appears at the output. Then when the clock goes low, the output retains the data. For instance, suppose that the data input is
D3D2D1D0 = 0111
When the clock goes high, this work is loaded into the D latches, resulting in an output of
Q3Q2Q1Q0 = 0111
After the clock goes low, the output data is retained, or stored. As long as the clock is low, the D values can change without affecting the Q values.
The 7475 in Fig. 8-17 is a TTL MSI circuit that contains four D latches; it’s called a quad bistable latch. The 7475 is ideal for handling 4-bit nibbles of data. If more than one 7475 is used, words of any length can be stored.
SELF-TEST
4. What does an entry X mean in a flip-flop truth table?
5. What could you do to disable the flip-flop in Fig. 8-11?
6. Which flip-flop is easier to use, the RS or the D, as a clocked or gated latch to store data?
8-3 EDGE - TRIGGERED RS FLIP-FLOPS
The simple latch-type flip-flops presented in Sec. 8-1 are completely transparent; that is, the output Q immediately follows any change of state at the input (R, S, or D). The gated