synchronous (1) Pertaining to two or more processes that depend upon the occurrence of specific events such as common timing signals. (2) Occurring with a regular or predictable time relationship.
counter (1) A functional unit with a finite number of states each of which represents a number that can be, upon receipt of an appropriate signal, increased by unity or by a given constant. This device is usually capable of bringing the represented number to a specified value; for example zero.
So a "synchronous counter" is actually a functional unit with a certain number of states, each representing a number which can be increaced or decreased upon receiving an appropriate signal (e.g. a rising edge pulse), and is usually used to count to, or count down to zero from, a specified number N.
.Basically, any sequential circuit that goes through a prescribed sequence of states upon the application of input pulses is called a counter. The input pulses, called count pulses, may be clock pulses or they may originate from an external source and may occur at prescribed intervals of time or at random. The sequence of states in a counter may follow a binary count or any other sequence.
Why do we need counters?
In a digital circuit, counters are used to do 3 main functions: timing, sequencing and counting.
A timing problem might require that a high-frequency pulse train, such as the output of a 10-MHz crystal oscillator, be divided to produce a pulse train of a much lower frequency, say 1 Hz. This application is required in a precision digital clock, where it is not possible to build a crystal oscillator whose natural frequency is 1 Hz.
A sequencing problem would arise if, for instance, it became necessary to apply power to various components of a large machine in a specific order. The starting of a rocket motor is an example where the energizing of fuel pumps, ignition, and possibly explosive bolts for staging must follow a critical order.
Measuring the flow of auto traffic on roadway is an application in which an event (the passage of a vehicle) must increment a tally. This can be done automatically with an electronic counter triggered by a photocell or road sensor. In this way, the total number of vehicles passing a certain point can be counted.
How are counters made?
Counters are generally made up of flip-flops and logic gates. Like flip-flops, counters can retain an output state after the input condition which brought about that state has been removed. Consequently, digital counters are classified as sequential circuits. While a flip-flop can occupy one of only two possible sattes, a counter can have many more than two states. In the case of a counter, the value of a state is expressed as a multidigit binary number, whose `1's and `0's are usually derived from the outputs of internal flip-flops that make up the counter. The number of states a counter may have is limited only by the amount of electronic hardware that is available. The main types of flip-flops used are J-K flip-flops or T flip-flops, which are J-K flip-flops with both J and K inputs tied together. Before that, here's a quick reminder of how a J-K flip-flop works:
T flip-flops are used because set/reset ([1,0] [0,1]) functions are seldom used. Only the "do nothing" and toggle ([0,0] [1,1]) functions are used. Logic gates are used to decide when to toggle which outputs. Below is an example of a synchronous binary counter, implemented using J-K flip-flops and AND gates.
The difference between asynchronous and synchronous counters.
In an asynchronous counter, an external event is used to directly SET or CLEAR a flip-flop when it occurs. In a synchronous counter however, the external event is used to produce a pulse that is synchronised with the internal clock. An example of an asynchronous counter is a ripple counter. Each flip-flop in the ripple counter is clocked by the output from the previous flip-flop. Only the first flip-flop is clocked by an external clock. Below is an example of a 4-bit ripple counter:
So what's wrong with asynchronous counters?
Dangers of asynchronous counters.
Although the asynchronous counter is easier to implement, it is more "dangerous" than the synchronous counter. In a complex system, there are many state changes on each clock edge, and some IC's (integrated circuits) respond faster than others. If an external event is allowed to affect a system whenever it occurs, a small percentage of the time it will occur near a clock transition, after some IC's have responded, but before others have. This intermingling of transitions often causes erroneous operations. What is worse, these problems are difficult to test for and difficult to forsee because of the random time difference between the events.
Different types of synchronous counters
Binary counters are the simplest form of counters. An N-bit binary counter counts from 0 to (2N - 1) and back to 0 again.
Instead of just counting up (up counter), counters can be made to count down (down counter) or both up and down (up-down counter). The diagram below shows an up-down counter. The counter counts up or down depending on which of the "up"and "down" inputs are high.
And instead of counting from 0, a counter can be made to count from a given initial value. This type of counter is called a loadable counter.
A BCD counter counts in binary-coded decimal from 0000 to 1001 and back to 0000. Because of the return to 0 after a count of 9, a BCD counter does not have a regular pattern as in a straight binary count.
A ring counter is a circular shift register with only one flip-flop being set at any particular time; all others are cleared. The single bit is shifted from one flip-flop to the other to produce the sequence of timing signals.
The Johnson counter, also called the twisted ring counter, is a variation of the ring counter, with the inverse output of the most significant flip-flop passed to the input of the least significant flip-flop. The sequence followed begins with all 0's in the register. The final 0 will cause 1's to be shifted into the register from the left-hand side when clock pulses are applied. When the first 1 reaches the most significant flip-flop, 0's will be inserted into the first flip-flop because of the cross-coupling between the output and the input of the counter.