Digital Counters

Digital counters are integrated circuits (ICs) that count events in computers and other digital systems. Because they must remember past states, digital counters include memory. Generally, digital counters consist of bistable devices or bistable multivibrators called flip-flops. The number of flip-flops and the way in which they are connected determines the number of states and the sequence of states that digital counters complete in each full cycle. The way in which devices are clocked determines whether digital counters are categorized as synchronous or asynchronous. In synchronous devices (such as synchronous BCD counters and synchronous decade counters), one clock triggers all of the flip-flops simultaneously. With asynchronous devices, often called asynchronous ripple counters an external clock pulse triggers only the first first-flop. Each successive flip-flop is then clocked by one of the outputs (Q or Q’) of the previous flip-flop. Digital counters are configured as UP (counting in increasing sequence), DOWN (counting in decreasing sequence) or Bidirectional (UP/DOWN).

Digital Counter Types

Several types of digital counters are available. Binary coded decimal (BCD) counters use a 4-bit code to represent each digit of a decimal number by its four-bit binary equivalent. A BCD decade counter is a 4-bit truncated counters that produce a counting sequence from 00002 (0) to 10012 (9). Bi-quinary counters are decade counters in which the most significant bit (MSB) is low for five states and then high for five states. This produces a 50% duty cycle on the output (square wave). A mod-n counter uses a number of different states. With non-truncated devices, the modulus (mod) number is given as 2n, where n is the number of stages (flip-flops). With truncated counters, n is the total number of different counting states. Other types of digital counters include Johnson counters and ring counters. A 4-bit Johnson counter creates sequences that resemble shift registers and restrict the division ratio to twice the number of flip-flops. A ring counter features a loop ring of counter stages in which only one flip-flop is set at any given time. An input trigger pulse causes the “on” state to move to the adjacent stage. Thus, the “on” state moves around the loop.