SlimeVR-Tracker-nRF

Wireless VR motion tracker firmware for the SlimeVR ecosystem, targeting Nordic Semiconductor nRF52 and nRF54L series SoCs.

Overview

SlimeVR-Tracker-nRF (also known as SlimeNRF) is an open-source firmware for low-latency, wireless IMU trackers used in VR full-body tracking. It runs on Zephyr RTOS and leverages Nordic's Enhanced Shock Burst (ESB) protocol for robust 2.4 GHz communication.

• Repository: SlimeVR/SlimeVR-Tracker-nRF
• Documentation: https://docs.slimevr.dev/smol-slimes
• Hardware PCB: SlimeVR-Tracker-nRF-PCB
• License: Dual-licensed under MIT and Apache-2.0

Supported Hardware

Development Kits

Board	SoC	Format
nrf52840dk/nrf52840	nRF52840	.hex
nrf52dk/nrf52832	nRF52832	.hex
nrf5340dk/nrf5340/cpuapp	nRF5340	.hex
nrf54l15dk/nrf54l15/cpuapp	nRF54L15	.hex
nrf21540dk/nrf52840	nRF52840 + FEM	.hex

SlimeVR Official Boards

Board	SoC	Format
slimenrf_r1/nrf52840	nRF52840	.hex
slimenrf_r2/nrf52832	nRF52832	.hex
slimenrf_r3/nrf52840	nRF52840	.uf2
slimevrmini_p1_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p2_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p3r6_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p3r7_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p4_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p4r9_uf2/nrf52833	nRF52833	.uf2
slimevrmini_p4r11_uf2/nrf52833	nRF52833	.uf2

Community Boards

Board	SoC	Bus	Format
promicro_uf2/nrf52840/i2c	nRF52840	I2C	.uf2
promicro_uf2/nrf52840/spi	nRF52840	SPI	.uf2
promicro_uf2/nrf52840/smspi	nRF52840	smSPI	.uf2
promicro_uf2/nrf52840/chrysalis	nRF52840	I2C	.uf2
xiao_ble/nrf52840	nRF52840	I2C	.uf2
xiao_ble/nrf52840/sense	nRF52840	I2C	.uf2
mochi (Mocha-Elec)	nRF52840	—	.uf2

Supported Sensors

IMUs

Sensor	Driver	Notes
BMI270	BMI270.c	Bosch; requires firmware blob
ICM-42688-P / ICM-42688-V	ICM42688.c	TDK InvenSense
ICM-45686 / ICM-45688-P	ICM45686.c	TDK InvenSense
ISM330BX	ISM330BX.c	ST (treated as LSM6DSV variant on server)
LSM6DSM	LSM6DSM.c	ST (LSM6DS3TR-C family compatible)
LSM6DSO	LSM6DSO.c	ST
LSM6DSV	LSM6DSV.c	ST

Magnetometers

AK09940, BMM150, BMM350, IST8306, IST8308, LIS2MDL, LIS3MDL, MMC5983MA, QMC6309

Architecture Highlights

• RTOS: Zephyr (via nRF Connect SDK v3.1-branch)
• Wireless: Nordic ESB (2.4 GHz proprietary) with automatic pairing
• Sensor Fusion: VQF (Kalman-based, default) or x-io Fusion (Madgwick-based, optional)
• Power Management: Active → Low Power → IMU Wake-Up → User Shutdown
• USB: HID data output + CDC-ACM console
• Calibration: 6-side accelerometer calibration, gyro bias, magnetometer hard/soft iron

Building from Source

Prerequisites

• Zephyr SDK (CI uses v0.16.9)
• Python 3 with west and ninja
• nRF Connect SDK (managed via west.yml, currently v3.1-branch)

Workspace Setup

west init -l SlimeVR-Tracker-nRF
west update --narrow -o=--depth=1
west zephyr-export
pip install -r zephyr/scripts/requirements.txt

Build

# Example: ProMicro I2C
west build \
  --board promicro_uf2/nrf52840/i2c \
  --pristine=always SlimeVR-Tracker-nRF \
  -- -DBOARD_ROOT=./SlimeVR-Tracker-nRF

# Example: XIAO BLE Sense
west build \
  --board xiao_ble/nrf52840/sense \
  --pristine=always SlimeVR-Tracker-nRF \
  -- -DBOARD_ROOT=./SlimeVR-Tracker-nRF

Output: build/SlimeVR-Tracker-nRF/zephyr/zephyr.uf2 or zephyr.hex

Project Structure

SlimeVR-Tracker-nRF/
├── src/
│   ├── main.c                 # Boot logic, reset handling, button/DFU
│   ├── console.c/h            # Shell console, settings, calibration UI
│   ├── config.c/h             # NVS persistent settings (4 config groups)
│   ├── globals.h              # Shared macros: sensor alignment, quaternion correction
│   ├── hid.c/h                # USB HID data output
│   ├── connection/            # Wireless communication
│   │   ├── esb.c/h            # Enhanced Shock Burst (ESB) radio
│   │   ├── timer.c/h          # Radio timing utilities
│   │   └── connection.c/h     # Connection state management
│   ├── sensor/                # IMU / magnetometer / sensor fusion
│   │   ├── sensor.c/h         # Main sensor thread, data pipeline
│   │   ├── interface.c/h      # Generic sensor interface abstraction
│   │   ├── calibration.c/h    # 6-side accel calibration, gyro bias
│   │   ├── scan.c/h           # I2C device auto-detection
│   │   ├── scan_spi.c/h       # SPI device auto-detection
│   │   ├── imu/               # IMU drivers
│   │   ├── mag/               # Magnetometer drivers
│   │   └── fusion/            # Sensor fusion algorithms (VQF, etc.)
│   └── system/                # Board support / power / status
│       ├── system.c/h         # System init, retained memory, die temp
│       ├── power.c/h          # Power management, DCDC/LDO, shutdown
│       ├── battery.c/h        # Battery voltage monitoring, SOC estimation
│       ├── led.c/h            # Status LED patterns / priorities
│       └── status.c/h         # System status flags
├── boards/                    # Custom board definitions (DTS, defconfig, board.yml)
│   ├── nordic/                # promicro_uf2, test54l, test805
│   ├── slimevr/               # slimennrf_r1/r2/r3, slimevrmini_p*
│   ├── mocha-elec/            # mochi
│   └── sctanf/
├── socs/                      # SoC-level DeviceTree overlays
├── pm_static/                 # Partition Manager static configs
├── vqf-c/                     # VQF sensor fusion library (external)
├── west.yml                   # West manifest (nRF Connect SDK v3.1-branch)
├── prj.conf                   # Default Kconfig project settings
├── Kconfig                    # Application-level Kconfig symbols
└── .github/workflows/         # CI builds for all supported boards

Configuration

Key Kconfig options (see Kconfig and prj.conf):

Option	Default	Description
SENSOR_USE_VQF	y	Sensor fusion algorithm (VQF)
SENSOR_USE_MAG	y	Enable magnetometer if present
USE_IMU_WAKE_UP	y	IMU wake-up motion detection
USER_SHUTDOWN	y	Allow user shutdown via button/reset
SENSOR_ACCEL_ODR	100	Accelerometer ODR (Hz)
SENSOR_GYRO_ODR	200	Gyroscope ODR (Hz)
RADIO_TX_POWER	8	Radio TX power (dBm)
CONNECTION_OVER_HID	n	Use USB HID as primary data output

Flashing

UF2 Bootloader (drag & drop)

1. Double-tap the reset button to enter bootloader mode.
2. A USB mass storage device appears.
3. Copy zephyr.uf2 to the drive.

nRF Connect Programmer / nrfjprog

For .hex files on DKs or custom boards without a UF2 bootloader:

nrfjprog --program zephyr.hex --sectorerase --reset

Contributing

1. Fork the repository and create a feature branch.
2. Follow the existing coding style (C, Zephyr APIs, LOG_MODULE_REGISTER for logging).
3. Ensure all SlimeVR-original files include the MIT license header.
4. Add your board to the CI matrix in .github/workflows/workflow.yml if applicable.
5. Submit a Pull Request.

Acknowledgments

• SlimeVR community and contributors
• Nordic Semiconductor for nRF Connect SDK and Zephyr integration
• Zephyr Project for the RTOS foundation
• VQF sensor fusion library authors

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For detailed hardware guides and tracker assembly instructions, visit the SlimeVR Docs.