Electronic Design

SoC Test System Speeds Design Verification, Cuts Test Cost

Quickly validate design-for-test logic on SoC designs with an economical benchtop test system.

As System-On-A-Chip complexity increases, testing the millions of gates that get integrated on the chip has become an ever more challenging and more expensive task. On-chip test support logic and built-in self-test (BIST) circuits have been developed to handle the design-for-test (DFT) methodologies that are now almost mandatory for new designs. Such test capabilities are needed because many functions embedded in the complex chips are unreachable from external pins, or because the time and cost of the test systems to access the blocks makes the testing very slow and very expensive.

Typically, when the first system-on-a-chip (SoC) comes off the manufacturing line, extensive testing is performed to ensure that the DFT circuits function as desired. But the multimillion dollar test systems that are often required to perform the analysis are usually kept very busy on the production test floor. Finding time to perform engineering verification frequently means staying overnight at the factory to get some time on the large production tester. On top of that, these "big iron" test systems aren't optimized to handle the internal test structures inside the SoCs.

A fresh approach to address these issues is a compact, low-cost system, the Validator 500. Developed by Teseda Corp., the system allows verification of the DFT structures in the SoCs. By validating that the DFT structures are correct, the system guarantees that the SoC's testability features are correct. That will result in high-quality tests that effectively reject faulty products. The system can be used by design engineers, a DFT specialist, a product engineer, or anyone else who has the responsibility of making sure that either the chip's DFT is working properly, or the DFT tests are correct, or both.

If the cores employed on the chip weren't fully DFT-enabled, the ASIC designers could leverage work done by the IEEE P1500 committee, which is currently developing a standard for embedded core test (SECT). The standard will define a sort of wrapper that helps isolate the core and permits the system to test it (see "P1500: The Standard For Embedded Test,").

The Teseda solution is a small desktop unit. It's the first of a family of DFT-focused test systems that enable engineering validation and circuit de- bugging at a cost of about $60,000 per test system plus the appropriate wafer probe or test adapter. The Validator 500 focuses on dc-scan testing and can reduce the cost of testing early prototypes by 70× compared to the large automated-test-equipment (ATE) systems. Teseda plans to introduce additional test options that will allow validation of ac scan, IDDQ, and additional support for BIST and boundary-scan validation.

These versions will allow designers to perform failure analysis and characterization. Furthermore, the ability to loop-back failure information to an ATPG for fault isolation and analysis is rapid and painless because the system naturally provides information in a format familiar to the ATPG. The system will also find use in manufacturing test and wafer sorting.

To clarify what the Validator 500 does, it exercises the on-chip test circuits. Through the use of test programs and data streams, it can verify that the test structures on the chip are working and are connected according to the specifications. In other words, it lets engineers debug DFT test data and circuit structures.

When the test patterns are run, the tools will let users determine the general location or conditions of a problem and make online changes to further isolate the problem. The software is called DFT-intelligent because it preserves the structural information supplied by the design automation tools and evaluates whether or not the DFT is operating correctly.

For example, the DFT tools can help identify errors that are almost obscured on traditional ATE systems. Consider a test with failures detected on cycle #579, #10,253, and #299,021. On a "flat" machine, these failures can appear random and unrelated. But on a DFT-focused system that knows the structure of the DFT tests and can reconstitute them, relating the failures to the logic can show a link. These failures could perhaps have been shown to originate in the same register in a particular core on the chip during three separate scan tests. The ability of the software tools to navigate back and forth between the hierarchical design view and the flat view makes it easier to spot links between errors.

The Validator doesn't perform functional testing of the blocks. That task remains in the domain of the larger ATE systems. However, by evaluating the DFT structures and data early in the validation process through a low-cost system that you can keep on your lab bench, you can find logic bugs or errors that would waste the valuable time of the large ATE systems.

To accomplish its job, the Validator simplifies and speeds the test preparation by directly importing automatic test-pattern generation (ATPG) scan test data in IEEE 1450 (STIL) format. The files include vector data and DFT structural descriptions. When performing the DFT evaluation, the Validator can check out devices with as many as 128 internal scan chains and four clock domains. Data can be clocked through the chip at a maximum rate of 50 MHz, and a maximum of 32 million pattern vectors can be validated at a time. The system can also generate a million BIST clock pulses with a single tester vector.

Output results from the tests can show the relationship among hierarchical scan test structure information, data generated by ATPG software, and the traditional table of test vectors applied to the device. With this information, designers can quickly detect and identify erroneous DFT behavior. The tools provided with the Validator allow easy navigation across both structural and tabular views of the same test data.

About the size of a large book, just 350 × 300 × 60 mm, the Validator 500 provides 272 signal pins for testing a chip (see the figure). Of those pins, 256 are used for up to 128 scan chains, eight pins are available for clock driving, and eight pins provide Scan-Enable signals. Inside the system are all the pin electronics to drive the scan chains, clocks, and Scan-Enable signals; software-controllable power supplies; the pattern memory; a 10/100-Mbit/s Ethernet interface; and execution control logic.

On the top of the Validator system are two connectors that provide the interface to a test board that can be plugged directly into the system. The device under test can be socketed on the test board. Or, the board can provide an interface between the Validator and a wafer probe system if you want to check out the chips at the wafer level.

System Specs: Test data can be clocked through the system at 5- to 50-MHz rates (20 to 200 ns), adjustable in 5 ns increments. Clock edge-to-edge accuracy is ±250 ps, while the clocks' pulse placement resolution is 250 ps. DFT data being driven to the device under test has timing set at the [email protected] boundary (NRZ), while data on the receive side has a strobe placement resolution of 250 ps. The edge placement resolution of the Scan-Enable signal is 250 ps using DNRZ coding. I/O logic levels for the device under test can be set at either 3.3 or 2.5 V, while core supply voltage can be set at any value from 0.7 to 3.8 V.

The Validator comes with DFT-intelligent software that provides an intuitive graphical user interface for easy control of interactive functions. The 10/100-Mbit/s Ethernet port on the Validator links the system to a host computer. The host can typically be a PC that runs Microsoft Windows 2000 and packs 512 Mbytes of RAM, a 1-GHz CPU, a 20-Gbyte hard drive, a 10/100-Mbit/s Ethernet port, and a CD-ROM drive.

The host PC runs the software and controls the Validator over the Ethernet link. Included in the software package are a pattern interface and software modules that provide edit and debug, a structural view, a tabular vector view, and result analysis. Via the software and the graphical user interface, you can quickly and easily adjust device timing and voltage levels and immediately observe the effect of the changes.

The company also is setting up partnerships with major electronic-design-automation tool suppliers to better link the Validator software to design and analysis tools from Synopsys, Mentor Graphics, LogicVison, and other vendors.

Price & Availability
The Validator 500 will be available by the end of this year. The first version of the software performs dc-scan testing, and the hardware/software combination costs about $60,000 (host computer additional). Users will have to create custom device-under-test boards or interfaces to wafer/chip probers.

Teseda Corp., 812 S.W. Washington, 5th floor, Portland, OR 97295-3232; (503) 223-3315; www.teseda.com.

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