FreeLists / si-list / [SI-LIST] Re: METASTABILITY

["Mike Greim" <mike.greim@xxxxxxxxxxx>]

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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