Microsoft Word - Digital Logic Design v_4_6a

4.2. SR Flip-Flops

SR flip-flops are the simplest form of single-bit memory (latch). A Latch is also known as a bi-stable
memory device. Flip-flop is used to store value of input.

 SR flip-flops have set and reset inputs. The circuit for a SR flip-flop is shown below:

 Q+ is used to refer to the next output state when the current output state is Q


 Unlike combinational logic, sequential logic Q+ is dependent on the value of previous output Q.

Q+ (S, R, Q) = S+Q+R

Note: Q + is dependent on inputs S and R as well as current state, q.

 Present-State/Next-State (PS/NS) Table
The PS/NS table is the sequential circuit equivalent to combinational circuit’s truth table. As it can be
seen from the following example, instead of output, PS/NS table has current and next state:

Present External

State(PS) Input Signal

Q S R

Next State

(NS)

Q+ Comment

Additional Comment

0 0 0 0 hold 0 Q is stable , Q new = Q

0 0 1 0 hold 0 Q is stable , Q new = Q

0 1 0 1 set Q is unstable, Q new = Q+ after ∆t = Q

0 1 1 0 normally not allowed Q is stable , Q new = Q, reset dominant

1 0 0 1 hold 1 Q is stable , Q new = Q

1 0 1 0 reset Q is unstable, Q new = Q+ after ∆t = Q

1 1 0 1 hold 1 Q is stable , Q new = Q

1 1 1 0 normally not allowed Q is unstable , Q new = Q+ after ∆t = Q,^

reset dominant

 Compressed Characteristic Table
Compressed Characteristic Table is another way to describe sequential circuit that is simpler to
generate than PS/NS table while provide most of the information required. Compressed
Characteristic table is commonly used in sequential circuit design and analysis. Below is an example
of Compressed Characteristic table for SR flip-flop.

∆t

(delay)

Q Q

+

R

S

Q+=S+Q+R

“Characteristic equation”

Feedback signal, Q

Feed Forward Signal

S+Q