“Asynchronous V/S Synchronous Circuits”

Topics: Digital electronics, Synchronous circuit, Sequential logic Pages: 9 (2579 words) Published: September 9, 2012


(CSE 2011)



I would like to take this opportunity to thank certain people without whom I would not have been able to make this project report. Firstly, I would like to express my gratitude towards my teacher, Mr. Harshpreet Singh Walia, Assistant Professor, Department of CSE, for his kind co-operation and encouragement which helped me in completion of this project. I am highly indebted to my friends for their guidance and constant supervision as well as for providing necessary information regarding the project. Lastly, I would like to thank my family members who always support me keep encouraging me to do better. -CHHAVI SHARMA


1.1 Abstract
2. 1Asynchronous circuits
2.1.1 Advantages
2.1.2 Disadvantages
3. Clock Signals
4. Clock Gating
5. Synchronous circuits
i) Digital Electronics
a) Advantages
b) Disadvantages
i) Sequential machines
a) Moore machine
b) Mealy machine
4. Synchronous v/s Asynchronous circuits
5. Conclusion
6. References


The term paper is about synchronous and asynchronous circuits and their differences. The advantages and disadvantages of both synchronous and asynchronous circuits are discussed further. The clock gating and clock signals are explained. Further, the sequential machines moore and mealy are discussed. In the end, the differences between the two types of circuits are highlighted. Also, the term paper is summed with a conclusion in the end.

An asynchronous circuit is a circuit in which the parts are largely autonomous. They are not governed by a clock circuit or global clock signal, but instead needs to only wait for the signals that indicate completion of instructions and operations. These signals are specified by simple data transfer protocols. [1]This digital logic design is contrasted with a synchronous circuit which operates according to clock timing signals. Asynchronous logic is the logic required for the design of asynchronous digital systems. This functions without a clock signal and so individual logic elements cannot be relied upon to have a discrete true/false state at any given time. Benefits

1) Robust handling of metastability of arbiters.
* Metastability in electronics is the ability of a digital electronic system to persist for an unbounded time in an unstable equilibrium or metastable state. * In metastable states, the circuit may be unable to settle into a stable '0' or '1' logic level within the time required for proper circuit operation. As a result, the circuit can act in unpredictable ways, and may lead to a system failure. Flip- flops:

In electronics, the flip-flop is a device that is susceptible to metastability. It has two well-defined stable states, traditionally designated 0 and 1, but under certain conditions (setup or hold time violation) it can hover between them for longer than a clock cycle. This condition is known as metastability. Such a metastable "state" is considered a failure mode of the logic design and timing philosophy or implementation. The most common cause of metastability is violating the flip-flop's setup and hold times. During the time from the setup to the hold time (capture window), the data input of the flip-flop should remain in a stable logic state; a change of the data input in that time will have a probability of setting the flip-flop to a metastable state[2].

Causes of metastability
1. The target clock having a different frequency than the source flip-flop, in which case the setup and hold time of the target flip-flop will be violated eventually, or 2. The target and source clock having the same frequency, but a phase alignment that causes the data to arrive at the target flip-flop during its...
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