Synchronous & Asynchronous Sequential Circuits

[Dineshkumar005]

What are the major differences between Synchronous & Asynchronous Sequential Circuits?

 

[FoxyRick]

Asynchronous Sequential circuits do not use a clock and can change their output state as fast as the signal path's propagation delay from the input allows. This means they can be faster than Synchronous Sequential circuits. However, they are considerably more likely to suffer from race conditions (inputs arriving at different times causing different output states) and intermediate output states (as the outputs change from one state to the next final state) than Synchronous Sequential circuits.

Synchronous Sequential circuits use a clock signal to alleviate the two problems mentioned above. The outputs can only change state with the clock and are designed such that all propagation delays are satisfied before the outputs are allowed to change. This however makes them potentially slower (because the whole circuit must run at the speed of the slowest path in it) and consumes significantly more power due to the extra circuitry required by distributing the clock to all flip-flops, and the continual switching.

Homework done!

 

[jeffrey samuel]

Nice home work here pal

The only significant difference is that the control passes in an Async counter in the form of ripples as the previous stage triggers the next

The sync devices are controlled solely by the clock pulse directly so no delay in responses by different parts of the circuit

 

[bhatt_tanmay]

Nearly all sequential logic today is clocked or synchronous logic. In a synchronous circuit, an electronic oscillator called a clock generates a sequence of repetitive pulses called the clock signal which is distributed to all the memory elements in the circuit. The basic memory element in sequential logic is the flip-flop. The output of each flip-flop only changes when triggered by the clock pulse, so changes to the logic signals throughout the circuit all begin at the same time, at regular intervals, synchronized by the clock.
The output of all the storage elements (flip-flops) in the circuit at any given time, the binary data they contain, is called the state of the circuit. The state of a synchronous circuit only changes on clock pulses. At each cycle, the next state is determined by the current state and the value of the input signals when the clock pulse occurs.
The main advantage of synchronous logic is its simplicity. The logic gates which perform the operations on the data require a finite amount of time to respond to changes to their inputs. This is called propagation delay. The interval between clock pulses must be long enough so that all the logic gates have time to respond to the changes and their outputs "settle" to stable logic values, before the next clock pulse occurs. As long as this condition is met (ignoring certain other details) the circuit is guaranteed to be stable and reliable. This determines the maximum operating speed of a synchronous circuit.
Asynchronous sequential logic is not synchronized by a clock signal; the outputs of the circuit change directly in response to changes in inputs. The advantage of asynchronous logic is that it can be faster than synchronous logic, because the circuit doesn't have to wait for a clock signal to process inputs. The speed of the device is potentially limited only by the propagation delays of the logic gates used.
However, asynchronous logic is more difficult to design and is subject to problems not encountered in synchronous designs. The main problem is that digital memory elements are sensitive to the order that their input signals arrive; if two signals arrive at a logic gate at almost the same time, which state the circuit goes into can depend on which signal gets to the gate first. Therefore the circuit can go into the wrong state, depending on small differences in the propagation delays of the logic gates. This is called a race condition. This problem is not as severe in synchronous circuits because the outputs of the memory elements only change at each clock pulse. The interval between clock signals is designed to be long enough to allow the outputs of the memory elements to "settle" so they are not changing when the next clock comes. Therefore the only timing problems are due to "asynchronous inputs"; inputs to the circuit from other systems which are not synchronized to the clock signal.
Asynchronous sequential circuits are typically used only in a few critical parts of otherwise synchronous systems where speed is at a premium, such as parts of microprocessors and digital signal processing circuits.
The design of asynchronous logic uses different mathematical models and techniques from synchronous logic, and is an active area of research.

 

[Dineshkumar005]

Please do not PM me with questions, unless it is a personal matter.

what did you mean by that?

 

[jeffrey samuel]

I am sure PMs are helpful but some people misuse the PM facility to torment others

 

[FoxyRick]

Re: Synchronous & Asynchronous Sequential Circuits

what did you mean by that?

Some people like to use Personal Messages to ask for help, instead of asking on the forum.

1. This is specifically against the rules of edaboard.
2. It prevents other people from sharing the benefit of any such communications.
3. It can become quite annoying, especially when certain people do it regularly despite being repeatedly asked not to.
4. Everyone here is 'volunteering' their time, and should not be subjected to personal 'demands' for attention by PM.

While it is possible to block PMs, I prefer not to do so in case someone wants to contact me for a reasonable reason (e.g. 'I think I know you from another forum. Is that you?') instead of just demanding help.

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Just to add: If you are asking because the line is on my post replying to you, that is just because it is in my signature. It appears on every post I make and is not aimed particularly at you, rather at everyone who reads my posts.

I have only put it there because I have had problems with the issue, in the hope that some people read it (as you have) and realise not to do it.