A decade counter is very common in today’s electronics. Most commonly available as IC CD7490, contains multiple flip flops to convert BCD-to-decimal and is incorporated as part of larger integrated circuits. Basically, counters can be implemented quite easily using register type circuits. Besides decade counter, there are various others that are also used regularly. Let’s take a look.
An asynchronous counter is a simple D-Flip flop, with the output fed back as input. The output changes state for each clock input. This creates a circuit that can store one bit of information. This counter will increment once for every clock cycle and takes two clock cycles for a transition from 0 to 1 and a transition from 1 to 0 creating a new clock with a 50% duty cycle.
The clock inputs of all the flip-flops are connected together and are triggered by the input pulses. Thus, all the flip-flops change state simultaneously. An advantage of synchronous counters is that there is no cumulative time delay because all flip-flops are triggered in parallel.
A ring counter is a shift register with the output of one flip flop connected to the input of the next 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 is initiated such that only one of its flip-flops is the state one while others are in their zero states
A Johnson counter is a kind of modified ring counter, where the output of the last stage is inverted before being fed back into the first flop. The register cycles through a sequence of bit-patterns, whose length is equal to twice the length of the shift register, continuing indefinitely. It is very commonly found in digital-to-analog converters.
The basic decade counter is an electronic circuit with a 4-bit binary output and an input signal (called a clock). With each clock pulse the outputs advance to the next higher value, resetting to 0000 when the output is 1001 and a subsequent clock pulse is received. Decade counters are used in clock circuits, frequency dividers, state machines, and sequencers, just to name a few applications.
For more detailed lesson on counters, have a look at the presentation below;