Timing Diagram for Positive Edge-Triggered Flip-Flops

Decade Counters

• A decade counter has a modulus of 10 and is used in display applications where BCD is required for conversion to a decimal readout.
• A 4-bit counter is required to implement a decade counter, as 2^3 (or 3 flip-flops) is insufficient.

Asynchronous Counters

• Asynchronous counters are also known as ripple counters.
• The events in an asynchronous counter do not have a fixed time relationship with each other and do not occur at the same time.
• In an asynchronous counter, the flip-flops do not change states at exactly the same time because they do not have a common clock pulse.

Synchronous Counters

• Synchronous counters have a fixed time relationship with each other and do occur at the same time.
• In a synchronous counter, all the bits in the count change synchronously with the assertion of the clock.

Counters

• A counter can be constructed by a synchronous circuit or by an asynchronous circuit.
• Counters can count up or count down or count up and down depending on the input control.
• A counter may have a limited range due to its limited word length.
• The count sequence usually repeats itself.
• External logic can be used to cause the counter to start at any count and terminate at any count.

Uses of Counters

• Counters are typically used to count the number of times that a certain event takes place.
• Counters are used to control a fixed sequence of actions in a digital system.
• Counters are used to generate timing signals.
• Counters are used to generate clocks of different frequencies.

Flip-Flops

• Flip-flops can be connected together to perform counting operations.
• The number of flip-flops used and the way they are connected determine the number of states (modulus) and the specific sequence of states that the counter goes through during each complete cycle.