Asynchronous vs synchronous reset

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Vijay
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Location: Bangalore, India

Asynchronous vs synchronous reset

Post by Vijay » Sun Apr 13, 2014 11:37 pm

What is the difference between asynchronous and synchronous reset? Explain the advantages and disadvantages of both?

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Narveer
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Location: Bangalore, INDIA

Re: Asynchronous vs synchronous reset

Post by Narveer » Mon Apr 14, 2014 8:27 am

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. The clock works as a filter for small reset glitches; however, if these glitches occur near the active clock edge, the flip-flop could go metastable.
In some designs, the reset must be generated by a set of internal conditions. A synchronous reset is recommended for these types of designs because it will filter the logic equation glitches between clock.

Disadvantages of synchronous reset:
Problem with synchronous resets is that the synthesis tool cannot easily distinguish the reset signal from any other data signal.
Synchronous resets may need a pulse stretcher to guarantee a reset pulse width wide enough to ensure reset is present during an active edge of the clock if you have a gated clock to save power, the clock may be disabled coincident with the assertion of reset. Only an asynchronous reset will work in this situation, as the reset might be removed prior to the resumption of the clock. Designs that are pushing the limit for data path timing, can not afford to have added gates and additional net delays in the data path due to logic inserted to handle synchronous resets.

Asynchronous reset :
The biggest problem with asynchronous resets is the reset release, also called reset removal. Using an asynchronous reset, the designer is guaranteed not to have the reset added to the data path. Another advantage favoring asynchronous resets is that the circuit can be reset with or without a clock present.

Disadvantages of asynchronous reset: ensure that the release of the reset can occur within one clock period. if the release of the reset occurred on or near a clock edge such that the flip-flops went metastable.

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