Meta-stability
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 the aphorism that 'digital circuits are built out of analog devices'.)
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.
One type of circuit in which meta-stability issues are a particular concern are synchronizers of all kinds, particularly arbiters.
The problem was independently re-discovered several times before the LINC group at WUSTL came across it, and after studying it intensively, was able to interest the rest of the electrical engineering profession in it.
It is thought that meta-stability issues may have caused 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.
Further reading
- Severo M. Ornstein, "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
- 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
