Up/down counter

An up/down counter is one that is capable of progressing in either direction through a certain sequence. An up/down counter, sometimes called a bidirectional counter, can have any specified sequence of states. A 3-bit binary counter that advances upward through its sequence (0, 1, 2, 3, 4, 5, 6, 7) and then can be reversed so that it goes through the sequence in the opposite direction (7, 6, 5, 4, 3, 2, 1, 0) is an illustration of up/down sequential operation. After completing this section, you should be able to u Explain the basic operation of an up/down counter u Discuss the 74HC190 up/down decade counter In general, most up/down counters can be reversed at any point in their sequence. For instance, the 3-bit binary counter can be made to go through the following sequence: UP UP 0, 1, 2, 3, 4, 5, 4, 3, 2, 3, 4, 5, 6, 7, 6, 5, etc. DOWN DOWN The arrows indicate the state-to-state movement of the counter for both its UP and its DOWN modes of operation. An examination of Q0 for both the up and down sequences shows that FF0 toggles on each clock pulse. Thus, the J0 and K0 inputs of FF0 are J0 = K0 = 1 For the up sequence, Q1 changes state on the next clock pulse when Q0 = 1. For the down sequence, Q1 changes on the next clock pulse when Q0 = 0. Thus, the J1 and K1 inputs of FF1 must equal 1 under the conditions expressed by the following equation: J1 = K1 = (Q0 # UP) + (Q0 # DOWN) For the up sequence, Q2 changes state on the next clock pulse when Q0 = Q1 = 1. For the down sequence, Q2 changes on the next clock pulse when Q0 = Q1 = 0. Thus, the J2 and K2 inputs of FF2 must equal 1 under the conditions expressed by the following equation: J2 = K2 = (Q0 # Q1 # UP) + (Q0 # Q1 # DOWN) Each of the conditions for the J and K inputs of each flip-flop produces a toggle at the appropriate point in the counter sequence. A basic implementation of a 3-bit up/down binary counter using the logic equations just developed for the J and K inputs of each flip-flop. Notice that the UP/DOWN control input is HIGH for UP and LOW for DOWN.