Make your J-Link less annoying with this two-wire 3.3V power hack

I love the J-Link — it’s extremely fast, reliable, and works with basically everything. The J-Link EDU has been a big hit with students and hobbyists who just want to get stuff done (instead of messing with open-source debug adapters) — and at only $60, it’s a no-brainer.

While Segger tried to build a debug adapter that can be everything to everyone, they made some engineering decisions that are well-intentioned, yet somewhat annoying:

  • No 3.3V output to power your target. Even though 99.9% of MCUs you hook up to this thing are going to run off 3.3V, the only voltage output Segger provides is a software-switchable 5V supply (that no vendor IDE allows you to easily enable).
  • You must feed the IO voltage level back into VTref. This allows the fancy logic-level converters to operate at the proper voltage for your target — which 99.9% of the time is running off 3.3V.

If you have two wires and a soldering iron, this is easily remedied, though.

We’re going to pop off the high-side switch controlling the 5V supply, and repurpose that pin to carry 3.3V from the internal regulator. We’ll route this to the NC across from VTref so we can bridge it with a standard 0.1″ jumper. Here’s the game plan:

I tried this on my personal EDU version of the J-Link, but this should work great on other J-Link models (which mostly share identical hardware).

Start by prying open your J-Link by targeting the flexures on the side (don’t bother popping off the rubber feet or sticker on the bottom — there’s no screws under there).

Resist your urge to feel angry that you blew lots of money on a microcontroller and some 15-cent level shifters.

After you crack open the J-Link, you’ll notice a 3.3V regulator on the right (supplied from USB VBUS). You’ll notice a transistor in a high-side switch configuration in the top-left next to the screw.

Simply pop off that transistor (we don’t want to inadvertently back-feed the 3.3V regulator with 5V!), and solder a wire from the regulator to the connector:

Next, flip the board over, and connect pin 19 to pin 2:

Put it back together, install a jumper, and use your silliest-looking chicken-scratch handwriting to label your accomplishment:

Once it’s done, you won’t be needing that bench supply anymore — just wire up your debugger to your target and go!

Notes: You can always switch back to an arbitrary VTref by removing the jumper. I didn’t take the time to try to identify the 3.3V regulator, but given its SOT-23 package, I wouldn’t pull more than 100 mA from it without inspecting the package for overheating. If you have an older model with an 1117 regulator on it, you’re good to 800 mA. In any case, use your brain, and remember this hack offers no overload current protection, and if you were to accidentally back-feed this with something other than 3.3V, at best, you’d fry the regulator, and at worst, you’d fry the $3 MCU that turns your $300 debugger into a paperweight.

Happy JTAGing!

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