Difference Between Synchronous and Asynchronous Counter

# Difference Between Synchronous and Asynchronous Counter

Esha Gupta
Associate Senior Executive
Updated on Oct 25, 2023 13:20 IST

A counter in digital electronics is a sequential logic circuit that goes through a predetermined sequence of states upon the application of input pulses. Counters can be broadly classified into two categories based on their operation: synchronous counters and asynchronous counters. Both types are used to count input pulses, but they differ in their operation and design. Let’s read more!

A synchronous counter is a type of digital counter where the same clock signal drives all the flip-flops that store the binary value, and an asynchronous counter, also known as a ripple counter, is a type of digital counter where the flip-flops are not clocked at the same time. Instead, the output of one flip-flop serves as the clock input for the next flip-flop in the series.

## Difference Between Synchronous and Asynchronous Counter

The following are the differences between synchronous and asynchronous counters

## What is Synchronous Counter?

A synchronous counter is a type of digital counter where all the flip-flops (or bistable elements) that make up the counter are driven by the same clock signal. This means that all the flip-flops change their states simultaneously with each clock pulse.

Bistable elements are fundamental components in digital electronics, characterized by their ability to remain stable in one of two distinct states. These two states typically represent the binary values 0 and 1.

### Working Using the Diagram

1. Start of the Process

The process begins at the Clock (C), which is the primary driver of the counter’s operation. This is marked as “Start” in the diagram.

2. Clock Triggers the Counter

• The Clock (C) sends a trigger signal to the Counter (CT). This is typically in response to a rising edge of the clock pulse.
• The Counter (CT) increments its count based on this trigger. The note “Count on clock’s rising edge” next to the Counter indicates this action.

3. Counter Processes and Sends to Decoder

• Once triggered, the Counter (CT) processes the current count and sends a binary count to the Decoder (D).
• The binary count represents the current state or value of the Counter in binary form.

4. Decoder Converts Binary to Decimal

• The Decoder (D) receives the binary count and decodes it into a decimal output. This conversion is necessary for displaying the count in a human-readable format.
• The note “Decode binary to decimal” explains the Decoder’s function.

5. Output Displays the Count

• The decoded decimal output is then sent to the Output (O), where it is displayed. This could be a digital display or another form of output.
• The Output shows the count as a decimal number, as indicated by the note “Display decimal count.”

6. Feedback Loop for Next Count

• After displaying the count, the Output (O) signals back to the Clock (C) to prepare for the next count. This creates a feedback loop, ensuring continuous counting with each clock pulse.
• The Clock waits for the next pulse (“Wait for next clock pulse”) before repeating the cycle.

7. End of Cycle (Optional)

• Optionally, the Clock (C) can send a reset or stop signal to the Counter (CT). This action either resets the count to its initial state or stops the counting process.
• This is indicated by the note “Counter stops or resets,” showing the control the Clock has over the counting process.

### Component Roles

• C – Clock: Initiates and controls the counting process.
• CT – Counter: Counts in binary, incrementing with each clock pulse.
• D – Decoder: Converts the binary count from the Counter into a decimal or other readable format.
• O – Output: Displays the final count.

This sequence diagram effectively illustrates the synchronous nature of the counter, where each step is dependent on the clock signal, ensuring coordinated and timely operations.

Explore How to Convert Binary to Decimal

## What is Asynchronous Counter?

An asynchronous counter, also known as a ripple counter, is a type of digital counter in which the flip-flops that make up the counter are not triggered by the same clock signal. Instead, each flip-flop is triggered by the output of the preceding flip-flop. This design leads to a “ripple” effect in the counting sequence, where the change in state of one flip-flop triggers the change in the next, and so on.

### Diagram

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### Working Using the Diagram

1. Start of the Process

The process begins with the Clock (C). The note “Start” over the Clock indicates the initiation of the counting sequence.

2. Clock Pulse to Flip-Flop 1

The Clock (C) sends a pulse to Flip-Flop 1 (FF1). The note next to FF1 (“Toggle on Clock’s Edge”) indicates that FF1 changes its state (from 0 to 1 or from 1 to 0) whenever it receives a clock pulse.

3. Sequential Triggering of Flip-Flops

• After FF1 toggles its state, it sends a trigger to Flip-Flop 2 (FF2). The note next to FF2 (“Toggle on FF1’s Edge”) signifies that FF2 will change its state based on the change in state (edge) of FF1, not directly by the clock pulse.
• Similarly, FF2, once it toggles, triggers Flip-Flop 3 (FF3). The note next to FF3 (“Toggle on FF2’s Edge”) shows that FF3’s state change is dependent on the state change of FF2.

4. Output Update

Once FF3 toggles its state, it updates the Output (O). The note next to the Output (“Display Binary Count”) indicates that the Output shows the binary count based on the states of FF1, FF2, and FF3.

5. Awaiting the Next Pulse

After updating the Output, the system is ready for the next clock pulse to repeat the counting process. The note “Clock Drives Next Count” left of the Clock (C) indicates that the entire process is cyclic and driven by the clock pulses.

### Component Roles

• C – Clock: Initiates and controls the counting process, providing the timing pulses that drive the flip-flops.
• FF1 – Flip-Flop 1: Counts in binary, incrementing with each clock pulse. It toggles its state on each clock edge and triggers Flip-Flop 2.
• FF2 – Flip-Flop 2: Counts in binary, incrementing with each trigger from Flip-Flop 1. It toggles its state on the edge of Flip-Flop 1’s output and triggers Flip-Flop 3.
• FF3 – Flip-Flop 3: Counts in binary, incrementing with each trigger from Flip-Flop 2. It toggles its state on the edge of Flip-Flop 2’s output and updates the Output.
• O – Output: Displays the final count in binary form, reflecting the combined states of the flip-flops.

This sequence diagram effectively illustrates the asynchronous nature of the counter, where each flip-flop is triggered sequentially rather than directly by the clock signal, leading to a ripple effect through the counting process.

## Conclusion

Thus, the primary difference between asynchronous and synchronous counter in digital electronics lie in their design, operation speed, complexity, and applications.

Keep learning, keep exploring!