Boundary-Scan Tutorial:

Extest Instruction

The Extest instruction selects the boundary-scan register when executed, preparatory to interconnect testing. The code for Extest used to be defined to be the all-0s code. This requirement has been relaxed in the 2001 version of the Standard.

Bypass Instruction

The Bypass instruction must be assigned an all-1s code and when executed, causes the Bypass register to be placed between the TDI and TDO pins.

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