synchronizer doubt - single flip flop instead of double

[mkanimozhivlsi]

synchronizer doubt

Hi Experts,
I have one doubt in synchronizer block design ,we normally use the synchronizer block for communicate between the two asynchronous block,and all the design every where using the back to back flip flop , why can't we use the single flip flop instead of double FF,what is the necessity,can anyone clarify my doubt.

Regards,
Kanimozhi.M

 

[ckaa]

Re: synchronizer doubt

When you try to communicate between two asynchronous blocks you could possibly have metastability(because of unknown phase relationships between clock and data in your design). This happens when data changes within the set-up/hold time window. You try and account for metastability and reduce the probability of failures by using synchronizers. Every flip flop added to your synchronizer increases your MTBF(Mean Time Between Failures). You can never totally eliminate a failure but can reduce the probability of occurance of a failure or increase your MTBF. There are mathematical equations to calculate your MTBF and if you go through those you see that using two flip-flops in your synchronizer almost squares the probability of a failure.In most designs your MTBF with a single stage synchronizer may not be acceptable\safe, but usually the MTBF with a 2 stage synchronizer is good enough.