Sometimes, one half will work when plugged in by itself, and the other half will work when plugged in by itself (probably with its keys mirrored), but they won't work when connected together with a TRRS cable between them.
Make sure you are using a TRRS cable. It has four contacts: the tip, two rings and the sleeve, which is the ring furthest from the tip.
Most audio cables are TRS cables, which means they only have three contacts. Most split keyboards will only work with a TRRS cable.
First of all, you'll want to make sure the solder joints on your TRRS jacks and microcontroller are good. Adafruit has an excellent overview of common soldering problems, of which the image below is taken. Compare your solder joints with those in the guide, and if any have problems, rework them to be alright again.
You may also want to check the orientation of your RGB LEDs. The LEDs are square, with one corner being cut into a triangle. This triangle should align with the dot that's printed next to the LED on your PCB. Are they not aligned, then you should desolder the LED and reinstall it in the correct orientation.
If you socketed your microcontroller, and have not found the issue after following the steps in this article, detach your microcontroller from the sockets. There's a slight chance solder flowed into the sockets, possibly joining them together. It's a very slight chance though, so check the other steps first.
Reflash your keyboard
You might want to try flashing your keyboard again, but this time, unplug your keyboard from the computer, then unplug both halves from one another. Then, plug only a single half back in, press its reset button, then click on "Reset EEPROM" in the QMK Toolbox. After, flash it, and repeat these steps for the other half.
After this full reset, connect both halves again (with them still not connected to the computer), then plug your keyboard back into your computer.
If you have purchased a Pro Micro from SparkFun or from a source other than splitkb.com, your Pro Micro may come with the J1 jumper presoldered.
The J1 jumper will bypass the on-board voltage regulator on a SparkFun Pro Micro board, and may have undefined behaviour on Pro Micro clones. One of the ways QMK can detect which side should act as the master half of a split keyboard is by means of reading the voltage on the VBUS pin of the microcontroller.
Now, by default, the jumper is soldered. When you connect both halves, the keyboard will feed VCC through to the other half, which then loops back through the jumper, the fuse into UVCC, which in turn is connected on the Pro Micro to VBUS. This causes the slave side to also think it's connected to the USB port, which means QMK then decides it should act as a master too. Desoldering the jumper will fix this, making QMK able to properly detect which side is connected to the USB cable once again.
Desolder the J1 jumper on the side you intend to use as the slave only. If you desolder the jumper on both sides, voltage will pass through the regulator and your board will only get 3.3V, which is likely not enough juice for your firm- and hardware. To get around this and be able to use either side as the master side, you might be able to add a small piece of wire between the VCC and RAW pins on both sides, which bypasses the voltage regulator, but still prevents current from flowing back to UVCC and thus VBUS by means of the onboard diode D2. For this to work, desolder the J1 jumper on both sides. This workaround is untested but should, in theory, work.
Often, the cause of only one half working has to do with continuity: the ability for electricity to flow between two points. You'll want to make sure there is continuity between the right points, to ensure you have no solder bridges or defective components.
To check for continuity, you can use a multimeter that has a continuity check mode. Often, the multimeter can emit a tone so you can hear it when there's a connection between two points.
Check for continuity on the following pins:
- When connected with a TRRS cable, each pin of the TRRS jack should connect with its corresponding pin on the other half of the keyboard and to no other pin on the same TRRS jack.
- The pins on the TRRS jack should connect with the appropriate pins on the microcontroller. You can find the schematics for the Kyria at Which microcontroller pins does the Kyria use?
If you still weren't able to find the problem after taking these steps, please get in touch: send an email to firstname.lastname@example.org and I'll help find a solution.