SR flip-flop

Also called: clocked SR flip-flop, S-R flip-flop, RS flip-flop, SR flip flop, clocked SR

An edge-triggered flip-flop with set (S) and reset (R) inputs that act only at the clock edge. Q⁺ = S + R'Q, with S = R = 1 not allowed.

An SR flip-flop (clocked SR flip-flop) works like an SR latch whose inputs are only looked at on the active clock edge.

At each rising edge:

S = 0, R = 0: hold.
S = 1, R = 0: set, Q⁺ = 1.
S = 0, R = 1: reset, Q⁺ = 0.
S = 1, R = 1: not allowed. The result is undefined.

Between edges, Q holds, whatever S and R do.

Its characteristic equation is Q⁺ = . That comes from a K-map in which the two S = R = 1 rows are treated as don't cares, so the equation is only valid as long as S and R are never 1 together.

The SR flip-flop is mostly a stepping stone. Its forbidden input is awkward, so practical designs use:

the JK flip-flop, which turns S = R = 1 into a toggle, or
the D flip-flop, which has only one data input and so cannot be given conflicting commands.

Its excitation table: 0 → 0 needs S = 0, R = X; 0 → 1 needs S = 1, R = 0; 1 → 0 needs S = 0, R = 1; 1 → 1 needs S = X, R = 0.

S\RQ	00	01	11	10
0	0m0	1m1	0m3	0m2
1	1m4	1m5	Xm7	Xm6

Worked examples

Example

Deriving Q⁺ = S + R'Q

Use the K-map above, with variables S, R, Q (S is the MSB). Q⁺ = 1 at minterms 1 (hold a 1), 4 and 5 (set). Minterms 6 and 7 are S = R = 1, so they are don't cares.

1.
Group 4, 5, 6, 7 (S = 1): .
2.
Group 1 and 5 (R = 0, Q = 1): .
3.
Q⁺ = .
4.
Check a row: S = 0, R = 1, Q = 1 gives 0 + 0·1 = 0, a reset. ✓

Example

Tracing an SR flip-flop

Q starts at 0. Rising edges in slots 2, 4, 6, 8 and 10.

1.
Slot 2: S = 1 → set. Q = 1.
2.
Slot 4: S = R = 0 → hold. Q = 1.
3.
Slot 6: R = 1 → reset. Q = 0.
4.
Slot 8: hold at 0.
5.
Slot 10: S = 1 → set. Q = 1.

Common mistakes

Treating it as a latch. Its inputs only count at the clock edge.
Applying Q⁺ = S + R'Q to S = R = 1. The equation assumes that never happens.
Confusing it with the JK flip-flop, whose 11 row is a legal toggle.

Practice SR flip-flop

Interactive questions with instant feedback and a worked solution for every wrong answer.

Latches and Flip-Flops lesson full course

A clocked SR flip-flop has Q⁺ = (S and R are never both 1). In each slot you are given S, R and the present Q. Draw N, the value Q will take after the next edge.

Practice exams

Latches and Flip-Flops: Exam 4 · 1 question

Learn it step by step

SR flip-flop is taught in Latches and Flip-Flops.

Latches and Flip-Flops →

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