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Metastability in electronics is the ability of a non-equilibrium electronic state to persist for a long period of time (see asynchronous circuit). Equilibrium or balance is any of a number of related phenomena in the natural and social sciences. ...
An asynchronous circuit is a circuit in which the parts are largely autonomous. ...
Flip-flops
In electronics, the flip-flop is a device that suffers from metastability. It has two well-defined stable states, traditionally designated 0 and 1, but under certain conditions it can hover between them for longer than a clock cycle. Two digital voltmeters The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. ...
In electronics and digital circuits, the flip-flop or bistable multivibrator is a pulsed digital circuit capable of serving as a one-bit memory. ...
Synchronous circuits Synchronous circuit design techniques make digital circuits that are immune to the failure modes that can be caused by metastability. A "clock domain" is defined as a group of flip flops with a common clock. Such architectures can form a circuit guaranteed free of metastability (below a certain maximum clock frequency, above which first metastability, then outright failure occur). A synchronous circuit is a circuit in which the parts are synchronized by means of a clock subcircuit. ...
When synchronous design techniques are used, protection against metastable events causing systems failures need only be provided when transferring data between different clock domains. The common style of request and acknowledge "mailbox flag" handshaking is one way to accomplish this, (at the cost of uncertainty as to whether the data will arrive in time to be transferred on any particular clock tick, or will have to wait for a later one).
Synchronous systems with one clock have another reliability advantage from an electrical noise point of view that is different than metastability and is distinguished from it. On each clock cycle of the system, before the clock is applied to the flip flops, there has been enough time since the last clock for all flip flop outputs to be at a stable 0 or 1 level, and for all the signals derived from these levels to propagate through the gating to form stable electrical levels at the data input of all flip flops in the system. At the moment the clock pulse arrives at the flip flops, they read in these stable values. During this brief part of the cycle the flip flops are sensitive to electrical noise distorting the correct value of the data input. After a slight delay the outputs begin to change to the just read in input values. This is followed by a large amount of electrical switching noise as these changes propagate through the gates. Eventually, after the maximum propagation time through the combinatorial logic, as set by the design, all flip flop data inputs will be stable once again. After a slight delay the next clock tick arriving at the flip flops will repeat this process. In effect, the electrical noise is synchronous with the clock, and the flip flops take their new value at the quietest time in the cycle. When additional clocks not synchronized to the first are introduced, the electrical noise associated with these clocks will drift through time relative to the first clock. With straightforward statistics based on the probability of overlapping in time, this noise will challenge the data input of flip flops during the vulnerable moment they are reading in their new values. Metastability is a distinct issue, different from this electrical noise issue, although they are sometimes confused, as they both involve flip flops loading erroneous values. and point to the need to minimize the number of independent clock sources in a circuit.
Failure modes As metastability is well understood and architectural techniques to control it are known, why does it persist as a failure mode in equipment? A failure mode is a characterization of the way a product or process fails. ...
Serious computer and digital hardware bugs caused by metastability have a fascinating social history; many engineers refusing to believe a bistable device can enter into a state that is neither "true" nor "false" and remain there for some period of time, albeit with the probablity that it will remain indefinite exponentially decreasing over time. Yet metastability is an inevitable consequence of quantum indeterminacy, because if the inputs to an arbiter (flip flop) arrive almost simultaneosly, the cicuit most likely will traverse a point of metastability. Remarkably, semiconductor engineers whose livelihood is based on the quantum mechanical realities often cannot grasp the concept of indeterminacy, and have "pet circuits" said to "solve" or "filter out" the metastability. Typically they just shift the metastability from one place to another. Current engineering solutions to this problem are often the well-characterized, multistage common-clock shift registers discussed in the links below. Something that is bistable can be resting in two states. ...
Quantum Mechanical indeterminacy, or often just quantum indeterminacy refers to the same fundamental physics phenomenon as does the more frequently used Heisenberg uncertainty principle. ...
See also In electrical engineering, ground bounce is a phenomenon associated with transistor switching where the gate voltage can appear to be less than the local ground potential, causing the unstable operation of a logic gate. ...
To a large extent, the design of a CPU, or central processing unit, is the design of its control unit. ...
Professor Carver Andress Mead (born 1 May 1934, in Bakersfield, California) is a prominent U.S. computer scientist. ...
Lynn Conway is a U.S. computer scientist and inventor. ...
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