The selected scan chain is placed in test mode by the EXTEST
instruction.
The EXTEST instruction connects the selected scan chain between TDI and TDO.
When the instruction register is loaded with the EXTEST instruction,
all of the scan cells are placed in their test mode of operation:
In the CAPTURE-DR state, inputs from
the system logic and outputs from the output scan cells to the system
are captured by the scan cells.
In the SHIFT-DR state, the previously captured test
data is shifted out of the scan chain using TDO,
while new test data is shifted in using the TDI input.
This data is applied immediately to the system logic and system
pins.
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