nRF52840 Support¶

Joypad OS supports nRF52840 as a build target for BT2USB (BLE to USB adapter) and USB2USB (USB host via MAX3421E to USB device). The nRF52840 has BLE and USB, making it a compact standalone wireless controller adapter.

Overview¶

Feature	Pico W / Pico 2 W	ESP32-S3	nRF52840
Bluetooth	Classic BT + BLE	BLE only	BLE only
USB Output	USB Device (RP2040)	USB OTG Device	USB Device
USB Host	PIO-USB	No	Via MAX3421E (SPI)
Controllers	All BT controllers	BLE controllers only	BLE controllers only
Build System	pico-sdk / CMake	ESP-IDF / CMake	nRF Connect SDK / west / CMake
Firmware Update	Drag-and-drop (.uf2)	Drag-and-drop (.uf2) via TinyUF2	Drag-and-drop (.uf2)

BLE-only limitation: nRF52840 only supports BLE controllers. Classic Bluetooth controllers (DualShock 3/4, DualSense, Switch Pro in BT mode) will not work. Most modern controllers support BLE.

Supported Controllers (BLE)¶

Controller	Status
Xbox One / Series (BLE mode)	Supported
8BitDo controllers (BLE mode)	Supported
Switch 2 Pro (BLE)	Supported
Generic BLE HID gamepads	Supported

Not Supported (Classic BT only)¶

Controller	Why
DualShock 3	Classic BT only
DualShock 4	Classic BT only
DualSense	Classic BT only
Switch Pro	Classic BT only
Wii U Pro	Classic BT only

These controllers pair over Classic BT and require the Pico W build.

Hardware¶

Supported Boards¶

Board	UF2 Drive	UART Pins	Apps	Notes
Seeed XIAO nRF52840	`XIAO-SENSE`	TX=D6 (P1.11), RX=D7 (P1.12)	bt2usb	Default board
Adafruit Feather nRF52840	`FTHR840BOOT`	TX=D1 (P0.25), RX=D0 (P0.24)	bt2usb, usb2usb	Feather form factor

XIAO nRF52840¶

Ultra-compact (0.84" x 0.70"), USB-C
Built-in BLE radio
UF2 bootloader (double-tap reset to enter)
Optional SSD1306 OLED via expansion board (I2C)
Purchase

Adafruit Feather nRF52840¶

Feather form factor (2.0" x 0.9"), USB-C
Compatible with FeatherWings (e.g., MAX3421E USB Host FeatherWing for usb2usb)
UF2 bootloader (double-tap reset to enter)
Optional SSD1306 OLED FeatherWing (I2C)
Purchase

Build & Flash¶

Prerequisites¶

# One-time setup: install nRF Connect SDK v3.1.0
make init-nrf

This installs the nRF Connect SDK workspace via west init and west update. Requires Python 3 and pip.

Build Commands¶

From the repo root:

# BT2USB (default)
make bt2usb_seeed_xiao_nrf52840                    # Build for XIAO
make bt2usb_adafruit_feather_nrf52840              # Build for Feather

# USB2USB (Feather + MAX3421E FeatherWing only)
make usb2usb_feather_nrf52840                      # Build for Feather

# Flash via UF2 bootloader
make flash-bt2usb_seeed_xiao_nrf52840              # Flash XIAO
make flash-bt2usb_adafruit_feather_nrf52840        # Flash Feather

# UART serial monitor (115200 baud)
make monitor-bt2usb_seeed_xiao_nrf52840            # Monitor XIAO
make monitor-bt2usb_adafruit_feather_nrf52840      # Monitor Feather

Or from the nrf/ directory:

cd nrf
make build                                          # Build bt2usb for XIAO (default)
make build BOARD=adafruit_feather_nrf52840          # Build for Feather
make build APP_TYPE=usb2usb BOARD=adafruit_feather_nrf52840  # usb2usb
make flash-uf2                                      # Flash via UF2 drive
make monitor                                        # UART serial monitor

Flashing¶

UF2 Bootloader (recommended):

Double-tap the reset button on the board
A USB drive appears (XIAO-SENSE or FTHR840BOOT)
Copy the .uf2 file to the drive, or run make flash-<target>
Board reboots automatically into new firmware

J-Link (alternative):

cd nrf && make flash-jlink

Requires a Segger J-Link debug probe connected to the board's SWD pins.

Usage¶

Pairing¶

Flash the bt2usb firmware
The adapter starts scanning for BLE devices automatically on boot
Put your controller into BLE pairing mode
Once paired, the controller reconnects automatically on subsequent use

Button Controls¶

Action	Function
Click	Start 60-second BLE scan
Double-click	Cycle USB output mode
Triple-click	Reset to default HID mode
Hold	Disconnect all devices and clear bonds

Status LED¶

LED State	Meaning
Blinking	No device connected (scanning/idle)
Solid on	Device connected

USB Output Modes¶

Double-click the button to cycle through output modes: XInput, DInput, Switch, PS3, PS4/PS5.

Debugging¶

UART Console¶

UART debug output is the primary debugging method. Connect a USB-to-UART adapter to the board's UART pins:

Board	TX Pin	RX Pin	Baud Rate
XIAO nRF52840	D6 (P1.11)	D7 (P1.12)	115200
Feather nRF52840	D1 (P0.25)	D0 (P0.24)	115200

make monitor-bt2usb_seeed_xiao_nrf52840   # Auto-detects serial port

UART output works from boot, in fault handlers, and from ISR context — no debug probe required.

Crash Diagnostics¶

Crash dumps include register state and stack trace. Key fault codes:

K_ERR_KERNEL_PANIC (reason=4): Software panic from k_panic() or abort() — typically a failed assertion in BTstack
BusFault / HardFault: Hardware fault — check for null pointer dereference or stack overflow
MPU fault: Stack overflow detected by MPU stack guard

Architecture¶

Threading Model¶

Main Thread                    BTstack Thread
  USB device polling             BLE scanning/pairing
  LED status updates             HID report processing
  Button input                   Controller data → router
  Storage persistence

BTstack runs in its own Zephyr thread (k_thread_create). All BLE operations happen in that thread. The main thread handles USB output, LED, and button input, yielding via k_msleep(1).

Key Differences from RP2040¶

Zephyr RTOS instead of bare-metal dual-core
Raw HCI passthrough — Zephyr provides the BLE controller, BTstack acts as BLE host
TinyUSB owns USB — Zephyr USB stack is disabled; TinyUSB's dcd_nrf5x.c drives USB directly
NVS storage — Flash persistence uses Zephyr NVS (shared between app settings and BTstack bonds)
MAX3421E for USB host — SPI-connected USB host IC (usb2usb only), using direct register access for ISR safety