Synchronous and Asynchronous Counter: Key Differences Explained

A logical circuit consists of several flip-flops designed to tally negative and positive edge shifts. Counters can be categorized as either synchronous or asynchronous. The distinction between the two types of counters is determined by the specific flip-flops activated. This new technology article overviews the difference between synchronous and asynchronous counter.

What is a Synchronous Counter?

A synchronous counter operates in a way where every flip-flop receives its clock input from a single source, leading them to generate an output simultaneously.

Synchronous Counter Diagram

The figure below shows a logic diagram for a three-stage (modulo-8) synchronous counter. All the flip-flops are connected to the same clock input to minimize any potential counting discrepancies. The J and K inputs of the first flip-flop (FF1) are wired to a HIGH state to enable toggling. FF1's output is linked to the J and K inputs of the second flip-flop (FF2), one input of an AND gate, and the A indicator. The output from FF2 is connected to the second input of the AND gate and the B indicator, while the AND gate's output connects to the J and K inputs of the third flip-flop (FF3). The C indicator is exclusively linked to the output of FF3.

Logic diagram

While discussing the circuit, refer to the logic diagram and the pulse patterns displayed beneath it.

Timing diagram

Synchronous Counter Features

1. All flip-flops update their state at the same time;

2. The status of a flip-flop is influenced by the present state of its preceding stage and the upcoming state of its following stage;

3. Although this structure requires gate circuitry in contrast to asynchronous counter designs, it boasts a quicker counting rate;

4. Circuit carry comes in two variations: sequential and simultaneous. Using the simultaneous method can boost the counting velocity even more.

For additional intricate details about synchronous counter types, you can explore this guide.

What is an Asynchronous Counter?

The asynchronous counter, also known as a ripple counter, is a type of sequential circuit. In these counters, the output isn't reliant on just one clock signal. Instead, they utilize multiple clock signals to generate the designated asynchronous outputs. Unlike a standard universal clock, only the main clock drives the first flip-flop. For subsequent flip-flops, the output from the preceding one serves as the clock input.

Asynchronous Counter Circuit Diagram

The following circuit represents a 4-bit binary ripple counter. Each JK flip-flop adjusts its condition upon detecting a declining shift in its clock input.

Moreover, one flip-flop's output becomes the input clock for the following one. Hence, whenever a bit transitions from 1 to 0, it propels the carry-forward to the immediate higher bit.

4-bit binary ripple counter

Asynchronous Counter Using JK Flip Flop

A primary challenge during the adjustment is ensuring the 4-bit binary counter omits the 6 states from 1010-1111 during its count sequence. Let's assume that when the J and K circuits of the chosen flip-flops are not connected (floating), they're equivalent to a logic level of 1.

Starting the count from Q3Q2Q1Q0-0000, as depicted in the diagram, it's evident that the J and K signal input terminals for flip-flops FF0, FF1, and FF2 remain consistently at 1. This mirrors the behavior of a T' flip-flop. Its operation remains consistent with the asynchronous binary addition counter until the 8th count pulse is introduced. Throughout this phase, even though the pulse produced by Q0 is also directed to flip-flop FF3, with J3=Q2Q1=0 and K3=1 every time Q's falling edge appears, flip-flop FF3 continuously retains its 0 state.

Upon introducing the eighth count pulse (with the counter's status at Q3Q2Q1Q0-0111), J3 and K3 equal 1. Thus, after Q0's falling edge occurs, Q3 transitions from 0 to 1. Concurrently, with the change in Q3, J1 turns to 0. After inputting the 9th count pulse, the circuit's state becomes Q3Q2Q1Q0-1001. Following the 10th count pulse, flip-flop FF0 flips to 0, and Q0's falling edge resets flip-flop FF3 to 0. As a result, the circuit moves from 1001 back to 0000, bypassing the 6 states of 1010-1111, effectively operating as a decimal counter.

Asynchronous Up Counter

An asynchronous up counter begins its count at 0 and continues until it reaches its upper limit. The number of flip-flops in play determines the scope of its count.

You can construct an asynchronous using both T-Flip flop and D-Flip flop. The design approach for counters can vary based on the specific flip-flop type utilized.

Take, for instance, a 3-bit counter where the outputs Q0, Q1, and Q2 correspond to the flip-flops FF0, FF1, and FF2, respectively. The corresponding state table for this 3-bit counter is provided below:

State	Q2	Q1	Q0
0	0	0	0
1	0	0	1
2	0	1	0
3	0	1	1
4	1	0	0
5	1	0	1
6	1	1	0
7	1	1	1

Asynchronous Down Counter

This counter descends from its maximum count to 0 before cycling back to the highest value.

Envision a 3-bit down counter where the Q0, Q1, and Q2 outputs are linked to flip-flops FF0, FF1, and FF2, respectively. The associated state table for this down counter is presented below:

State	Q2	Q1	Q0
0	0	0	0
1	0	0	1
2	0	1	0
3	0	1	1
4	1	0	0
5	1	0	1
6	1	1	0
7	1	1	1

Asynchronous Up/Down Counter

This counter can count upwards and downwards, contingent upon the position of the mode select switch. This switch toggles between upward and downward counting functions. If M is set to 1, the counter increments, whereas if M is set to 0, it decrements.

What distinguishes the up-counter from the down-counter is the nature of the clock signal. Hence, a 2_1 Mux will be employed to choose between the two clock signals.

A diagram of the asynchronous Up/Down counter can be found below:

Asynchronous BCD Counter

A BCD (Binary Coded Decimal) counter is a decade counter with a modulus (Mod) of 10, signifying it has ten states. It counts decimal digits from 0 through 9, then reverts to its starting value 0. With each clock tick, this counter increments its decimal tally.

The Asynchronous/Ripple BCD counter functions similarly to the Ripple Up counter, but there's a key distinction: once the BCD counter reaches a count of 10, it resets its internal flip-flops.

Taking Q0, Q1, Q2, and Q3 as the four bits of the counter, the state table for the Ripple BCD counter can be outlined.

State	Q3	Q2	Q1	Q0
0	0	0	0	0
1	0	0	0	1
2	0	0	1	0
3	0	0	1	1
4	0	1	0	0
5	0	1	0	1
6	0	1	1	0
7	0	1	1	1
8	1	0	0	0
9	1	0	0	1
10 (reset)	1	0	1	0

Based on this table, it behaves largely as a regular up counter, with the 10th state acting as a reset trigger.

Should the count arrive at the 10th state, the counter must automatically refresh its flip-flops. A specific combinational circuit is required for this action. Typically, flip-flops possess an active-low reset. When the counter hits the 1010 state, this circuit should generate a logic' 0'. Interestingly, a NAND gate will also emit a logic '0' when all input signals are high.

Thus, a NAND gate becomes the tool to reset the flip-flop once a count of 10 is reached.

Clear = (Q3 AND Q1)’

The Asynchronous/Ripple BCD counter's design can employ T-flip or D-flip flops.

Key Differences Between Synchronous and Asynchronous Counter

The table below outlines the key distinctions between Synchronous and Asynchronous Counters.

	Synchronous Counter	Asynchronous Counter
Also called	Parallel Counter	Serial Counter
Trigger	In a synchronous counter, continuous clock inputs are provided, and flip-flops generate the output.	In asynchronous counters, distinct clock signals are utilized to generate the output.
Operation Speed	In synchronous counters, the processing speed is faster	In asynchronous counters, the processing speed is slower.
Error Prone	Synchronous counters are less error-prone due to their design. Each flip-flop is individually clocked, so they rarely introduce decoding errors or error-prone outcomes.	Asynchronous counters exhibit a greater "error-prone" tendency, often leading to both decoding issues and error-phone incidents within the system.
Complexity	In a synchronous counter, all flip-flops align with the clock, rendering its design notably more complex than its asynchronous counterpart.	In an asynchronous counter, the output of one flip-flop becomes the input for the next, rendering its design and deployment notably simple.
Sequence	Synchronous counters are adaptable to a variety of counting sequences.	Asynchronous counters operate with a predetermined set of counting sequences.
Delay	In Synchronous Counters, propagation delay is absent.	For asynchronous counters, a propagation delay occurs as the signal moves from one flip-flop to the next.
Overall settling time	The longest settling time among all the flip-flops determines the overall maximum settling time in the setup.	Totaling the settling times of every distinct flip-flop.

What are the Advantages of Synchronous vs Asynchronous Counter?

Advantages of Synchronous Counter

• It functions concurrently.

• There's an absence of propagation delay.

• Its speed surpasses that of the asynchronous counter.

• Designing it is more straightforward than its asynchronous counterpart.

• Errors are minimized as logic gates dictate the counting sequence.

Disadvantages of Synchronous Counter

• The execution requires considerable supplementary logic.

• As state numbers escalate, the circuitry grows intricate.

• A singular clock pulse governs all flip-flops in an asynchronous counter.

• Asynchronous counters, they demand a higher number of components and infrastructure.

Advantages of Asynchronous Counter

• Truncated counters enable the generation of any modulus number count.

• Each output in the sequence requires a state transition from the preceding flip-flop output.

• "Divide-by-n" counter circuits can be employed for their design.

• Often termed a ripple counter, an asynchronous counter appears to have data "ripple" from one flip-flop's output to the subsequent one's input.

Disadvantages of Asynchronous Counter

• To count sequences not equivalent to 2^n, extra feedback logic becomes essential.

• Counting inaccuracies manifest at elevated clock frequencies.

• Given that all flip-flops in synchronous counters respond to a concurrent clock pulse, they are quicker and more dependable.

• If many bits are involved, the time lapse between phases might extend to an undesirable length.

Conclusion

In summary, the discussion underscores that synchronous and asynchronous counters exhibit distinct attributes due to the varied application of the clock signal. As a result, they are suited for diverse applications.

Read More

• What is a 4k7 Resistor？4.7k ohm Resistor Color Code

• A Comprehensive Guide to Counter IC

• Synchronous Counter: Circuit, Types, and How it Works

• Exploring IC 7493: A Deep Dive into 4 Bit Binary Counter Circuit

• CD4060 IC: A Ultimate Guide to datasheet, circuit & application

Previous: CD4017BE CMOS Counter: Circuit, Pinout and Datasheet

Next: The Ultimate Guide to Ring Counter: Working, Types & Applications