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SCAN PATH TO INJECT OR READ OUT MICROCODE

IP.com Disclosure Number: IPCOM000005585D
Original Publication Date: 1985-Oct-01
Included in the Prior Art Database: 2001-Oct-17
Document File: 1 page(s) / 66K

Publishing Venue

Motorola

Related People

Grady Giles: AUTHOR

Abstract

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.

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MOTOROLA Technical Developments Volume 5 October 1985

SCAN PATH TO INJECT OR READ OUT MICROCODE

by Grady Giles

   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.

   The design of the Motorola MC68951 Paged Memory Management Unit employs a scan technique to accomplish a logical separation of the microrom and execution unit for test. The microrom output latches are modified to be master-slave shift register cells and strung together into a serial scan path. Also linked into th...