Hello, to identify the port jtag not need any init.
You should locate the ID of the CPU.
I think the probem may be the lack of some component (pull-up resistors etc. ..).
If you look at the picture I posted, lack multiple resistance, which connects several jtag pin to Gnd. After lack a capacitor and a resistor that will Vcc.
I'll put the photo jtag port with its components of a test etrex20/30 Garmin.
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Here is a photo of a model jtag port Garmin 60csx, where you can see the name of 3 of the pins. Tdi, Tms, Tck.
Maybe someone can help us know if you need to place these resistors, or something similar.
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