Dual flop synchronizers and mtbf

I started reading on dual flop synch and i found like for metastable data to settle , we add dual flop to give once cycle time to it.

my question is "After the first edge If the first flop goes metastable and settles to an incorrect value, then it will be propogated on flop 2".

1) How to solve this problem ?
2) what is the second flop goes metastable ? Do we add any control signal to for the second clock domain to capture the data after the certain clk edges?

please pardon me if i am made my queries unclear.

Thanks
Alauddin

my question is "After the first edge If the first flop goes metastable and settles to an incorrect value, then it will be propogated on flop 2".
1) How to solve this problem ?

Click to expand...

Your idea of an "incorrect value" is the problem...both values - either '0' or '1' shall be considered correct.
Think of the following game for analogy:
Someone is showing you a set of photos he took on his camera. he shows you a new photo every 20 seconds.
You keep your eyes open for 1 second and shut for 4 second - the process of closing your eyes takes around 0.25 seconds...
When your eyes are shut - your job is to think about and imagine the last photo you saw before you close them.

You play the game for a few minutes and everything goes fine...but than, just as you close your eyes the guy in front of you switches the photos.
At first you're a little confused and don't really know what photo to think about (the old one or the new one) - but than after a few moments you make up your mind and decide about one.

If you decide to imagine the old one - it's OK. It was a real photo with true information - not something made up. And the new image will still be there when you open your eyes - so you'll have a chance to imagine it too the next time you close your eyes.

If you decide to imagine the new one - it's also fine (as you already had the chance to think about the old one).

No matter your decision - there's no wrong choice.

This paper has diagrams that may help: http://www.sunburst-design.com/papers/CummingsSNUG2008Boston_CDC.pdf