Teensy 4.1 board with JTAG supported out-of-the-box?


New member
I'm seeing a bunch of discussions about modifications of boards being needed.

Is there a dev kit/board somewhere with jtag pins officially exposed?

I'm the kinda guy that needs to have a traditional pause/step debugger...
No. I believe the way PJRC boards work is that their proprietary bootloader chip is attached to the JTAG pins and can thus force software updates on the main MCU by flashing it over JTAG.

So, you can't have connection to JTAG because the bootloader chip is already actively using those pins. I've floated the idea that maybe they could implement JTAG passthrough but I don't know whether that's something they'd consider or not.

Short of that, you kind of have to do board mods to get at the JTAG pins and be able to isolate them from the bootloader chip on demand. Or, cut out the bootloader entirely and then you always get to use a ST-Link or something to do flashing.
There are some posts showing how to connect an external JTAG connector on Teensy pads+cutting traces.
There is even a project of a flexible pcb with JTAG connector. It is soldered on the Teensy pcb close to the debug chip.
You will find them with the JTAG word