Synchronous Counter: Circuit, Types, and How it Works

A counter is a device used for tallying specific occurrences, hinging on prior events. In the realm of computers or digital logic systems, its pivotal role lies in tracking and storing event frequencies tied to a CLK signal. There exist diverse counter variants: synchronous, asynchronous, synchronous/asynchronous decade, and synchronous/asynchronous up-down counters.

Prominent among counters is the sequential logic circuit, equipped with a solo CLK and multiple outputs. These outputs correspond to binary or decimal representations, modulated by binary code. With each CLK signal, the number either increments or decrements. This new technology article offers an overview of synchronous counters, delving into their operation and applications.

What is a Synchronous Counter?

A Synchronous counter is one in which all flip-flops utilize a shared clock input source, resulting in output generation occurring simultaneously. Each flip-flop (FF) in this counter is synchronized with a shared clock signal, triggering simultaneous activation. 

   

In contrast to synchronous design, the process of configuring in an asynchronous manner is considerably uncomplicated. However, asynchronous counters face a restriction in terms of their maximum operational frequency. To address this constraint, the strategy often involves implementing concurrent clocking, thereby synchronizing output alterations with the input clock signal.

Synchronous Counter Circuit Diagram

The explanation and functioning of the synchronous counter are provided below. The circuit diagram for the 3-bit synchronous counter is depicted, employing 2 AND logic gates, 3 J-K FFs, and a CLK signal that triggers the Flip Flop.

In this configuration, an active high signal triggers flip flop A at the falling edge of each CLK input, causing it to toggle. A similar setup is used for flip flop B, wherein the output hinges on the preceding FFs input and output, with B toggling when the AND gate is activated by the high output of flip-flop A.

3 bit Synchronous Counter Circuit Diagram

Consequently, flip-flop C is driven by the output of the second AND gate, toggling once the A2 logic gate activates. Activation of the A1 logic gate and Flip Flop-B outputs, both being high, triggers the A2 logic gate.

The operation of a 3-bit synchronous counter is outlined as follows: Initially, the flip-flops are set at 0, rendering their outputs QCQBQA = 000. At the falling edge of the first CLK signal, flip-flop A transitions from zero to one. However, flip-flops B and C remain unchanged until the subsequent CLK signal.

Upon providing the first CLK signal, the flip-flop outputs become QCQBQA = 001. Prior to the second clock signal, both flip flops A and B are set to 1 due to the A1 gate output. At the falling edge of the second CLK signal, both flip flops toggle again, altering FFA's output from one to zero and FFB's output from zero to one. Consequently, the output becomes 010, and gates A1 and A2 are deactivated.

With the application of the third clock signal, flip-flop A toggles, activating logic gates A1 and A2, yielding an output of 011. Upon the fourth clock signal, all three FF inputs become high. As a result, the falling edge of the fourth flip flop toggles all outputs: QA and QB shift to 0, while QC transitions to 1. The overall output for this CLK signal becomes 100, and gates A1 and A2 are turned off.

Upon the subsequent CLK signal, at its falling edge, flip-flop A toggles again, transitioning the outputs QCQBQA to 10, activating gates A1 and A2.

The sixth CLK signal prompts flip-flop A to toggle at its falling edge, altering its output from 1 to 0. Concurrently, the input to flip-flop B turns high, causing its output to change from 0 to 1. This results in an output of 110 for QCQBQA. This pattern continues, and at the falling edge of the 8th CLK signal, all FF outputs reset to 000.

In synchronous counters, it's crucial for all FFs to reset simultaneously. The counter setting time corresponds to the propagation delay of each flip-flop. This counter can effectively function with a high-frequency CLK signal.

Truth Table

The truth table for the 3-bit synchronous counter is displayed here, derived from the preceding explanation.

CLK	QC	QB	QA	Decimal Equivalent
Firstly	0	0	0	0
1st Falling Edge	0	0	1	1
2nd Falling Edge	0	1	0	2
3rd Falling Edge	0	1	1	3
4th Falling Edge	1	0	0	4
5th Falling Edge	1	0	1	5
6th Falling Edge	1	1	0	6
7th Falling Edge	1	1	1	7
8th Falling Edge	0	0	0	0

The timing diagram of the synchronous counter is shown below.

Timing Diagram of Synchronous Counter

Synchronous Counter Types

In digital electronics, an array of synchronous counters exists, encompassing diverse categories such as binary counters, 4-bit synchronous UP and DOWN counters, 4-bit synchronous UP/DOWN counters, BCD counters, Synchronous decade counters, 2-bit and 3-bit counters, loadable counters, Johnson counters, and ring counters. Several of these variants are explored in the subsequent sections.

Binary Counters

A binary tally device is a circuitry comprised of flip-flops, where the output of one flip-flop is directed into the subsequent flip-flop consecutively. Depending on how the flip flops (FFs) are interlinked, a binary counter can function in either a synchronous or asynchronous manner. In synchronous operation, all FFs are activated concurrently via a uniform CLK signal.

Conversely, an asynchronous counter is commonly referred to as a ripple counter. In this counter variant, the CLK signal is initially fed into the first FF, and subsequently, it propagates to the following FFs with a certain delay.

4- Bit Synchronous Up Counter

Crafting a 4-Bit synchronous up counter closely parallels the methodology employed for a 3-bit synchronous up counter, differing only in the count of flip flops engaged. In this counter, a quartet of JK flip flops is harnessed for its construction. The rationale for employing this flip flop lies in its characteristic toggling upon simultaneous high inputs, all in tune with the CLK signal.

Uniformly, the external CLK signal permeates all four flip-flops in unison. The counter encompasses 16 distinct output states, diligently progressing from 0000 to 1111. Much like its 3-bit counterpart, the timing diagram and operational dynamics remain consistent within this iteration.

4- Bit Synchronous Down Counter

The primary purpose of this tally mechanism entails a decremental numerical sequence. In contrast to the incrementing design of an up counter, the down counter follows a parallel structure, albeit with a requisite decrement. As such, the inputs of the JK flip-flops are routed towards Q' (complement of Q), while an identical external CLK signal synchronizes the four flip-flops integrated into the configuration.

Upon initiation of the countdown progression, the FF inputs are initially elevated to a high state, aligning with the descent of the numerical sequence. Hence, commencing from 1111, the count regresses until halting at 0000, akin to the behavior of an up counter. It is pertinent to observe that in this variant, should the foremost flip-flop generate a low logic output, the antecedent flip-flop experiences a straightforward toggle.

2 bit Synchronous Counter

Constructing a 2-bit synchronous counter involves the utilization of a pair of bidirectional JK-flip flops, alongside two Feynman gates. In this context, the Feynman gate corresponds to a CNOT (Controlled-NOT) gate, harnessed to replicate signals due to the constraints imposed by reversible logic circuits on fan-out. Hence, this gate functions as a means to copy signals akin to a fan-out process.

The CLK input interfaces with the Feynman gate, with the ensuing output linked to a subsequent gate as an input, concurrently establishing a connection with the bidirectional JK flip flop by serving as a CLK input.

Synchronous Up/Down Counter or Bidirectional Counter

The synchronous tally system is engineered to enable bidirectional counting by responding to control signals, thereby facilitating count progression in diverse directions. Employing a JK flip-flop as a T flip-flop bolsters its bit retention capabilities.

For instance, a 3-bit bidirectional counter encompasses 8 feasible output configurations. Depending on the governing control input, the counter adeptly navigates count progression. If initiated in the upward trajectory, the count transverses from 0 to 7. Upon altering the control input, the counter seamlessly reverses its course, descending from 7 back to 0.

In essence, the counter's operational orientation hinges upon the control input. When the control input registers as 1, it deactivates the 2nd and 4th AND gates, concurrently activating the 1st and 3rd gates. Consequently, the counter commences its upward counting trajectory.

Triggering the Counter

Synchronous Tally Systems employ flip-flops triggered by specific edges, transitioning on the "positive-edge" (upswing) or "negative-edge" (downswing) of the clock pulse received via the controlling input. This elicits a solitary count alteration when the clock input undergoes a state transformation.

Ordinarily, the rising-edge, the shift from low to high in the clock signal, is the preferred point for synchronous counters to tally. Conversely, asynchronous ripple counters tabulate during the falling-edge, signifying the shift from high to low in the clock signal.

The employment of the falling-edge for ripple counters might appear counterintuitive. Yet, it simplifies the interlinking of counters. This is because the most significant bit (MSB) of one counter can dictate the clock input of its successor.

This approach functions by necessitating the subsequent bit to change states upon the prior bit's transition from high to low—a juncture demanding a carry into the next bit. Synchronous counters typically integrate carry-out and carry-in pins, facilitating inter-counter connections devoid of introducing any delays in signal propagation.

What are the Advantages and Disadvantage of Synchronous Counter?

We have now gained an understanding of the Synchronous counter and discerned the disparities between the Asynchronous and Synchronous counter configurations. The Synchronous counter effectively addresses numerous limitations inherent in the Asynchronous counterpart.

The merits of the Synchronous counter encompass:

1. Simpler design compared to the Asynchronous counter.

2. Simultaneous operation.

3. Absence of propagation delay.

4. Count sequence manipulation through logic gates, leading to reduced error probabilities.

5. Expedited performance when juxtaposed with the Asynchronous counter.

Nonetheless, amid the many advantages, it is crucial to acknowledge a significant drawback of the Synchronous counter – its operation necessitates considerable additional logic implementation.

Applications of Synchronous Counter

The utility of synchronous counters spans various applications, including:

• Machine Motion control

• Motor RPM counter

• Rotary Shaft Encoders

• Digital clock 

• Pulse generators

• Digital Watch 

• Alarm systems

Top 4 Popular Synchronous Counters at Jotrin

74191 IC: Synchronous Up/Down 4-Bit Binary Counter with Mode Control.

74193 Counter: Synchronous 4-Bit Up-Down Binary Counter.

74161 Counter: 4-Bit Synchronous Binary Counter.

74163 Counter: 4-Bit Binary Up Counter with Synchronous Load and Synchronous Clear.

Conclusion

Therefore, this encapsulates an outline of the synchronous counter, which can be constructed through Toggle or D-type flip-flops. To synthesize key points regarding Synchronous Counters:

• Synchronous Counters are assembled from Toggle or D-type flip-flops.

• Designing synchronous counters proves more straightforward than designing asynchronous counterparts.

• Dubbed synchronous counters due to the synchronized clocking of flip-flop clock inputs using a shared clock signal.

• The uniform clock pulse results in simultaneous state transitions for all output states.

• Unified clock input eliminates inherent delays in propagation.

• Synchronous counters are at times denoted as parallel counters, given the parallel clock input to all flip-flops.

• Inherent memory mechanisms retain the counter's ongoing state.

• Control over the count sequence is exerted through logic gates.

• In comparison to Asynchronous counters, quicker overall operation is attainable.

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