S R Y X Figure 4.12 An S-R latch 106 Chapter 4 Gates and Circuits A circuit called a demultiplexer (or demux ) performs the opposite oper-ation. That is, it takes a single input and routes it to one of 2 n outputs, depending on the values of the n control lines. 4.5 Circuits as Memory Another important role of digital circuits is that they can be used to store information. These circuits form a sequential circuit, because the output of the circuit is also used 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). There are several ways an S-R latch circuit could be designed using various kinds 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
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