# Sequential circuits

Sequential circuits are able to remember what has happened to them and can use it to alter their future behaviour. One of the simplest sequential circuit is a D flip-flop which can remember whether a single bit is one or zero. This single-bit memory can be used as the basis of all other sequential circuits.

The logic circuitry for a clocked D flip-flop is shown in the following figure.

Where the inputs on the left are D and the clock, and the outputs on the right are Q and Q'.
D = 0 makes Q = 0 when the clock is high. When D = 1, then Q = 1. So when the clock is high, Q = D. When the clock is low, the D flip-flop stores the last value that was on D when the clock was high.
The following is the flip-flop's transition table :

 D(t) Q(t) Q(t+1) 0 0 0 0 1 0 1 0 1 1 1 1

And the state-transition diagram is the following :

state-transition diagram for D flip-flop

Last modified 1/1/2000 by Juan de Lara ( Juan.Lara@ii.uam.es, http://www.ii.uam.es/~jlara) need help for using this courses?.

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