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##### 18.8 4-BIT BINARY UP/DOWN COUNTERS

A 4-bit binary up/down counter counts sequence from 0000 to 1111 and 1111 to 0000. The state table is given in Table 18.32 and the logic diagram using T flip-flop is shown in Fig. 18.28. The circuit operation can be explained as follows:

1. When the external input UP is equal to 1, no matter what the DOWN input is, the circuit operates as an UP counter and counts sequence from 0000 to 1111.
2. When the external input DOWN is equal to 1 and UP is equal to 0, the circuit operates as a DOWN counter and counts sequence from 1111 to 0000.
3. If both the inputs UP and DOWN are equal to 0, then the output of the flip-flop remains unchanged.

TABLE 18.32 The state table of a 4-bit up/down counter using T flip-flops

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