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Multiple JTAG TAP Controllers for Debug While Maintaining Single Chip Visibility for IEEE 1149.1 Boundary Scan Testing

IP.com Disclosure Number: IPCOM000029123D
Publication Date: 2004-Jun-16
Document File: 2 page(s) / 40K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for debugging that eliminates the need for additional software. Benefits include reducing software costs.

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Multiple JTAG TAP Controllers for Debug While Maintaining Single Chip Visibility for IEEE 1149.1 Boundary Scan Testing

Disclosed is a method for debugging that eliminates the need for additional software. Benefits include reducing software costs.

Background

Currently, there are two reasons why IEEE 1149.1 is used: for open/short/continuity testing for board/card manufacturing fault detection, and for debugging the control of chips. The majority of high-volume board manufacturers require IEEE 1149.1 for open/short/continuity testing. Given multi-chip modules model of chips and embedded CPUs, debugging chips is used less frequently, and uses custom software provided by or developed for that chip.

Note. There is a major conflict between open/short/continuity testing and debugging chips, involving many instructions required and supported by the IEEE 1149.1 standard.

Board vendors expect to be able to treat the part as a single TAP controller. There is software which allows them to control multi-TAP controllers I na chip, but they must purchase it above and beyond their normal flow and it is expensive. Unless this special software is purchased, multi-internal TAP controller chips are not compliant for the IDCODE, BYPASS, etc. instructions.

The code for IDCODE is all 1's except the last bit 0, and BYPASS is all 1's.  The requirement for BYPASS is that the chip appears as one flop from TDI to TDO.  For IDCODE, the ID must be 32-bits.  For multi-internal TAP controller chips configuration, BYPASS is n flops and IDCODE is 32+ (n-1)-bits.  This occurs because the instruction register length is defined as the sum off all the TAP controller instruction registers in the multi-internal TAP controller chip. For IDCODE and BYPASS, these places all, or all but one, TAP in BYPASS making the n-1 bits too long and non-compliant.

General Description

The disclosed method “tricks” the software by making the boundary scan chain n-1 internal-only pads longer, which the tools do not test (see Figure 1). These pads are actually the...