[SI-LIST] Re: METASTABILITY

Hi,
 

See Dr. Howard Johnson explanations:

 

http://www.sigcon.com/Pubs/news/4_2.htm

 

Edi

 

> -----Original Message-----

> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]

> On Behalf Of Somesh Dhavala

> Sent: Thursday, January 05, 2006 7:15 PM

> To: Mike Greim

> Cc: si-list@xxxxxxxxxxxxx

> Subject: [SI-LIST] Re: METASTABILITY

> 

> Hi  Mike,

> Thank you for your Reply.

> 

> How to reduce the metastability by connecting two or more flip flops
in

> chain when first flip flop latching wrong data?

> 

> On 1/5/06, Mike Greim <mike.greim@xxxxxxxxxxx> wrote:

> 

> >

> > Clocked flip-flops are prone to a problem called metastability,
which

> > happens when a data or control input is changing at the instant of
the

> clock

> > pulse. The result is that the output may behave unpredictably,
taking

> many

> > times longer than normal to settle to its correct state, or even

> oscillating

> > several times before settling. In a computer system this can cause

> > corruption of data or a program crash.

> >

> > In many cases, metastability in flip-flops can be avoided by
ensuring

> that

> > the data and control inputs are held constant for specified periods

> before

> > and after the clock pulse, called the setup time (tsu) and the hold
time

> > (th) respectively. These times are specified in the data sheet for
the

> > device, and are typically between a few nanoseconds and a few
hundred

> > nanoseconds for modern devices.

> >

> > Unfortunately, it is not always possible to meet the setup and hold

> > criteria, because the flip-flop may be connected to a real-time
signal

> that

> > could change at any time, outside the control of the designer. In
this

> case,

> > the best the designer can do is to reduce the probability of error
to a

> > certain level, depending on the required reliability of the circuit.
One

> > technique for suppressing metastability is to connect two or more
flip-

> flops

> > in a chain, so that the output of each one feeds the data input of
the

> next,

> > and all devices share a common clock. With this method, the
probability

> of a

> > metastable event can be reduced to a negligible value, but never to
zero.

> >

> > So-called metastable-hardened flip-flops are available, which work
by

> > reducing the setup and hold times as much as possible, but even
these

> cannot

> > eliminate the problem entirely. This is because metastability is
more

> than

> > simply a matter of circuit design. When the transitions in the clock
and

> the

> > data are close together in time, the flip-flop is forced to decide
which

> > event happened first. However fast we make the device, there is
always

> the

> > possibility that the input events will be so close together that it

> cannot

> > detect which one happened first. It is therefore logically
impossible to

> > build a perfectly metastable-proof flip-flop.

> >

> >

> > -----Original Message-----

> > From: si-list-bounce@xxxxxxxxxxxxx

> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Somesh Dhavala

> > Sent: Thursday, January 05, 2006 8:54 AM

> > To: si-list@xxxxxxxxxxxxx

> > Subject: [SI-LIST] METASTABILITY

> >

> >

> > Hi All,

> > I am very new to the SI.

> > I am unable to understand the concept of *METASTABILITY*, I am very

> thank

> > ful to you if you explain me in detail.

> > Please suggest me some documents.

> >

> > Thanks & Regards

> > Somesh Dhavala

> > CG-CoreEl

> >

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> >

> >

> 

> 

> --

> Somesh Dhavala

> CG-CoreEl

> 

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