Next, we will look at the SR (Set-Reset) flip flop. It has two inputs,
S and R, which have the same meaning as those for the SR latch. Its
behavior is described by the truth tables of Table 6.5 and state diagram of
Figure 6.16. The Set (S) input causes a 1 to be stored in the flip flop at the
next active clock edge; the Reset (R) input causes a 0 to be stored. The S
and R inputs are never made 1 at the same time. Although that would not
damage the flip flop, as in the case of the latch, it is not certain what happens
when S and R both go back to 0. Note that in the diagram, each label
is two digits; the first is the value of S and the second the value of R.
Two labels are on the path from 0 to 0 since either 00 or 01 will cause the
flip flop to return to state 0. (There are also two input combinations that
cause the flip flop to go from state 1 to 1.)
In Map 6.1, we map q (from the first truth table). Notice that two of
the squares are don’t cares, since we will never make both S and R equal
to 1 at the same time. This allows us to write an equation for the next