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The book System-on-Chip Test Architectures, Wang et.al., mentions the need of double latch based design on boundary scan cell:

The bidirectional buffer illustrates the need for the double-latched scan chain design of the basic BSC to prevent back driving of other bidirectional buffers on a PCB while shifting in test patterns and shifting out test responses. The Update latch holds all values (including tri-state control value) stable at the pads during the shifting process.

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The figure is taken from the book itself (p. 10)

Q1. What does it mean by "back driving"?

Q2. The Update latch is needed for INTEST instruction, but with controls available for 2 muxes, there should not be any interference on the PCB lines while scan shifting, so what does the statement from the book mean?

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  1. Backdriving driving an output backwards or injecting current on an output often done by external ATE to measure output impedance.

In this case it would mean contention for the bus by more than one output.

  1. avoid crosstalk on MUX and scan inputs.
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