Counter
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This article is about the term counter used in electronics and computing. For other uses, see Counter (disambiguation).
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 signal.
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Electronic counters
In electronics, counters can be implemented quite easily using register-type circuits such as the flip-flop, and a wide variety of classifications exist:- Asynchronous (ripple) counter – changing state bits are used as clocks to subsequent state flip-flops
- Synchronous counter – all state bits change under control of a single clock
- Decade counter – counts through ten states per stage
- Up/down counter – counts both up and down, under command of a control input
- Ring counter – formed by a shift register with feedback connection in a ring
- Johnson counter – a twisted ring counter
- Cascaded counter
Occasionally there are advantages to using a counting sequence other than the natural binary sequence—such as the binary coded decimal counter, a linear feedback shift register counter, or a Gray-code counter.
Counters are useful for digital clocks and timers, and in oven timers, VCR clocks, etc.[1]
Asynchronous (ripple) counter
An asynchronous (ripple) counter is a single JK-type flip-flop, with its J (data) input fed from its own inverted output. This circuit can store one bit, and hence can count from zero to one before it overflows (starts over from 0). This counter will increment once for every clock cycle and takes two clock cycles to overflow, so every cycle it will alternate between a transition from 0 to 1 and a transition from 1 to 0. Notice that this creates a new clock with a 50% duty cycle at exactly half the frequency of the input clock. If this output is then used as the clock signal for a similarly arranged D flip-flop (remembering to invert the output to the input), you will get another 1 bit counter that counts half as fast. Putting them together yields a two-bit counter:Cycle | Q1 | Q0 | (Q1:Q0)dec |
---|---|---|---|
0 | 0 | 0 | 0 |
1 | 0 | 1 | 1 |
2 | 1 | 0 | 2 |
3 | 1 | 1 | 3 |
4 | 0 | 0 | 0 |
The use of flip-flop outputs as clocks leads to timing skew between the count data bits, making this ripple technique incompatible with normal synchronous circuit design styles.
Synchronous counter
A simple way of implementing the logic for each bit of an ascending counter (which is what is depicted in the image to the right) is for each bit to toggle when all of the less significant bits are at a logic high state. For example, bit 1 toggles when bit 0 is logic high; bit 2 toggles when both bit 1 and bit 0 are logic high; bit 3 toggles when bit 2, bit 1 and bit 0 are all high; and so on.Synchronous counters can also be implemented with hardware finite state machines, which are more complex but allow for smoother, more stable transitions.
Hardware-based counters are of this type.
Decade counter
A decade counter is one that counts in decimal digits, rather than binary. A decade counter may have each digit binary encoded (that is, it may count in binary-coded decimal, as the 7490 integrated circuit did) or other binary encodings (such as the bi-quinary encoding of the 7490 integrated circuit). Alternatively, it may have a "fully decoded" or one-hot output code in which each output goes high in turn (the 4017 is such a circuit). The latter type of circuit finds applications in multiplexers and demultiplexers, or wherever a scanning type of behavior is useful. Similar counters with different numbers of outputs are also common.The decade counter is also known as a mod-counter when it counts to ten (0, 1, 2, 3, 4, 5, 6, 7, 8, 9). A Mod Counter that counts to 64 stops at 63 because 0 counts as a valid digit.
Up/down counter
A counter that can change state in either direction, under the control of an up/down selector input, is known as an up/down counter. When the selector is in the up state, the counter increments its value. When the selector is in the down state, the counter decrements the count.Ring counter
Main article: Ring counter
A ring counter is a circular shift register which is initiated such
that only one of its flip-flops is the state one while others are in
their zero states.A ring counter is a Shift Register (a cascade connection of flip-flops) with the output of the last one connected to the input of the first, that is, in a ring. Typically, a pattern consisting of a single bit is circulated so the state repeats every n clock cycles if n flip-flops are used. It can be used as a cycle counter of n states.
Gaffar Counter
Johnson counter
Main article: Ring counter
A Johnson counter (or switchtail ring counter, twisted-ring counter,
walking-ring counter, or Moebius counter) is a modified ring counter,
where the output from the last stage is inverted and fed back as input
to the first stage.[2][3][4]
The register cycles through a sequence of bit-patterns, whose length is
equal to twice the length of the shift register, continuing
indefinitely. These counters find specialist applications, including
those similar to the decade counter, digital-to-analog conversion, etc.
They can be implemented easily using D- or JK-type flip-flops.Computer science counters
Main article: Register machine
In computability theory, a counter is considered a type of memory. A counter stores a single natural number (initially zero) and can be arbitrarily many digits long. A counter is usually considered in conjunction with a finite-state machine (FSM), which can perform the following operations on the counter:- Check whether the counter is zero
- Increment the counter by one.
- Decrement the counter by one (if it's already zero, this leaves it unchanged).
- Deterministic or non-deterministic FSM plus two counters
- Non-deterministic FSM plus one stack
- Non-deterministic FSM plus one counter
- Deterministic FSM plus one counter
- Deterministic or non-deterministic FSM
The first machine, an FSM plus two counters, is equivalent in power to a Turing machine. See the article on counter machines for a proof.
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