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Unformatted text preview: instructions may include: INTEST: here the boundary scan register is connected between TDI and TDO and the core logic input and output states are captured and controlled by the register. Note that the inputs are driven to the complement of the values supplied. Other wise the operation is similar to EXTEST. This instruction exists to support the testing of the core logic. PCB testing The principal goal of the JTAG test circuit is to enable the continuity of tracks and the connectivity of solder connections on printed circuit boards (PCBs) to be tested. This has become difficult since the introduction of surface mount packages which do not require through-holes in the circuit board. Previously 'bed of nails' testers could contact all the pins on each 1C package from the back of the PCB to check continuity; surface mount packages typically have the pins closer together and the tracks accessible only on the component side of the board, making the 'bed of nails' approach inapplicable. When the surface mount components have a JTAG test interface, this can be used (using the EXTEST instruction) to control outputs and observe inputs independently of the normal function of the chip, so board-level connectivity can readily be checked from chip to chip. If the board also contains components which do not have a JTAG interface these will require 'bed of nails' contacts, but these can be used together with the JTAG interfaces where available to minimize the cost and difficulty of building the production test equipment. High-complexity integrated circuits require extensive production testing to identify faulty devices before they get built into product. A production 1C tester is a very expensive piece of equipment, and the time each device spends on the tester is an important factor in its manufacturing cost. Since the JTAG test circuitry operates through serial access, it is not a high-speed way to apply test vectors to the core VLSI testing 230 Architectural Support for System Development logic. Furthermore, it is not possible to apply test vectors through the JTAG port at the normal operating...
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- Spring '09