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Design for Testability 55 2.4 SCAN CELL DESIGNS As mentioned in the previous section, in general, a scan cell has two different input sources that can be selected. The first input, data input , is driven by the combinational logic of a circuit, while the second input, scan input , is driven by the output of another scan cell in order to form one or more shift registers called scan chains . These scan chains are made externally accessible by connecting the scan input of the first scan cell in a scan chain to a primary input and the output of the last scan cell in a scan chain to a primary output. Because there are two input sources in a scan cell, a selection mechanism must be provided to allow a scan cell to operate in two different modes: normal/capture mode and shift mode . In normal/capture mode, data input is selected to update the output. In shift mode, scan input is selected to update the output. This makes it possible to shift in an arbitrary test pattern to all scan cells from one or more
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This note was uploaded on 05/16/2011 for the course ENGINEERIN mp108 taught by Professor Elbarki during the Spring '08 term at Alexandria University.

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