The count is one of the most simple basic computing counter logic circuit is to realize this operation,
the counter in digital systems is to count the number of pulses to achieve the measurement and counting and control functions, also has the function of frequency division, the counter is composed of basic counting unit and a control gate, a all kinds of trigger counting unit is composed of a series of information storage function, the trigger RS flip-flop, T flip-flop, D flip-flop, JK flip-flop etc.. The counter is widely used in digital systems, such as the address of the instruction count in the computer controller, in order to remove the order of the next instruction, in the arithmetic unit for multiplication and division operations to write down the number of addition and subtraction, and as in the digital instrument of pulse counting etc..
The counter can be used to display the working status of the product, are commonly used to indicate the product has completed a number of copies of the folding with page. Its main indicator is the number of counters, common 3 and 4. Obviously, the 3 digit counter maximum can be displayed to 999, the maximum number of 4 digits can be displayed to 9999.