Counting is the simplest and most basic operation. The counter is a logic circuit to realize this operation. In the digital system, the counter mainly counts the number of pulses to realize the functions of measurement, counting and control, and also has the function of frequency division. The counter consists of a basic counting unit and some control gates. The counting unit consists of a series of various triggers with the function of storing information, such as rs trigger, T trigger, D trigger and JK trigger.

Counters are widely used in digital systems, such as counting instruction addresses in the controller of an electronic computer to take out the next instruction in sequence, recording the number of additions and subtractions in multiplication and division in an arithmetic unit, and counting pulses in a digital instrument. Counter can be used to display the working status of products. Generally speaking, it is mainly used to indicate how many copies of folding and matching work have been completed. Its main index lies in the number of digits of the counter, generally 3 digits and 4 digits. Obviously, the 3-digit counter can display 999 at most, and the 4-digit counter can display 9999 at most.