Combining VXI and Boundary Scan for More Effective System Test

A comprehensive and seamless system test suite has become something of a Holy Grail for many test engineers. Certainly functional, structural, analog, at-speed, instrumentation, and other types of tests are not difficult to devise. But integrating these diverse test methods into an effective system test suite that functions efficiently within the context of a manufacturing flow is another issue.

One solution that more and more test engineers are turning to integrates different test capabilities on a VXI platform. The key types of testing to be integrated are the structural and functional testing provided by boundary scan test techniques. Achieving a seamless integration of VXI and boundary scan requires some forethought and planning, but the benefits in terms of increased manufacturing and test productivity are significant.

Based on Solid Hardware

As the first step toward the integration of VXI and boundary scan, the architecture of the VXI/boundary scan platform must be examined carefully. This can have a major effect on the throughput of system tests on the manufacturing floor. Figure 1 illustrates the architecture of such a VXI/boundary scan platform with memory large enough for high-speed test execution.

Adequate on-board memory allows test vectors to be downloaded into the platform’s memory. Once they are placed in memory, test vectors can be executed without involving the VXI system processor.

With sufficient on-board memory, the VXI/boundary scan platform can execute as many as 100 different tests out of its own memory instead of downloading test vectors from the VXI system memory and executing one test at a time. In addition, some boundary scan test systems support test-pattern compression algorithms that further streamline the process.

It also is important that the VXI/boundary scan platform features the processing power of a dedicated digital signal processor (DSP) for real-time pattern comparisons as well as full VXI backplane synchronization to other modules.

Seamless User Interface

How boundary scan is integrated on a VXI platform from the standpoint of the user interface also will be critical to the efficiency of the system test process. Most test technicians using a VXI-based system are accustomed to one of the commercially available test executives. Integrating boundary scan tests under the same test executive will assure that all of the vectors in a system test suite are executed quickly and efficiently. Without this seamless integration, test technicians would have to switch from one computing environment to another to execute different types of tests.

To achieve seamless integration under a test executive like HP-VEE™, LabVIEW^®, or LabWindows/CVI^®, the boundary scan environment must support the capability to create dynamic link library (DLL) modules for the boundary scan test objects. Integration of boundary scan test at the DLL level eliminates many of the problems associated with other integration techniques such as communications between instruments via ASCII files and invoking separate executables for each test process. Integration at the DLL level also allows a closer coupling of VXI-based instrumentation with the boundary scan tests.

Bringing the Pieces Together

At the level of system test, boundary scan brings dynamic test management capabilities to a VXI platform. Many electronic systems use a base design which may have hardware modules such as daughter cards, MCMs, memory SIMMS added or taken away, depending upon the configuration being assembled. System testing of a structurally hierarchical system is difficult to perform because the typical bed-of-nails fixture will not have access to the hardware modules installed on a system-on-a-board.

With boundary scan on a VXI platform, the access problem can be obviated by routing a scan path to all possible add-on hardware modules and designing boundary scan test capabilities into the system. But just as importantly, a VXI/boundary scan system test platform can dynamically manage and assemble the appropriate test modules that correspond to the hardware modules configured in a particular system.

The Pyramid Approach

Combining VXI and boundary scan also brings the benefits of portability to the entire test process. Tests may migrate with the system as it moves from one stage in its life cycle to another. For example, tests generated during the product design stage can be reused during manufacturing and field service.

Design and debug tests, which often are developed in the engineering or R&D lab, usually verify only portions of the system since the entire system has not been integrated yet. Having boundary scan capabilities on a VXI platform allows these low-level tests to migrate into manufacturing. There a modular system test suite can be constructed by consolidating the tests that were used during the design phase on segments of the system.

The result of this process is a thorough system test suite incorporating both structural and functional testing. This test suite is hierarchical in nature and resembles a pyramid that is based on low-level structural test modules but also incorporates higher-level functional system tests (Figure 2).

This type of approach assures that the system test suite provides very high test coverage and includes the capabilities needed to diagnose problems in the event the system fails a high-level go, no-go test. Since this pyramid or hierarchical test strategy incorporates test modules for all of the building blocks in the system, the diagnosis of a no-go problem can begin by testing the basic building blocks and progress to higher levels of the system until the cause of a problem is found.

Adding Value at the Structural Level

Adding boundary scan tests to a system test suite on a VXI platform ensures that the structural aspects of the system are sound. One major manufacturer of fault-tolerant computer systems found that more than 80% of system test failures were caused by structural failures of one sort or another. A seamlessly integrated VXI/boundary scan test platform cannot only detect structural problems but also diagnose any failures.

Using the pyramid approach, the test strategy builds from a very basic level. The first step is to verify the integrity of the scan path.

Many boundary scan test systems feature automatic test pattern generation (ATPG) tools that simplify the testing of the scan path. Essentially, scan-path verification places all of the boundary scan devices that are on the scan path into a bypass mode. Test patterns subsequently are sent down the path, and the results are observed.

The next level of structural testing involves interconnect testing. It verifies that components are connected properly to the printed circuit board (PCB) and that the etches in the board that connect the devices are, indeed, functional.

Most boundary scan test systems also feature a set of ATPG tools to generate interconnect tests for boundary scan devices. When a board has a high percentage of boundary scan devices, the test coverage will be greater. When nonboundary scan devices are present, interconnect tests still can be done on the board, but these tests cannot be generated automatically.

Some boundary scan environments feature a library of interconnect test macros that can be used to construct interconnect tests on nonboundary scan devices. The more advanced boundary scan test environments also include C++ programming capabilities for generating tests.

Board-Level Functional Tests and In-System Programming

Boundary scan test techniques also can add value to functional tests to be completed by the VXI-based system test suite. The first type of test that a VXI-based station can perform with boundary scan is a device-level functional test. The testing of devices might have begun during the system’s design phase and, because of the portability of boundary scan test methods, these same test patterns may migrate with the system as it moves into manufacturing.

Tests that verified a device’s functionality before it was assembled on a PCB can be used to test its functionality after it has been soldered to a board. Boundary scan also can be used to verify a device’s functionality within the context of the board’s architecture.

Built-in self test (BIST), the fastest method of device-level testing, also is available when boundary scan is included on a VXI-based test station. BIST can be used to perform go, no-go tests at the individual device level. Today, the most advanced boundary scan test environments include the capability to run BIST and obtain pass/fail results. Some advanced boundary scan environments soon will support BIST error reporting that is more sophisticated than simple go, no-go results.

The fact that not all components are compatible with boundary scan is not an obstacle to using boundary scan to perform board-level functional tests on VXI stations. A test method known as cluster testing can be used to verify the functionality of nonboundary scan devices.

Cluster tests treat nonboundary scan devices as virtual components on a boundary scan net. Virtual primary inputs to the cluster can be compared with virtual primary outputs from the cluster to ascertain the integrity of the devices in the cluster.

System manufacturers recently have begun incorporating nontraditional functions into system test suites because they can be done effectively at that point in the manufacturing flow when system tests are applied. One such function is in-system programming (ISP), or the loading of software or data into PLDs, PALs, FPGAs, and flash memories after these devices are connected to a PCB. Since boundary scan often is the access method for ISP, a VXI/boundary scan platform with deep on-board memory can be used to quickly program devices out of its own memory without requiring the intervention of the VXI system CPU.

Teaming VXI and Boundary Scan

The increasing complexity of electronic systems is inevitable. Systems will do more, what they do will be done faster, and they will be smaller. This complexity increases the importance of system-level tests.

Combining boundary scan in VXI-based test stations allows the development of system test suites with greater test coverage and more powerful diagnostic capabilities without disrupting the manufacturing flow. The result will be more reliable, better performing electronic products that are manufactured most efficiently.

About the Authors

Glenn Woppman was named president and CEO of ASSET InterTech when the company was founded. Prior to ASSET, he directed the ASSET business unit within Texas Instruments. Mr. Woppman has a B.S.I.E. degree from the University of South Florida and an M.B.A. degree from Southern Methodist University.

Tim Caffee has served as director of customer applications and VXI product manager since ASSET was founded. Mr. Caffee was a technical product specialist and director of software development for Texas Instruments for 10 years before joining ASSET. He has a B.A. degree in computer science from Old Dominion University.

ASSET InterTech, 2201 N. Central Expressway, #105, Richardson, TX 75080, (972) 437-2800.

September 1997