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Browse Prior Art Database

JTAG Controlled Testpoint Select Mechanism

IP.com Disclosure Number: IPCOM000111311D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Bergey, AL: AUTHOR

Abstract

This invention uses a JTAG Test Access Port to control a testpoint select multiplexer. It is useful for analysis and debug of computer logic chips.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 89% of the total text.

JTAG Controlled Testpoint Select Mechanism

      This invention uses a JTAG Test Access Port to control a
testpoint select multiplexer.  It is useful for analysis and debug of
computer logic chips.

      Analysis and debug of modern, highly integrated computer logic
chips is difficult because so much of the logic is imbedded deep
within the chip.  Such a chip may contain a large number - perhaps a
few hundred - test points of interest.  However, only a small number
- perhaps eight - outputs are available for testpoint purposes.

      The invention, shown in the Figure, uses a multiplexer 1 to
select a small group of test points 2 that may be observed at any one
time from among a much larger group of internal test points 3.  The
multiplexer is controlled by a selection register 4, which determines
which group of test points is selected.  The selection register is
loaded through a JTAG Test Access Port (TAP) 5, which is in turn
manipulated through the JTAG module inputs 6.

      A JTAG Test Access Port is a mechanism described in the IEEE
Standard Test Access Port and Boundary-Scan Architecture specifi-
cation (IEEE Std 1149.1-1990).  The JTAG (Joint Test Action Group)
test access port provides for access to various on-component test
functions by means of a five-wire serial interface.

      The multiplexer selection register may be loaded or modified at
any time through the JTAG port, during functional operation, without
disturbing normal operati...