Device Modeling Is Critical for
Fast Time-to-Test

ScanWorks' First Pass

Once the test engineer or technician is ready for ScanWorks to take its first pass at generating a test, the test-generation engine begins its analysis of the design. ScanWorks examines the cluster models for all non-boundary-scan devices to determine each one’s operating modes. For example, a typical “245” eight-bit transceiver would have three modes: isolation, where no data is being transferred because all outputs have been turned off; A-to-B mode, where A pins are enabled as inputs and B pins are enabled as outputs; and B-to-A mode, where B pins are enabled as inputs and A pins are enabled as outputs. (See Figure 1.)

Figure 1. Modes of a "245" Device

Subsequently, the ScanWorks test generator begins to build a test by selecting each device’s operating mode so that, first, the safety of the printed circuit board will be ensured and, second, test coverage for shorts will be maximized. To do this, ScanWorks must examine each device within the context of the design.

For example, if ScanWorks has access to a non-boundary-scan cluster device from one direction but does not have enough information about the device and the design to know what is beyond the device, then it cannot safely drive test patterns beyond the device. To do so would be to jeopardize the safety of the board by setting up a situation where bus contention might arise. In this case, boundary-scan test coverage would extend to the cluster device that is accessible from a boundary-scan device but not beyond it. Test coverage can be extended later, at the discretion of the test engineer or technician who is developing the test. (In Figure 2 below, test coverage can be extended beyond a non-boundary scan device because the model of the interconnected device has been included in the ScanWorks test.)

Figure 2. Test Coverage Blocking

This is a complex process because the operating mode selected for each device has a ripple effect on other devices in the design. The algorithms used by ScanWorks’ test-generation engine are continually fine-tuned to optimize this process. Once operating modes are selected for all devices in the design, the test vectors for shorts are generated.

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