Up-Down Counter

Counters are used in many different applications. Some count up from zero and provide a change in state of output upon reaching a predetermined value; others count down from a preset value to zero to provide an output state change.

However, some counters can operate in both up and down count mode, depending on the state of an up/down count mode input pin. They can be reversed at any point within their count sequence. Dual purpose ICs such as the TTL 74LS190 and 75LS191 are available which implement both Up and Down count functions.

The TTL 74LS190 is a 4-bit device that can be switched between Up and Down modes, and provides a BCD decade output; the 74LS191 is a binary counter. The counters are synchronous, but they are asynchronously presettable. Four data inputs (A – D) allow the preset target to be loaded. The counter is decremented or incremented synchronously with the low to high transition of the clock. The counters can be cascaded in high-speed mode.

A simple three-bit Up/Down synchronous counter can be built using JK flip-flops configured to operate as toggle or T-type flip-flops giving a maximum count of zero (000), advancing through 001, 010 to seven (111) and back to zero again.