Meta-stability - Revision history

Jnc: /* Further reading */ link to online copies

2024-02-28T00:11:03Z

Jnc: /* See also */ +M7821 Interrupt Control - a DEC arbiter card which likely had meta-stability issues

2024-02-20T05:40:02Z

See also: +M7821 Interrupt Control - a DEC arbiter card which likely had meta-stability issues

← Older revision		Revision as of 05:40, 20 February 2024
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	It is thought that meta-stability issues may have caused many computer 'crashes' early on, but since meta-stability failures are very hard to reproduce outside of carefully controlled laboratory situations, likely most such crashes went un-recognized for what they were.		It is thought that meta-stability issues may have caused many computer 'crashes' early on, but since meta-stability failures are very hard to reproduce outside of carefully controlled laboratory situations, likely most such crashes went un-recognized for what they were.

			==See also==

			* [[M7821 Interrupt Control]] - a DEC arbiter card which likely had meta-stability issues

	== Further reading==		== Further reading==

Jnc: Avoid redir

2023-03-26T12:48:54Z

Avoid redir

← Older revision		Revision as of 12:48, 26 March 2023
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	'''Meta-stability''' refers to a situation in which a [[digital]] [[circuit]] element (usually one involving feedback - i.e. outputs are connected back to inputs) which is theoretically [[binary]] is actually in a state which is neither '0' nor '1'. (This is an instance of [[Vonada's Maxim]] that 'digital circuits are made from analog parts'.)		'''Meta-stability''' refers to a situation in which a [[digital]] [[circuit]] element (usually one involving feedback - i.e. outputs are connected back to inputs) which is theoretically [[binary]] is actually in a state which is neither '0' nor '1'. (This is an instance of [[Vonada's Engineering Maxims\|Vonada's Maxim #2]] that 'digital circuits are made from analog parts'.)

	Generally, if the output is 'toward' one of those two values, the circuit moves quickly (through feedback) all the way to that value. However, if it is balanced precisely in the middle, it may take a while before it fluctuates to one side or the other, and then rapidly moves to that value.		Generally, if the output is 'toward' one of those two values, the circuit moves quickly (through feedback) all the way to that value. However, if it is balanced precisely in the middle, it may take a while before it fluctuates to one side or the other, and then rapidly moves to that value.
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	This situation may resolve itself quickly (i.e. considerably faster than the speed of the overall circuit), in which case it is generally not an issue; or it may persist for some time (in theory, for an un-bounded period), in which case it can be a significant problem.		This situation may resolve itself quickly (i.e. considerably faster than the speed of the overall circuit), in which case it is generally not an issue; or it may persist for some time (in theory, for an un-bounded period), in which case it can be a significant problem.

	Note that there is no way to avoid this situation (and considerable theoretical work has gone into studying it, and 'proves' that there is no solution). The best one can do is create ~~circits~~ that ask 'is it still trying to decide', and if so, wait.		Note that there is no way to avoid this situation (and considerable theoretical work has gone into studying it, and 'proves' that there is no solution). The best one can do is create circuits that ask 'is it still trying to decide', and if so, wait.

	==Latches and arbiters==		==Latches and arbiters==

Jnc: +link

2020-02-12T14:03:52Z

+link

← Older revision		Revision as of 14:03, 12 February 2020
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	The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.		The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.

	As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual [[propagation delay]]. However, if several inputs change at the same time (i.e. a ~~'''~~race~~'''~~ condition), if the timing in the race is perfect (sic), it can cause meta-stability.		As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual [[propagation delay]]. However, if several inputs change at the same time (i.e. a [[race]] condition), if the timing in the race is perfect (sic), it can cause meta-stability.

	One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which can cause a latch to go meta-stable.		One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which can cause a latch to go meta-stable.

Jnc: slight copy-edit

2018-12-25T00:59:05Z

slight copy-edit

← Older revision		Revision as of 00:59, 25 December 2018
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	'''Meta-stability''' refers to a situation in which a digital circuit element (usually one involving feedback) which is theoretically binary is actually in a state which is neither '0' nor '1'. (This is an instance of [[Vonada's Maxim]] that 'digital circuits are made from analog parts'.)		'''Meta-stability''' refers to a situation in which a [[digital]] [[circuit]] element (usually one involving feedback - i.e. outputs are connected back to inputs) which is theoretically [[binary]] is actually in a state which is neither '0' nor '1'. (This is an instance of [[Vonada's Maxim]] that 'digital circuits are made from analog parts'.)

	Generally, if the output is 'toward' one of those two values, the circuit moves quickly (through feedback) all the way to that value. However, if it is balanced precisely in the middle, it may take a while before it fluctuates to one side or the other, and then rapidly moves to that value.		Generally, if the output is 'toward' one of those two values, the circuit moves quickly (through feedback) all the way to that value. However, if it is balanced precisely in the middle, it may take a while before it fluctuates to one side or the other, and then rapidly moves to that value.
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	==Latches and arbiters==		==Latches and arbiters==

	The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback ~~- i.e. outputs are connected back to inputs~~. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.		The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.

	As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a '''race''' condition), if the timing in the race is perfect (sic), it can cause meta-stability.		As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual [[propagation delay]]. However, if several inputs change at the same time (i.e. a '''race''' condition), if the timing in the race is perfect (sic), it can cause meta-stability.

	One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which can cause a latch to go meta-stable.		One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which can cause a latch to go meta-stable.

Jnc: +cat

2018-12-14T19:58:57Z

+cat

← Older revision		Revision as of 19:58, 14 December 2018
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	* Thomas J. Chaney, Charles E. Molnar, "''Anomalous Behavior of Synchronizer and Arbiter Circuits''", IEEE Transactions on Computers, Vol. C-22, pp 421-422, 1973 - One of the earliest papers in the literature once meta-stability had been accepted in the field as a real issue		* Thomas J. Chaney, Charles E. Molnar, "''Anomalous Behavior of Synchronizer and Arbiter Circuits''", IEEE Transactions on Computers, Vol. C-22, pp 421-422, 1973 - One of the earliest papers in the literature once meta-stability had been accepted in the field as a real issue
	* Thomas J. Chaney, "''My Work on All Things Metastable: Me and My Glitch''", 2012 - Extensive notes about work on meta-stability during a professional career		* Thomas J. Chaney, "''My Work on All Things Metastable: Me and My Glitch''", 2012 - Extensive notes about work on meta-stability during a professional career

			[[Category: Theory]]

Jnc: /* Latches and arbiters */ Bold redir target

2018-09-18T03:08:19Z

Latches and arbiters: Bold redir target

← Older revision		Revision as of 03:08, 18 September 2018
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	The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback - i.e. outputs are connected back to inputs. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.		The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback - i.e. outputs are connected back to inputs. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.

	As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a 'race' condition), if the timing in the race is perfect (sic), it can cause meta-stability.		As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a '''race''' condition), if the timing in the race is perfect (sic), it can cause meta-stability.

	One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which ~~may~~ cause a latch to go meta-stable.		One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which can cause a latch to go meta-stable.

	A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ensue isn't harmful.		A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ensue isn't harmful.

Jnc: Link latch

2018-03-10T19:08:27Z

Link latch

← Older revision		Revision as of 19:08, 10 March 2018
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	==Latches and arbiters==		==Latches and arbiters==

	The most common circuit to display this issue is the latch, which is a circuit with feedback - i.e. outputs are connected back to inputs. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.		The most common circuit to display this issue is the [[flip-flop\|latch]], which is a circuit with feedback - i.e. outputs are connected back to inputs. In line with the axiom above, this feedback is not digital (i.e. restricted to '0' or '1'), but may be any intermediate voltage.

	As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a 'race' condition), if the timing in the race is perfect (sic), it can cause meta-stability.		As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a 'race' condition), if the timing in the race is perfect (sic), it can cause meta-stability.

Jnc: /* Latches and arbiters */ typo

2017-07-07T21:47:36Z

Latches and arbiters: typo

← Older revision		Revision as of 21:47, 7 July 2017
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	As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a 'race' condition), if the timing in the race is perfect (sic), it can cause meta-stability.		As long as only one input to a latch changes at a time, there is no problem; the output will change as expected, after the usual propagation delay. However, if several inputs change at the same time (i.e. a 'race' condition), if the timing in the race is perfect (sic), it can cause meta-stability.

	One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which may cause a latch ~~may~~ go meta-stable.		One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which may cause a latch to go meta-stable.

	A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ensue isn't harmful.		A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ensue isn't harmful.

Jnc: /* Latches and arbiters */ typo

2017-04-23T17:27:30Z

Latches and arbiters: typo

← Older revision		Revision as of 17:27, 23 April 2017
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	One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which may cause a latch may go meta-stable.		One class of circuit in which meta-stability issues are a particular concern are [[synchronizer]]s of all kinds, particularly [[arbiter]]s. These are often constructed using latches, and their inputs may contain races (indeed, the function of an arbiter is precisely to resolve races). Unfortunately, it is precisely races which may cause a latch may go meta-stable.

	A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ~~ensure~~ isn't harmful.		A designer cannot avoid all races (some are inherent in what a particular circuit is trying to do), but they can try and move them to places (in time and/or the logic) where the meta-stability which may ensue isn't harmful.

	==History==		==History==

← Older revision		Revision as of 00:11, 28 February 2024
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	== Further reading==		== Further reading==

	* Severo M. Ornstein, "''Computing in the Middle Ages''", AuthorHouse, 2002 - An autobiography which describes early WUSTL work with meta-stability		* Severo M. Ornstein, [https://archive.computerhistory.org/resources/access/text/2019/03/102785079-05-01-acc.pdf ''Computing in the Middle Ages''"], AuthorHouse, 2002 - An autobiography which describes early WUSTL work with meta-stability
	* Warren Mackie Littlefield, Thomas J. Chaney, "''The Glitch Phenomenon''", Technical Memo No. 10, Washington University in St. Louis, 1966 - An extensive look at early WUSTL work on meta-stability		* Warren Mackie Littlefield, Thomas J. Chaney, "''The Glitch Phenomenon''", Technical Memo No. 10, Washington University in St. Louis, 1966 - An extensive look at early WUSTL work on meta-stability
	* Thomas J. Chaney, Charles E. Molnar, "''Anomalous Behavior of Synchronizer and Arbiter Circuits''", IEEE Transactions on Computers, Vol. C-22, pp 421-422, 1973 - One of the earliest papers in the literature once meta-stability had been accepted in the field as a real issue		* Thomas J. Chaney, Charles E. Molnar, [https://itlab.us/pubs/molnar_glitch.pdf ''Anomalous Behavior of Synchronizer and Arbiter Circuits''], IEEE Transactions on Computers, Vol. C-22, pp 421-422, 1973 - One of the earliest papers in the literature once meta-stability had been accepted in the field as a real issue
	* Thomas J. Chaney, "''My Work on All Things Metastable: Me and My Glitch''", 2012 - Extensive notes about work on meta-stability during a professional career		* Thomas J. Chaney, [https://www.arl.wustl.edu/~jon.turner/cse/260/glitchChaney.pdf ''My Work on All Things Metastable: Me and My Glitch''], 2012 - Extensive notes about work on meta-stability during a professional career

	[[Category: Theory]]		[[Category: Theory]]