Arbiter - Revision history

Jnc: +link

2021-03-03T07:35:37Z

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	and decides which of them arrived first.		and decides which of them arrived first.

	For signals that arrive more or less simultaneously (i.e. a so-called [[race]] condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.		For signals that arrive more or less simultaneously (i.e. a so-called [[race]] condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide [[mutual exclusion]]; e.g. to prevent two different clients from trying to use a single resource at the same time.

	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

Jnc: link to new page

2020-02-12T14:09:30Z

link to new page

← Older revision		Revision as of 14:09, 12 February 2020
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	and decides which of them arrived first.		and decides which of them arrived first.

	For signals that arrive more or less simultaneously (i.e. a so-called [[~~Meta-stability#Latches and arbiters\|~~race]] condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.		For signals that arrive more or less simultaneously (i.e. a so-called [[race]] condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.

	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

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2020-02-06T05:09:35Z

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	A '''arbiter''' is a device which examines two or more ~~signals~~ which are either:		A '''arbiter''' is a device which examines two or more [[signal]]s which are either:

	* synchronous, but using ~~clocks~~ which are not fixed relative to one another; or		* [[synchronous]], but using [[clock]]s which are not fixed relative to one another; or
	* at least one is asynchronous		* at least one is [[asynchronous]]

	and decides which of them arrived first.		and decides which of them arrived first.

	For signals that arrive more or less simultaneously (i.e. a so-called 'race' condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.		For signals that arrive more or less simultaneously (i.e. a so-called [[Meta-stability#Latches and arbiters\|race]] condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.

	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

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2019-11-11T16:45:38Z

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	If a specialized three-output flip-flop is not available, one common approach is to use a two-flop design, of the kind used in [[synchronizer]]s.		If a specialized three-output flip-flop is not available, one common approach is to use a two-flop design, of the kind used in [[synchronizer]]s.

			==See also==

			* [[Bus Arbitration on the Unibus and QBUS]]

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

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2018-12-14T20:00:33Z

+cat

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	* Warren Mackie Littlefield, Thomas J. Chaney, "''The Synchronizer "Glitch" Problem''", Technical Report No. 47, Washington University in St. Louis, 1974 - Includes perhaps the earliest description of the 'three-output flip-flop' solution		* Warren Mackie Littlefield, Thomas J. Chaney, "''The Synchronizer "Glitch" Problem''", Technical Report No. 47, Washington University in St. Louis, 1974 - Includes perhaps the earliest description of the 'three-output flip-flop' solution
	* David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution		* David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution

			[[Category: Hardware Basics]]

Jnc: Add 'race' terminology

2017-04-23T17:24:43Z

Add 'race' terminology

← Older revision		Revision as of 17:24, 23 April 2017
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	and decides which of them arrived first.		and decides which of them arrived first.

	For signals that arrive more or less simultaneously, it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.		For signals that arrive more or less simultaneously (i.e. a so-called 'race' condition), it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.

	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

Jnc at 19:50, 2 December 2016

2016-12-02T19:50:54Z

← Older revision		Revision as of 19:50, 2 December 2016
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	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

	One ~~common~~ solution is to use a special-purpose component called a 'three-output flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another).		One early solution is to use a special-purpose component called a 'three-output flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another).

	Even this kind of arbiter is not ''guaranteed'' to produce an answer within a fixed, bounded time (it is currently believed that this is theoretically impossible), but at least the user is aware that such an arbiter is still dithering. By suitable engineering, it is however possible to create arbiters that do meet a modest timing limit an extremely high percentage of the time (with failure rates on the order of centuries).		Even this kind of arbiter is not ''guaranteed'' to produce an answer within a fixed, bounded time (it is currently believed that this is theoretically impossible), but at least the user is aware that such an arbiter is still dithering. By suitable engineering, it is however possible to create arbiters that do meet a modest timing limit an extremely high percentage of the time (with failure rates on the order of centuries).

			If a specialized three-output flip-flop is not available, one common approach is to use a two-flop design, of the kind used in [[synchronizer]]s.

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

Jnc: /* Further reading */ add second WUSTL report

2016-12-02T17:20:40Z

Further reading: add second WUSTL report

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	* 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, it includes an amusing analogy to describe the arbiter meta-stability problem		* 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, it includes an amusing analogy to describe the arbiter meta-stability problem
			* Warren Mackie Littlefield, Thomas J. Chaney, "''The Synchronizer "Glitch" Problem''", Technical Report No. 47, Washington University in St. Louis, 1974 - Includes perhaps the earliest description of the 'three-output flip-flop' solution
	* David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution		* David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution

Jnc: Mention close to perfection

2016-12-02T15:44:01Z

Mention close to perfection

← Older revision		Revision as of 15:44, 2 December 2016
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	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

	One common solution is to use a special-purpose component called a 'three-~~state~~ flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another).		One common solution is to use a special-purpose component called a 'three-output flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another).

	Even this kind of arbiter is not ''guaranteed'' to produce an answer within a fixed, bounded time (it is currently believed that this is theoretically impossible), but at least the user is aware that such an arbiter is still dithering.		Even this kind of arbiter is not ''guaranteed'' to produce an answer within a fixed, bounded time (it is currently believed that this is theoretically impossible), but at least the user is aware that such an arbiter is still dithering. By suitable engineering, it is however possible to create arbiters that do meet a modest timing limit an extremely high percentage of the time (with failure rates on the order of centuries).

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

Jnc: Add an important point

2016-12-02T15:36:32Z

Add an important point

← Older revision		Revision as of 15:36, 2 December 2016
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	* at least one is asynchronous		* at least one is asynchronous

	and decides which of them arrived first. (For signals that arrive more or less simultaneously, it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice.)		and decides which of them arrived first.

			For signals that arrive more or less simultaneously, it is usually more important that the arbiter pick one relatively quickly, without dithering too long over the choice. That is because the chief function of an arbiter is usually to provide mutual exclusion; i.e. to prevent two different clients from trying to use a single resource at the same time.

	If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).		If the incoming signals are aligned 'just right' (in other words, 'just wrong'), this process can be subject to [[meta-stability]] issues, if the circuitry is constructed of standard digital components ([[flip-flop]]s, etc).

	~~The~~ common solution is to use a special-purpose ~~componente~~ called a 'three-~~output~~ flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another)		One common solution is to use a special-purpose component called a 'three-state flip-flop', which has a 'not done yet' output in addition to the normal '0' and '1' outputs. The third output is produced by the addition of two analog comparators, which look at the output, and check to see if it is above a low margin, and below a high margin (i.e. the output has not yet settled into one state or another).

			Even this kind of arbiter is not ''guaranteed'' to produce an answer within a fixed, bounded time (it is currently believed that this is theoretically impossible), but at least the user is aware that such an arbiter is still dithering.

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

	* 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, it includes an amusing analogy to describe the arbiter meta-stability problem		* 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, it includes an amusing analogy to describe the arbiter meta-stability problem
			* David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution
	David J. Kinniment, John V. Woods, "''Synchronization and Arbitration Circuits in Digital Systems''", Proceedings of the IEEE, Vol. 123, pp. 961-966, 1976 - A crisp description of the three-output flip-flop solution