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Metastability in electronicsFrom Wikipedia, the free encyclopediaFor other uses of the term, see Metastability .

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Metastability in electronics is the ability of a digital electronic system to persist for anunbounded time in an unstable equilibrium or metastable state. [1] In metastable states, the circuitmay be unable to settle into a stable ' ' or '1' logic le!el "ithin the time required for proper circuitoperation. #s a result, the circuit can act in unpredictable "ays, and may lead to a system failure,sometimes referred to as a $glitch$. [%]&etastability is an instance of uridan's ass parado(.

&etastable states are inherent features of asynchronous digital systems , and of systems "ithmore than one independent clock domain. In self)timed asynchronous systems, arbiters aredesigned to allo" the system to proceed only after the metastability has resol!ed, so themetastability is a normal condition, not an error condition. [*] In synchronous systems "ithasynchronous inputs, synchroni+ers are designed to make the probability of a synchroni+ationfailure acceptably small. [ ] &etastable states are a!oidable in fully synchronous systems "hen theinput setup and hold time requirements on flip)flops are satisfied.

Contents [hide ]

• 1#rbiters

• %-ynchronous circuits

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*Failure modes

• -ee also

• /eferences

• 0 (ternal links

Arbiters [edit ]Main article: arbiter (electronics)

In electronics, an arbiter is a circuit designed to determine "hich of se!eral signals arri!e first. #rbiters are used in asynchronous circuits to order computational acti!ities for shared resourcesto pre!ent concurrent incorrect operations. #rbiters are used on the inputs of fully synchronoussystems, and also bet"een clock domains, as synchroni+ers for input signals. #lthough they canminimi+e the occurrence of metastability to !ery lo" probabilities, all arbiters ne!ertheless ha!emetastable states, "hich are una!oidable at the boundaries of regions of the input statespace resulting in different outputs. [ ]

Synchronous circuits [edit ]-ynchronous circuit design techniques make digital circuits that are resistant to the failure modesthat can be caused by metastability. # clock domain is defined as a group of flip)flops "ith acommon clock. -uch architectures can form a circuit guaranteed free of metastability 2belo" acertain ma(imum clock frequency, abo!e "hich first metastability, then outright failure occur3,

https://en.wikipedia.org/wiki/Metastabilityhttps://en.wikipedia.org/wiki/Metastabilityhttps://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttps://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edithttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edithttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edithttps://en.wikipedia.org/wiki/Help:Introduction_to_referencing_with_Wiki_Markup/1https://en.wikipedia.org/wiki/Help:Introduction_to_referencing_with_Wiki_Markup/1https://en.wikipedia.org/wiki/Equilibrium_pointhttps://en.wikipedia.org/wiki/Equilibrium_pointhttps://en.wikipedia.org/wiki/Metastabilityhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-1https://en.wikipedia.org/wiki/Logic_levelhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-2https://en.wikipedia.org/wiki/Buridan's_ass#Application_to_digital_logic:_Metastabilityhttps://en.wikipedia.org/wiki/Asynchronous_circuithttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-bainbridge-3https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-4https://en.wikipedia.org/wiki/Setup_time#Setup_and_hold_timeshttps://en.wikipedia.org/wiki/Setup_time#Setup_and_hold_timeshttps://en.wikipedia.org/wiki/Metastability_in_electronicshttps://en.wikipedia.org/wiki/Metastability_in_electronics#Arbitershttps://en.wikipedia.org/wiki/Metastability_in_electronics#Synchronous_circuitshttps://en.wikipedia.org/wiki/Metastability_in_electronics#Failure_modeshttps://en.wikipedia.org/wiki/Metastability_in_electronics#See_alsohttps://en.wikipedia.org/wiki/Metastability_in_electronics#Referenceshttps://en.wikipedia.org/wiki/Metastability_in_electronics#External_linkshttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=1https://en.wikipedia.org/wiki/Arbiter_(electronics)https://en.wikipedia.org/wiki/Arbiter_(electronics)https://en.wikipedia.org/wiki/Synchronizerhttps://en.wikipedia.org/wiki/SR_latch#Timing_considerationshttps://en.wikipedia.org/wiki/State_space_(controls)https://en.wikipedia.org/wiki/State_space_(controls)https://en.wikipedia.org/wiki/State_space_(controls)https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-5https://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=2https://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=2https://en.wikipedia.org/wiki/Synchronous_circuithttps://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edithttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edithttps://en.wikipedia.org/wiki/Help:Introduction_to_referencing_with_Wiki_Markup/1https://en.wikipedia.org/wiki/Equilibrium_pointhttps://en.wikipedia.org/wiki/Metastabilityhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-1https://en.wikipedia.org/wiki/Logic_levelhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-2https://en.wikipedia.org/wiki/Buridan's_ass#Application_to_digital_logic:_Metastabilityhttps://en.wikipedia.org/wiki/Asynchronous_circuithttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-bainbridge-3https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-4https://en.wikipedia.org/wiki/Setup_time#Setup_and_hold_timeshttps://en.wikipedia.org/wiki/Metastability_in_electronicshttps://en.wikipedia.org/wiki/Metastability_in_electronics#Arbitershttps://en.wikipedia.org/wiki/Metastability_in_electronics#Synchronous_circuitshttps://en.wikipedia.org/wiki/Metastability_in_electronics#Failure_modeshttps://en.wikipedia.org/wiki/Metastability_in_electronics#See_alsohttps://en.wikipedia.org/wiki/Metastability_in_electronics#Referenceshttps://en.wikipedia.org/wiki/Metastability_in_electronics#External_linkshttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=1https://en.wikipedia.org/wiki/Arbiter_(electronics)https://en.wikipedia.org/wiki/Synchronizerhttps://en.wikipedia.org/wiki/SR_latch#Timing_considerationshttps://en.wikipedia.org/wiki/State_space_(controls)https://en.wikipedia.org/wiki/State_space_(controls)https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-5https://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=2https://en.wikipedia.org/wiki/Synchronous_circuithttps://en.wikipedia.org/wiki/Metastability

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assuming a lo") ske" common clock. 4o"e!er, e!en then, if the system has a dependence onany continuous inputs then these are likely to be !ulnerable to metastable states. [0]

When synchronous design techniques are used, protection against metastable e!ents causingsystems failures need only be pro!ided "hen transferring data bet"een different clock domainsor from an unclocked region into the synchronous system. 5his protection can often take the form

of a series of delay flip)flops "hich delay the data stream long enough for the metastability toha!e statistically been remo!ed.

Failure modes [edit ] #lthough metastability is "ell understood and architectural techniques to control it are kno"n, itpersists as a failure mode in equipment.

-erious computer and digital hard"are bugs caused by metastability ha!e a fascinating socialhistory. &any engineers ha!e refused to belie!e that a bistable de!ice can enter into a state thatis neither true nor false and has a positi!e probability that it "ill remain indefinite for any gi!enperiod of time, albeit "ith e(ponentially decreasing probability o!er time. 4o"e!er, metastabilityis an ine!itable result of any attempt to map a continuous domain to a discrete one. 5here "ill

al"ays be points in the continuous domain "hich are equidistant 2or nearly so3 from the points ofthe discrete domain, making a decision as to "hich discrete point to select a difficult andpotentially lengthy process. [6] If the inputs to an arbiter or flip)flop arri!e almost simultaneously,the circuit most likely "ill tra!erse a point of metastability. &etastability remains poorlyunderstood in some circles, and !arious engineers ha!e proposed their o"n circuits said to sol!eor filter out the metastability7 typically these circuits simply shift the occurrence of metastabilityfrom one place to another. [8] 9hips using multiple clock sources are often tested "ith tester clocksthat ha!e fi(ed phase relationships, not the independent clocks drifting past each other that "illbe e(perienced during operation. 5his usually e(plicitly pre!ents the metastable failure mode that"ill occur in the field from being seen or reported. 9urrent engineering solutions to this problemare often the "ell)characteri+ed, multi)stage common)clock shift registers discussed in the linksbelo".

https://en.wikipedia.org/wiki/Clock_skewhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-6https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flophttps://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flophttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=3https://en.wikipedia.org/wiki/Failure_modehttps://en.wikipedia.org/wiki/Digital_hardwarehttps://en.wikipedia.org/wiki/Flip-flop_(electronics)https://en.wikipedia.org/wiki/Flip-flop_(electronics)https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-Lamport_1984-7https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-8https://en.wikipedia.org/wiki/Clock_skewhttps://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-6https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip-flophttps://en.wikipedia.org/w/index.php?title=Metastability_in_electronics&action=edit&section=3https://en.wikipedia.org/wiki/Failure_modehttps://en.wikipedia.org/wiki/Digital_hardwarehttps://en.wikipedia.org/wiki/Flip-flop_(electronics)https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-Lamport_1984-7https://en.wikipedia.org/wiki/Metastability_in_electronics#cite_note-8