I have been trying to get JTAG debugging to work with the Portenta, and I finally managed to get it to work, but it seems the documentation is incorrect.
Specifically, the PDF found on this page (the last link on the page, the full pinout diagram): Arduino Portenta Breakout
The full pinout diagram of the JTAG connector shows the first pin is 3.3V. When I measured the pin, this appeared incorrect. It is also obvious because there are no traces running to this pin.
However the pin on the other side (pin 20, according to the documentation is supposed to be unconnected), is actually 3.3V. It appears the pinout of the JTAG connector is mirrored in the documentation.
Is the documentation wrong, or am I missing something?