SCAN PATH TO INJECT OR READ OUT MICROCODE
Original Publication Date: 1985-Oct-01
Included in the Prior Art Database: 2001-Oct-17
Consider a conventional microcontroller design, consisting of, an execution unit where micro-controlled data manipulations are performed, a microsequencer, and a control ROM (also called amicrorom). Such a design presents fundamental problems for the development of a functional test, The execution unit is designed to present certain general capabilities and is controlled by the outputs of the microrom. The microrom is of limited size and is coded in such a way as to implement the intended function of the microcontroller. That is, the microrom is coded to op- timize microcontroller functionallity rather than testability. This makes the problem of developing a purely func- tional test at best difficult, and at worst, there may be no complete test possible with a given set of control firmware. Microcode is typically very subject to revision, and any test based on a particular sequence executed from the microrom may be invalidated by such a microcode change. Also the observability of the microrom contents in a functional test may be extremely poor. Therefore it is desirable to test the microrom contents and the execution unit independently of each other.