Arbiter (electronics) - Asynchronous Arbiters

Asynchronous Arbiters

An important form of arbiter is used in asynchronous circuits, to select the order of access to a shared resource among asynchronous requests. Its function is to prevent two operations from occurring at once when they should not. For example, in a computer that has multiple CPUs or other devices accessing computer memory, and has more than one clock, the possibility exists that requests from two unsynchronized sources could come in at nearly the same time. "Nearly" can be very close in time, in the sub-femtosecond range. The memory arbiter must then decide which request to service first. Unfortunately, it is not possible to do this in a fixed time .

Ivan Sutherland and Jo Ebergen, in their article "Computers without Clocks", describe Arbiters as follows:

An Arbiter is like a traffic officer at an intersection who decides which car may pass through next. Given only one request, an Arbiter promptly permits the corresponding action, delaying any second request until the first action is completed. When an Arbiter gets two requests at once, it must decide which request to grant first. For example, when two processors request access to a shared memory at approximately the same time, the Arbiter puts the requests into one order or the other, granting access to only one processor at a time. The Arbiter guarantees that there are never two actions under way at once, just as the traffic officer prevents accidents by ensuring that there are never two cars passing through the intersection on a collision course.

Although Arbiter circuits never grant more than one request at a time, there is no way to build an Arbiter that will always reach a decision within a fixed time limit. Present-day Arbiters reach decisions very quickly on average, usually within about a few hundred picoseconds. When faced with close calls, however, the circuits may occasionally take twice as long, and in very rare cases the time needed to make a decision may be 10 times as long as normal.