IEEE1149.1 JTAG Boundary Scan allows pin-level access independent of the device packaging technology.
IEEE1149.1 JTAG Boundary Scan is enabled when TST is tied to high, PD0 tied to low, and JTAGSEL tied to high during powerup. These pins must be maintained in their respective states for the duration of the boundary scan operation. The SAMPLE, EXTEST and BYPASS functions are implemented. In Serial Wire Debug mode, the ARM processor responds with a non-JTAG chip ID that identifies the processor. This is not IEEE1149.1 JTAG-compliant.
It is not possible to switch directly between JTAG Boundary Scan and SWJ Debug Port operations. A chip reset must be performed after JTAGSEL is changed.
A Boundary Scan Descriptor Language (BSDL) file to set up the test is provided on www.microchip.com.