Motivation

Most digital systems that interact with the external world must handle asynchronous inputs because events outside that system appear at random points in time with respect to the system's internal operation. As an example, a crankshaft angle sensor generates a pulse train regardless of the state of operation of the electronic engine management unit that processes those pulses.

Synchronization problems are not confined to electromechanical interfaces. Much the same situation occurs when electronic systems interact that are mutually independent, in spite — or precisely because — of the fact that each of them is operating in a strictly synchronous way. Just think of a workstation that exchanges data with a file server over a local area network (LAN), of audio data that are being brought to a D/A converter for output, or of a data bus that traverses the borderline from one clock domain to another. Obviously, the processing of asynchronous inputs is more frequent in digital systems than one would like.

The following episode can teach us a lot about the difficulty of accommodating asynchronously changing input signals. The account is due to the late Charles E. Molnar, who was honest enough to tell us about all misconceptions he and his colleagues went through until the problems of synchronization were fully understood.

Historical example

Back in 1963, a team of electronics engineers was designing a computer for biological researchers that was to be used for collecting data from laboratory equipment.