Flip-flops: latch

How to store a value with a circuit?

Simplest form: unclocked latch

Set-reset (S-R) latch

2 NOR gates with each output fed back

Note that this is not a valid combinational circuit

(contains a loop)

Assume:

R = 0

(Fig. B.12)

S = 0

Q = 1

Then output Q is fed back as input to the second NOR gate and negated,

producing the negation Q' = 0

Likewise, the output Q' is fed back to the first NOR gate and negated,

producing the output Q = 1

The output can be changed by asserting the value R or S:

If S = 1, then the second NOR gate produces 0, so Q continues to be 1

But, if R = 1, then the first NOR gate produces 0,

which becomes the new value of Q.