Vendor corrects errors about built-in self-testing.

FEEDBACK

Dear Editor,

I was very disappointed to find several glaring errors in the Focus Report on design-for-test tools in the September issue [p. 36]. Specifically, the following paragraph is totally incorrect:

"But BIST is a functional test designed to ensure that a circuit runs correctly through a limited set of test vectors. It doesn't test the physical integrity of the device or provide insight into the operation of complex, indeterminate functions. For those capabilities, designers must look elsewhere."

Here are the facts:

1. BIST is a manufacturing test, not a functional test.

2. Its purpose is to catch manufacturing defects. That means that it is used to verify the physical integrity of the device.

3. Unlike external testing or Iddq, BIST provides excellent insight into the operation of complex chips. LogicVision's BIST solutions are routinely used to test all on-board memories as well as all logic blocks. Additionally, because BIST is embedded within the chip, it provides a "dedicated channel" for gathering diagnostic information about the internal operation of the logic and memories.

4. There is no limit to the number of BIST test vectors that are run, since they are generated "on the fly" by the on-chip BIST control circuitry. Most designers will select a high enough vector count to get the desired fault coverage (typically greater than 95 percent and often 100 percent) at the lowest possible test time.

5. More importantly, LogicVision's BIST applies random patterns at speed for testing logic. Customer experience and research have shown that this approach yields higher-quality parts than the traditional ATPG approach. Specifically, at a given fault coverage, BIST-tested parts have a much lower rate of test escapes than the same parts tested using ATPG and an external tester. That's because the combination of random patterns and at-speed testing uncovers many defects that don't fit the standard stuck-at fault model, and that's precisely why BIST is so powerful at verifying the physical integrity of the device.