Mixed-signal IC testing - the link to quality for system on a chip

Electronic News, July 14, 1997 by Roger Blethen

When devices become systems, the testing paradigm also changes. The real specification of the device won't be what's printed on the data sheet, but rather the customer's expectation of the system's performance. Product quality, defined as fulfilling the customer's expectation, can only be guaranteed by testing to that new specification. As a result, there will be a mixture of embedded component tests and system-level tests applied to system-on-a-chip devices. A substantial portion of these tests will be mixed signal, and they will consume more than their fair share of both development time and test time. Designing efficient and thorough mixed-signal tests requires as much skill as designing devices, and arguably more experience. Semiconductor companies and test vendors which have invested in and developed strong mixed-signal test development capability have a strategic advantage over their competitors, and have amassed considerable test intellectual property as a result.

Intellectual property, or IP, will assume an expanded definition. Since it was first applied to embedded cores, it is often considered to be the province of virtual component designers, to the point where virtual component designs and IP have become synonymous. There are many other forms of IP, however. A test design, for example, contains algorithms and techniques embedded in the test engineer's code and device under test (DUT) interface design that represent his intellectual property, and is every bit as valuable as a virtual component. This IP can be sold, reused and can shorten time to market in exactly the same way as its component counterpart.

Meeting the system-on-a-chip challenge has significant implications for test equipment design. The demarcation between VLSI and mixed-signal testers will blur, just as it is between devices. Both devices and testers have developed their use of digital signal processing (DSP)--taking what used to be analog processing elements and realizing them instead as algorithms, retaining analog circuitry only for the interfaces to the real world. Since the number-crunching horsepower to execute these algorithms is implemented in embedded processor cores, mixed-signal testers will require digital test capability on a par with the VLSI testers; greater than 512 pins, embedded memory test, scan and high-speed serial capability. Add to this a full suite of mixed-signal instruments, and the ability to configure the machine anywhere from pure analog to pure digital at the right price points, and you have a design task that only the most innovative test vendors are likely to pull off.

Mixed-signal testing will play an important role in assuring the quality of these new system-on-a-chip devices, since how well these devices connect to the customer's environment is largely a function of the performance of its mixed-signal interfaces. It won't be acceptable to ship a highly integrated cellular phone on a chip that meets its parametric specification, if the customer drops calls because the manufacturer didn't include tests that emulate the phone's worst-case environment. These system-level tests are the kind of intellectual property that will be as important to the successful system-on-a-chip vendor as his embedded cores, and will help ensure that he ships a quality product.