A shield PCB for the nRF52840 Supermini/Pro Micro with an IMU, magnetometer, two buttons and a RGB LED, designed for nRF-based SlimeVR trackers

Chrysalis is a compact, back-mounted shield that solders directly to the underside of the nRF52840 Supermini or Pro Micro. It adds:

• 3-axis accelerometer/gyroscope (TDK InvenSense ICM-45686)
• 3-axis magnetometer (QMC6309)
• Two push-buttons for SW0 and RESET
• Configurable RGB LED (Currently requires custom built firmware for changing the color)
• Pads for battery wires

All exposed pads are marked with a star (*) and must be soldered.

• Assembled Chrysalis board
• nRF52840 Supermini or Pro Micro
• Soldering iron (chisel or knife-type tip recommended), solder wire, flux (optional but recommended)

Chrysalis requires:

• Castellated holes on two sides
• ENIG surface finish (for the QMC6309 BGA pads)
• Thin PCB stack-up (0.8 mm or 1.0 mm recommended)

Manufacturing is most cost-effective at volume. For small quantities, consider ordering from here.

1. Prepare the Supermini

   • Update your bootloader before soldering and verify functionality. SWD pads will be covered after soldering, so unbricking/recovery will require removing the shield.

2. Align Chrysalis

   • Flip the Supermini upside-down (chips side down).
   • Match the silkscreen markers on the backside of Chrysalis:
     • USB above → near the USB connector
     • Antenna below → near the antenna

3. Solder Every Exposed Pad

   • All castellated pads are marked with *.
   • Tack one corner pad to secure alignment, then solder the rest.
   • Use direct solder connections—no wires or pin-headers.
   • Inspect joints under magnification; check for cold or dry joints and bridges.

4. Consider the Wire-Mod for the Pro-Micro Antenna

   • https://docs.slimevr.dev/smol-slimes/hardware/smol-tracker.html#-copper-wire-for-wire-antenna-mod

Note:

• The module intentionally sources power from one of the nRF52840's GPIOs (P0.31), as the voltage of it is adjustable.
• The QMC6309 requires a minimum supply voltage of 2.5 V.

For the complete schematic and up-to-date pin mapping, see Chrysalis.kicad_sch.

1. Plug the assembled Supermini+Chrysalis combo into your host via USB.
2. Flash the Slime-nRF Chrysalis firmware:
   • Download the pre-compiled firmware:
     https://docs.slimevr.dev/smol-slimes/firmware/smol-pre-compiled-firmware.html#pcb
   • Follow the guide:
     https://docs.slimevr.dev/smol-slimes/firmware/smol-flashing-firmware.html
3. Open a serial console to verify IMU, magnetometer and button functionality.

Note:

• At the time of writing this, the magnetometer will not be detected, because the IMU's sensorhub functionality has not been fully implemented.

This project is licensed under the CERN Open Hardware Licence 2.0 – Strongly Reciprocal (CERN-OHL-S).
See LICENSE.md for full terms