Finding the pins
To connect to the JTAG pins, you first have to find them! It is a common practice among vendors not to label the pins on the debug ports, not to populate resistors on production boards to avoid JTAG access, or even not to route the pins at all! In this section, we will go through the best-case scenario (the debug port is a standard, recognizable port) to the worst practical case (pins are there but not labeled, or dispersed on test pads). It is possible that the pins are not routed at all (that is, no trace is connecting the chip's pins to an externally reachable connector), or even worse, a BGA chip has no trace when getting the debug signal from the underside of the chip (in this case, there isn't a lot you can do).
The PCB "plays nicely"
Sometimes typical debug ports can be found on the PCBs, labeled on the silkscreen or not.
Typical JTAG ports' pinouts look like this:
- ARM 10 pins (JTAG + SWD):
