Synchronous and Asynchronous Sequential Circuits utilize feedback for generating the next output. These circuits can be differentiated based on the type of feedback employed. A sequential circuit’s output depends on both present and past inputs.
Synchronous Sequential Circuits
Synchronous Sequential Circuits, when all outputs change in relation to the active transition of a clock signal, are known as Synchronous Sequential Circuits. In this type of circuit, all outputs change simultaneously. Therefore, the outputs of synchronous sequential circuits synchronize with either the positive or negative edges of the universal clock signal. Synchronous sequential circuits are reliable as their transition states are always predictable. However, their operational speed is slower due to the propagation delay of the clock signal throughout the circuit. Counters, shift registers, and memory units commonly use synchronous sequential circuits.
Asynchronous Sequential Circuits
Asynchronous Sequential Circuits are those in which some or all outputs do not change with respect to the active transition of a clock signal. Outputs in an asynchronous sequential circuit do not change simultaneously. Most outputs in asynchronous sequential circuits are not synchronized with the positive or negative edges of the universal clock signal. Asynchronous sequential circuits operate based on input-driven signals, meaning output variables change directly in response to input variable changes. However, these circuits can enter incorrect states due to timing differences between input arrivals, leading to race conditions. Consequently, asynchronous sequential circuits are less reliable.
Difference between Synchronous and Asynchronous Sequential Circuits
Here are the key differences between Synchronous and Asynchronous Sequential Circuits:
Synchronous Sequential Circuits:
- Employ clock signals for feedback using clocked flip-flops as memory units.
- States are always predictable and reliable.
- Design is relatively easier.
- Slower performance due to clock signal propagation delay.
- Examples include counters, shift registers, and memory units.
Asynchronous Sequential Circuits:
- Lack clock signal-based feedback and instead use unclocked flip-flops or time delays as memory elements.
- May enter incorrect states due to timing differences between input arrivals (race conditions).
- Design is more challenging due to instability issues caused by feedback among logic gates.
- Faster performance compared to Synchronous Sequential Circuits as there is no clock signal delay.
- Examples include low-power/high-speed operations such as simple microprocessors, digital signal processing units, and communication systems for email, internet access, and networking.
In conclusion, Synchronous Sequential Circuits rely on clock signals for feedback, while Asynchronous Sequential Circuits do not depend on clock signals for feedback in generating the next output.
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