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 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu