![]() In the simplest case, a one-bit counter consists of a single flip-flop. ![]() ![]() When implemented with discrete flip-flops, ripple counters are commonly implemented with JK flip-flops, with each flip-flop configured to toggle when clocked (i.e., J and K are both connected to logic high). Each flip-flop introduces a delay from clock edge to output toggle, thus causing the counter bits to change at different times and producing a ripple effect as the input clock propagates through the chain. The first flip-flop is clocked by rising edges all other flip-flops in the chain are clocked by falling clock edges. Shift register generator counter – based on a shift register with feedback.Ĭounters are implemented in a variety of ways, including as dedicated MSI and LSI integrated circuits, as embedded counters within ASICs, as general-purpose counter and timer peripherals in microcontrollers, and as IP blocks in FPGAs.Īsynchronous (ripple) counter Asynchronous counter created from two JK flip-flopsĪn asynchronous (ripple) counter is a "chain" of toggle (T) flip-flops wherein the least-significant flip-flop (bit 0) is clocked by an external signal (the counter input clock), and all other flip-flops are clocked by the output of the nearest, less significant flip-flop (e.g., bit 0 clocks the bit 1 flip-flop, bit 1 clocks the bit 2 flip-flop, etc.).Gray-code counter – outputs a sequence of Gray codes.Johnson counter – a twisted ring counter.Ring counter – formed by a "circular" shift register.Up/down counter – counts up and down, as directed by a control input, or by the use of separate "up" and "down" clocks.Decade counter – modulus ten counter (counts through ten states).Modulus counter – counts through a particular number of states.In asynchronous counters, each flip-flop has a unique clock, and the flip-flop states change at different times.Ĭounters are categorized in various ways. In synchronous counters, all flip-flops share a common clock and change state at the same time. For example, a four-bit counter can have a modulus of up to 16 (2^4).Ĭounters are generally classified as either synchronous or asynchronous. The maximum possible modulus is determined by the number of flip-flops. The modulus of a counter is the number of states in its count sequence. This is commonly used to implement counter cascading (combining two or more counters to create a single, larger counter) by connecting the Terminal Count output of one counter to the Enable input of the next counter. ![]() Some counters provide a Terminal Count output which indicates that the next clock will cause overflow or underflow. Load – copies parallel input data to the counts.Data – parallel input data which represents a particular counts value.Direction – determines whether counts will increment or decrement.Some IC manufacturers name it "clear" or "master reset (MR)". Many counters provide additional input signals to facilitate dynamic control of the counting sequence, such as: Examples of the latter include ring counters and counters that output Gray codes. Depending on the type of counter, the output may be a direct representation of the counts (a binary number), or it may be encoded. Internally, counters use flip-flops to represent the current counts and to retain the counts between clocks. When the counts have reached the end of the counting sequence (maximum counts when incrementing zero counts when decrementing), the next clock will cause the counts to overflow or underflow, and the counting sequence will start over. Upon each qualified clock edge, the circuit will increment (or decrement, depending on circuit design) the counts. Counters are a very widely used component in digital circuits, and are manufactured as separate integrated circuits and also incorporated as parts of larger integrated circuits.Īn electronic counter is a sequential logic circuit that has a clock input signal and a group of output signals that represent an integer "counts" value. Each pulse applied to the clock input increments or decrements the number in the counter.Ī counter circuit is usually constructed of several flip-flops connected in a cascade. The values on the output lines represent a number in the binary or BCD number system. The most common type is a sequential digital logic circuit with an input line called the clock and multiple output lines. In digital logic and computing, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock. Voltage changes on the five outputs of the binary counter counting from 00000, left to 11111 (or 31), right (vertically). For other uses, see Counter (disambiguation). This article is about the term counter used in electronics, computing, and mechanical counting devices.
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