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May 2014 • SMT Magazine 57 SIGNaL INTEGRITy IN TEST FIXTURES continues feATuRe What is boundary scan? JTAG boundary scan is an electrical test method designed to overcome problems in test access generally associated with complex, high density boards. Components such as FPGAs, CPLDs and CPUs contain boundary scan technology, which allows engineers to stimulate and test the circuitry digitally with a JTAG Controller and attached software, in order to pinpoint precise locations and causes of faults. Boundary scan reduces the requirement for test points on the circuit board, so the physi- cal access problems associated with ICT and some functional tests are no longer an issue. The test system and boundary scan cells are connected only by means of a four or five-wire test bus, which must be considered during board design to ensure testability. Many lead- ing vendors of JTAG boundary scan systems supply DFT guidelines to encourage design en- gineers to do so. How does it work? All the signals between a boundary scan compliant device's core logic and its pins are intercepted by a serial scan path known as the boundary scan register (BSR). In normal opera- tion these boundary scan cells have no effect; however, in test mode the cells can be used to set and/or read values from the device pins. A set of four or five JTAG Test Access Port (TAP) signals are used by boundary scan enabled de- vices to allow the boundary scan data to be transferred to and from the device, allowing controlling software to analyse the circuit's per- formance. SmT Rob Humphrey is a principal hardware engineer at XJTAg. He is responsible for the hardware and fPgA development of all XJTAg's products and also pro- vides consultancy services and the final level of technical support for customers. CliCk To View Video Interview Scienscope general Man- ager Dan Kelsey discusses the growth of his company over the past 20 years, and his drive to create high-end X-ray inspection equipment, especially for BgAs. Scienscope Introduces X-Ray Inspection Equipment by Real Time with... IPC APEX EXPO 2014

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