Arbiter
A arbiter is a device which examines two or more signals which are either:
- synchronous, but using clocks which are not fixed relative to one another; or
- 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.)
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-flops, 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)
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
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
