9.3.3. Boundary scan test vectors
If you use the JTAG boundary scan test methodology to apply
production test vectors, you might want to have independent external
access to each TAP controller. This avoids the requirement to merge
test vectors for more than one block in the device. One solution
to this is to adopt a hybrid, using a pin on the package that switches
elements of the device into a test mode. This can be used to break
the internal daisy chaining of TDO and TDI signals, and to multiplex out independent
JTAG ports on pins that are used for another purpose during normal
operation.