Digital circuits play another important role: They can store information. These circuits form a sequential circuit, because the output of the circuit also serves as input to the circuit. That is, the existing state of the circuit is used in part to determine the next state.

Many types of memory circuits have been designed. We examine only one type in this book: the S-R latch. An S-R latch stores a single binary digit (1 or 0). An S-R latch circuit could be designed using a variety of gates. One such circuit, using NAND gates, is pictured in FIGURE 4.12.

The design of this circuit guarantees that the two outputs X and Y are always complements of each other. That is, when X is 0, Y is 1, and vice versa. The value of X at any point in time is considered to be the current state of the circuit. Therefore, if X is 1, the circuit is storing a 1; if X is 0, the circuit is storing a 0.

Recall that a NAND gate produces an output of 1 unless both of its input values are 1. Each gate in this circuit has one external input (S or R) and one input coming from the output of the other gate. Suppose the current state of the circuit is storing a 1 (that is, X is 1), and suppose both S and R are 1. Then Y remains 0 and X remains 1. Now suppose that the circuit is currently storing a 0 (X is 0) and that R and S are again 1. Then Y remains 1 and X remains 0. No matter which value is currently being stored, if both input values S and R are 1, the circuit keeps its existing state.

This explanation demonstrates that the S-R latch maintains its value as long as S and R are 1. But how does a value get stored in the first place? We set the S-R latch to 1 by momentarily setting S to 0 while keeping R at 1. If S is 0, X becomes 1. As long as S is returned to 1 immediately, the S-R latch remains in a state of 1. We set the latch to 0 by momentarily setting R to 0 while keeping S at 1. If R is 0, Y becomes 0, and thus X becomes 0. As long as R is immediately reset to 1, the circuit state remains 0.

By carefully controlling the values of S and R, the circuit can be made to store either value. By scaling this idea to larger circuits, we can design memory devices with larger capacities.