RS flip-flop. Explain the output waveform Q.
Solution
Initially, the flip-flop is reset (Q = 0). At time t1 EN goes high; the flip-flop is now enabled, and it is immediately set (Q = 1) since R = 0 and S = 1. At time t2 EN goes low, and the flip-flop is disabled and latches in the stable state Q = 1.
Between t2 and t3 both R and S change states, but since EN is low, the flip-flop is still disabled and Q remains at 1.
Between t3 and t6, the flip-flop will respond to any change in R and S since EN is high. Thus at t3 Q goes low, and at t4 it goes back high. No change occurs at t5. At t6 the value Q = 1 is latched and no changes in Q occur between t6 and t7 even though both R and S change.
Between t7 and t8 no change in Q occurs since both R and S are low.