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:
- 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.
- 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.
- 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