To finding out the answer of question what is a counter? We should know as name suggests counter is a digital device which count something. Basically counters are one of the simplest types of sequential networks of digital electronics. And a counter is usually constructed from one or more flip flops that change output accordingly input pulses are received. So we can say a counter basically counts those pulses which are driven by a clock. Since the clock pulses occur at known intervals, the counter can be used as an instrument for measuring time and therefore period of frequency.
Now question is how many categories we can classify the Counters? And answer is we can be broadly classified into three categories:
(i) Asynchronous and Synchronous counters.
(ii) Single and multi mode counters.
(iii) Modulus counters.
See the block diagram of a counter in bellow.
The asynchronous counter is simple and straightforward in operation and construction and usually requires a minimum amount of hardware. In asynchronous counters, each flip flop is triggered by the previous flip flop, and hence the speed of operation is limited. In fact, the settling time of the counter is the cumulative sum of the individual settling times of the flip flops. This type of counters is also called ripple or serial counter.
As we know the problem of asynchronous counter it needs lots of time to perform its task. So the speed limitation of asynchronous counters can be overcome by applying clock pulses simultaneously to all of the flip flops. This causes the settling time of the flip-flops to be equal to the propagation delay of a single flip flop. The increase in speed is usually attained at the price of increased hardware. This type of counter is also known as a parallel counter.
Now as per our requirement counters can be designed such that the contents of the counter advances by one with each clock pulse; and is said to operate in the count-up mode. And some time we need the opposite which is also possible, when the counter is said to operate in the count-down mode. Now if the same counter circuit can be operated in both the UP and DOWN modes, it is called a multimode counters or we can say UP-DOWN Counter.
Now the last category is Modulus counters which can be defined based on the number of states they are capable of counting. This type of counter can again be classified into two types: Mod N and MOD < N. For example, if there are n bits then the maximum number counted can be 2n or N. If the counter is so designed that it can count up to 2n or N states, it is called MOD N or MOD