Synchronous Reset VS Asynchronous Reset

Why Reset?

A Reset is required to initialize a hardware design for system operation and to force an ASIC into a known state for simulation.

A reset simply changes the state of the device/design/ASIC to a user/designer defined state. There are two types of reset, what are they? As you can guess them, they are Synchronous reset and Asynchronous reset.

Synchronous Reset

A synchronous reset signal will only affect or reset the state of the flip-flop on the active edge of the clock. The reset signal is applied as is any other input to the state machine.


  • The advantage to this type of topology is that the reset presented to all functional flip-flops is fully synchronous to the clock and will always meet the reset recovery time.
  • Synchronous reset logic will synthesize to smaller flip-flops, particularly if the reset is gated with the logic generating the d-input. But in such a case, the combinational logic gate count grows, so the overall gate count savings may not be that significant.
  • Synchronous resets provide some filtering for the reset signal such that it is not effected by glitches, unless they occur right at the clock edge. A synchronous reset is recommended for some types of designs where the reset is generated by a set of internal conditions. As the clock will filter the logic equation glitches between clock edges.
  • The problem in this topology is with reset assertion. If the reset signal is not long enough to be captured at active clock edge (or the clock may be slow to capture the reset signal), it will result in failure of assertion. In such case the design needs a pulse stretcher to guarantee that a reset pulse is wide enough to be present during the active clock edge.
  • Another problem with synchronous resets is that the logic synthesis cannot easily distinguish the reset signal from any other data signal. So proper care has to be taken with logic synthesis, else the reset signal may take the fastest path to the flip-flop input there by making worst case timing hard to meet.
  • In some power saving designs the clocked is gated. In such designed only asynchronous reset will work.
  • Faster designs that are demanding low data path timing, can not afford to have extra gates and additional net delays in the data path due to logic inserted to handle synchronous resets.
Asynchronous Reset

An asynchronous reset will affect or reset the state of the flip-flop asynchronously i.e. no matter what the clock signal is. This is considered as high priority signal and system reset happens as soon as the reset assertion is detected.

  • High speeds can be achieved, as the data path is independent of reset signal.
  • Another advantage favoring asynchronous resets is that the circuit can be reset with or without a clock present.
  • As in synchronous reset, no work around is required for logic synthesis.
  • The problem with this type of reset occurs at logic de-assertion rather than at assertion like in synchronous circuits. If the asynchronous reset is released (reset release or reset removal) at or near the active clock edge of a flip-flop, the output of the flip-flop could go metastable.
  • Spurious resets can happen due to reset signal glitches.

Both types of resets have positives and negatives and none of them assure fail-proof design. So there is something called "Asynchronous assertion and Synchronous de-assertion" reset which can be used for best results. (which will be discussed in next post).


Yaqoot said...

Cud u plz explain the 2nd advantage & 3rd disadvantage in more detail??

Yaqoot said...

i meant ...of synchronous reset

Yaqoot said...

actually 3rd advantage n 2nd disadv....

VSD Team said...

VSD is an IIT Bombay Alumnus initiative which plays a role of bridging up the gap between the Concepts learned in Graduate School to the Knowledge applied in Industrial World. Building a Chip is like building a City. And the most important part of any Structure is the base i.e. Basic and Fundamentals of the CHIP Design.
Our idea is to build an Eco-System for Knowledge Sharing. We have tried to squeeze in the concepts related to Chip design, Physics of Semiconductor and Industrial flow into very simple Infographics Macro video.
Below link has the introductory lectures.
VSD Team

Vlsi Training said...

Wiztech offers best VLSI Training Institute in Chennai, that offers advanced practical oriented project training.
More details,

Vlsi Training said...

Welcome to ‘Wiztech Automation’ - a premier institute for higher learning in VLSI, Embedded System And PLC technologies. Wiztech Automation deliver the VLSI training courses in Chennai for students and working professionals who are pursuing a career in VLSI Core. The course syllabus is designed keeping in mind the requirements of the industry by using industry standard software tools for lab and projects.
More Details,
VLSI Training in Chennai

Vlsi Training said...

VLSI Training Institute in Chennai
VLSI Course in Chennai
Best VLSI Training Centre in Chennai
VLSI Project image gallery
VLSI Training Enquiry
Contact us

mallika said...

logical effort for cmos based dual mode logic gates

AceZed ITSolution said...

Nice Information you have written here. Really Great Stuff. I keep it bookmark for our future purpose.

We are also Web development Company in India who provide the services in Android App Development in Nagpur , SEO Company in Nagpur , Ecommerce Website Development in Nagpur. Visit Us today

AceZed IT Solution

prabhjyot Suri said...

Hi. Very nice post. Please help me understand how the reset recovery time will always be met with the synchronous reset?

Semicon Technologies said...

This blog is really informative for vlsi. We have best Teaching for the VLSI training. VLSI Training in Bangalore|VLSI Training Center in Bangalore

Sumit Kumar said...

nice post sharing get more information Abohar City


Your Blog is Very Nice...!!!!
Web Design and Development Company in India

Sathya G said...

Really very informative and creative contents. These concept is a good way to enhance the knowledge.I like it and help me to development very well.Thank you for this brief explanation and very nice information.
Digital Marketing Training in Chennai

Thamizharasi J said...

from my experience point of view score is must and needful for everygraduate students. thanks for valuable article.
SAT Training Centre in Chennai

Deeksha Shukla said...

best digital marketing company in lucknow
digital marketing company in lucknow
digital marketing agency in lucknow

dhivya priya said...

Truly a very good article on how to handle the future technology. After reading your post,thanks for taking the time to discuss this, I feel happy about and I love learning more about this topic
Isoft Innovations Facebook
Isoft Innovation Chennai

Abiya Carol said...

Great post! I am actually getting ready to across this information, It's very helpful for this blog.Also great with all of the valuable information you have Keep up the good work you are doing well.

Java Training in Chennai

aarthi rengarajan said...

Thanks for sharing this with us it is a worth read. xcellent post!!! Our Digital Marketing Training is tailored for beginners who want to learn how to stand out digitally, whether it is for their own business or a personal brand.

Digital Marketing Training in Chennai
Digital Marketing Course in Chennai
SEO Training in Chennai
What is Digital Marketing
Digital Marketing
Digital Marketing Company
SEO Services in India

Albert Smith said...

Thanks for sharing this article on Synchronous Reset VS Asynchronous Reset. I have learned the major difference between synchronous and asynchronous reset. I was unable to differentiate the two terms but this article has helped me to clearly understand the function of each reset. I will suggest this idea as a research topic title to students who access our professional Qualitative Capstone Project Paper Editing Help.

Lunameadow said...

I think great site for these post and i am read the most of contents have useful for my Carrier.Thanks for these useful information.Any information are commands like to share him.

Oracle Training in Chennai

Karthika Shree said...

Great post! I am see the great contents and step by step read really nice information.I am gather this concepts and more information. It's helpful for me my friend. Also great blog here with all of the valuable information you have.
Java Training in Chennai

Mahalyasree said...

Great articles, first of all Thanks for writing such lovely Post! Earlier I thought that posts are the only most important thing on any blog. But here at Shoutmeloud I found how important other elements are for your blog.Keep update more posts..
Architects in Chennai

Anonymous said...

Why stooped blogging just out of curiosity

sandhosh said...

Thank you for this great article which conveyed a good information about synchronous vs asynchronous reset.keep more updates.
SEO Services in India

 Save and Share: Digg Reddit Facebook Mixx Google YahooMyWeb blogmarks Blue Dot StumbleUpon Bumpzee Furl Sphinn Ma.gnolia MisterWong Propeller Simpy TwitThis Wikio BlinkList NewsVine