Counters

It is a Sequential device that  stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal.
counters can be implemented quite easily using register-type circuits such as the flip flop and a wide variety of designs exist, e.g.:

• 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

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