Switch Pico Controller Bridge

Raspberry Pi Pico firmware that emulates one or more Switch Pro controllers over USB. Input can come from the SDL3-to-UART computer bridge or, on Pico 2 W, directly from Bluetooth controllers through Bluepad32.

What you get

• Firmware (src/firmware/): acts as a Switch Pro controller (one on standard Pico, four on Pico 2 W AIO), accepting either UART bridge reports or the optional Pico 2 W Bluepad32 backend.
• Python bridge (switch_pico_bridge.controller_uart_bridge / CLI controller-uart-bridge): reads SDL3 controllers on the host, sends reports over UART, and applies rumble locally. Hot‑plug friendly and cross‑platform (macOS/Windows/Linux).
• Color configuration (src/firmware/platform/pico/controller_color_config.h): compile-time RGB colors for emulated controller grips and supported Bluetooth controller LEDs.
• Pico 2 W AIO firmware (firmware/switch-pico-aio.uf2): hosts four concurrent Bluetooth controllers and sends their controls, calibrated motion, rumble, and slot identity through four separate Switch Pro USB interfaces without a computer.

Source layout

Firmware code has one include root, src/firmware, with responsibility-based modules:

Path	Responsibility
src/firmware/main.cpp	Firmware entry point and backend orchestration
src/firmware/adapter/	USB mode selection, host probing, and managed reboot
src/firmware/configuration/	Persistent adapter configuration and transactions
src/firmware/core/	Shared controller identity, color, and input-state types
src/firmware/input/	Bluepad32 controller input backend and hotkeys
src/firmware/platform/pico/	Pico flash/BOOTSEL integrations and compile-time board configuration
src/firmware/profile/	Controller profiles, transforms, storage, and runtime
src/firmware/usb/	USB output boundary, management protocol, and per-protocol drivers

Internal includes are rooted at src/firmware, for example #include "profile/controller_profile.h". Host-side Python remains in src/switch_pico_bridge/; native firmware tests remain in tests/.

Quick start

1. Flash the Pico with firmware/switch-pico.uf2 (or build your own) using BOOTSEL drag-and-drop (see “Manual UF2 flashing” below).
2. Wire Pico UART1 to a USB↔UART adapter (GPIO4 TX, GPIO5 RX, GND) and plug that adapter into your host PC.
3. Enable System Settings → Controllers and Sensors → Pro Controller Wired Communication on the Switch.
4. Install the Python bridge (see “Python bridge”) and run controller-uart-bridge --interactive.
5. Connect the Pico to the Switch (dock USB-A or USB-C OTG); the Switch should see it as a wired Pro Controller.

Pico 2 W all-in-one Bluetooth option

Architecture

The AIO build accepts up to four concurrent Bluetooth controllers on a single Pico 2 W. TinyUSB and the four Switch report generators run on Core 0; Bluepad32, BTstack, and the CYW43439 radio run on Core 1. Each Bluetooth device index maps directly to one always-present USB Pro HID interface. Per-slot state snapshots and generation-tagged latest-value rumble mailboxes are the only cross-core data paths.

All four USB interfaces are always present to the Switch as separate Pro Controllers on one physical USB device. Input, motion, rumble, lifecycle, and displayed grip color remain isolated per slot.

Build and flash

Initialize the pinned Bluepad32 dependency once:

git submodule update --init external/bluepad32

Build and flash a Pico 2 W in BOOTSEL mode:

python3 build.py --aio

This uses an isolated build-aio/ CMake cache and publishes:

• firmware/switch-pico-aio.elf
• firmware/switch-pico-aio.uf2

The default python3 build.py command and firmware/switch-pico.* artifacts remain the UART/Pico build. The AIO build requires PICO_BOARD=pico2_w; it is not interchangeable with the original non-wireless Pico firmware.

Both build.py --aio and direct AIO CMake configuration copy the pinned Bluepad32 source into the active build directory and apply patches/bluepad32-sdl3-imu.patch there before compiling. The patch makes supported motion controllers use SDL3-equivalent axes and fixed-point units before conversion to Nintendo samples. The external/bluepad32 submodule remains pristine; patch or source-revision drift fails configuration.

Switch 2 wake from L + R + Home, PS, or Xbox

The AIO firmware can wake a sleeping Switch 2 when a connected controller's physical L + R + System chord becomes held: L + R + Home on Nintendo-style controllers, L1 + R1 + PS on PlayStation controllers, or LB + RB + Xbox on Xbox controllers. Plain Home, PS, or Xbox remains a normal button and does not start wake advertising. Setup needs one wake advertisement captured from a Joy-Con 2 already paired with that Switch 2. The generated configuration is console-specific and is intentionally ignored by Git.

The implementation replays the captured, unencrypted Switch 2 BLE wake advertisement for two seconds at a 20 ms base interval. The CYW43439 has one controller-wide public Bluetooth address, so the firmware temporarily changes from the Pico's normal identity to the captured Joy-Con identity for the wake burst and restores it afterward. The explicit chord confines the resulting input-controller disconnect to an intentional wake attempt. This follows the packet format documented by ndeadly/switch2_controller_research and the capture/replay approach demonstrated by alexvnesta/switch2controller and the MIT-licensed Switch2-Wake-Beacon-ESPHome.

Back up the complete Pico flash before replacing the AIO firmware with the temporary capture image:

picotool save -a -v switch-pico-before-wake-capture.uf2

Then:

1. Connect the Pico 2 W to the computer and build/flash the one-shot capture firmware:

   python3 build.py --wake-capture
   

   This also publishes firmware/switch-pico-wake-capture.elf and firmware/switch-pico-wake-capture.uf2.

2. Start the configuration tool. It auto-detects a single Pico USB serial port; use --port /dev/ttyACM0 when more than one Pico is attached:

   python3 tools/configure_switch2_wake.py
   

3. Detach a Joy-Con 2 that is already paired with the target Switch 2, put the console to sleep, and press that Joy-Con's Home button. Do not press its sync button. The capture firmware accepts the first public ADV_IND packet with Nintendo's Switch 2 wake flag and nonzero console address, stops scanning automatically, lights the onboard LED solid, and repeats the captured record until the tool receives it.

4. The tool validates the packet and atomically writes src/firmware/platform/pico/switch2_wake_config.h.

5. Restore the AIO firmware with the generated wake configuration:

   python3 build.py --aio
   

The capture tool also accepts a saved serial log:

python3 tools/configure_switch2_wake.py --input switch2-joycon-capture.log

With the configured AIO firmware powered while the console sleeps, first turn on the paired input controller with Home, PS, or Xbox and let it reconnect to the Pico. Then hold L + R and press its system button to send one wake burst. Holding the chord does not retrigger it; release at least one chord button before another attempt. Plain Home, PS, or Xbox is forwarded normally and does not disturb the radio.

The input controller disconnects during the intentional wake burst because the CYW43439 cannot retain its normal public identity while transmitting the captured controller's public identity. It can reconnect after the Pico restores its address. Avoiding that disconnect requires a second BLE radio dedicated to wake transmission; keeping the captured identity throughout gameplay caused severe Classic Bluetooth latency in hardware testing.

The Pico must remain powered for wireless wake. If the Switch or dock removes USB power during sleep, use a powered USB arrangement that preserves the Pico-to-Switch data connection. Keep the captured Joy-Con inactive during the two-second wake burst to avoid two radios using one address.

To target another Switch 2, repeat the capture and configuration steps. To disable wake, delete the generated switch2_wake_config.h and rebuild the AIO firmware. Restore the full-flash backup only if you need to recover the exact pre-capture firmware and persistent state.

Pairing up to four controllers

1. Flash and connect the Pico 2 W to the Switch.
2. Enable System Settings → Controllers and Sensors → Pro Controller Wired Communication.
3. Hold BOOTSEL for about two seconds until the onboard LED starts double-blinking. This enables new Bluetooth authentication for 60 seconds.
4. Put a controller into Bluetooth pairing mode:
   • DualSense: hold Create + PS.
   • DualShock 4: hold Share + PS.
   • Switch Pro: press its sync button.
   • Xbox Bluetooth controller: hold its pair button.
   • 8BitDo: use a Bluetooth mode supported by Bluepad32; use Switch/S mode when motion is required.
5. Wait for the controller's player light to settle. Repeat step 4 for additional controllers while the window remains open. Holding BOOTSEL again extends the deadline by 60 seconds from that point.

Pairing order determines the initial USB slot assignment. Up to four controllers map 1:1 to the four emulated Switch Pro Controller interfaces.

With no active controller, the Pico runs Bluepad32 discovery and autoconnect. After any controller becomes active, active discovery pauses to protect input, motion, and rumble latency; bonded controllers may still initiate incoming reconnects. Pairing keys persist across Pico power cycles, so reconnect a previously paired controller by pressing its normal Home, PS, or Xbox power button. Hold BOOTSEL for the bounded pairing window before pairing a new controller or a controller that requires host-side discovery. Outside that window, BTstack remains non-bondable and rejects new Classic and BLE authentication.

To clear every stored Classic and BLE pairing without a PC, hold BOOTSEL continuously for 10 seconds. The normal pairing window opens after two seconds; continuing to hold until the LED changes to a rapid blink clears all bonds, disconnects active controllers, publishes neutral state to every slot, and closes new authentication. Release BOOTSEL, open a new pairing window, and pair controllers again.

LED meanings and device state

The Pico 2 W onboard LED reports the overall Bluetooth state:

• Double blink: new controller authentication is enabled for the bounded pairing window.
• Rapid blink for two seconds: all stored pairings were cleared.
• Fast blink: a controller connection is still completing its handshake.
• Solid: at least one controller is active.
• Slow blink: no controller is active; Bluetooth discovery and autoconnect are running.
• Solid immediately after boot that never transitions: Bluepad32 initialization did not complete; check firmware flashing and UART logs.

Managing controller disconnect and reconnect

• Disconnect a controller: its slot immediately publishes neutral buttons, sticks, and motion. Other connected controllers are unaffected.
• Reconnect a paired controller: power it on normally with its Home, PS, or Xbox button.
• 8BitDo Ultimate Bluetooth reconnect: leave its selector in Bluetooth mode, press Home once, then shake it. After an abrupt controller power-off, the Pico can remain solid for up to four seconds while Bluetooth link supervision confirms the disconnect; scanning restarts immediately afterward.
• Pair a new controller: hold BOOTSEL until the LED double-blinks, then put the controller into its explicit Bluetooth pairing mode.
• Pairing window expires: new authentication and active discovery stop while a controller is active; remembered controllers may still initiate reconnects.
• Clear all pairings: hold BOOTSEL continuously for 10 seconds, through the initial double blink, until the rapid confirmation blink starts. All controllers are disconnected and must be paired again.

Managing configuration, profiles, and pairings from a PC

Connect the Pico 2 W to the PC while the AIO firmware is running normally; do not enter the ROM BOOTSEL drive. switch-pico-config uses versioned private vendor requests on USB endpoint 0, so it does not add an interface or depend on Linux hidraw nodes.

uv run switch-pico-config status
uv run switch-pico-config diagnostics
uv run switch-pico-config reboot bootsel
uv run switch-pico-config config show
uv run switch-pico-config config set --pairing-window-seconds 90
uv run switch-pico-config config reset --yes
uv run switch-pico-config mode auto
uv run switch-pico-config mode switch
uv run switch-pico-config mode xinput
uv run switch-pico-config mode dinput
uv run switch-pico-config mode mac
uv run switch-pico-config profiles list
uv run switch-pico-config profiles edit
uv run switch-pico-config profiles export 1 profile.json --identity 0
uv run switch-pico-config profiles import 2 profile.json --identity 0
uv run switch-pico-config profiles activate 2 --identity 0
uv run switch-pico-config profiles reset all --identity 0 --yes
uv run switch-pico-config pairings list
uv run switch-pico-config pairings clear --yes

Adapter configuration records use version, size, generation, and CRC fields in two dedicated flash sectors. Profiles use separate append-only arenas before the adapter and Bluepad32 bond regions. Profile writes are individually checksummed, recover the previous record after interruption or corruption, compact atomically between arenas, skip unchanged data, and are rate-limited.

Output mode is selected before TinyUSB starts and never changes while mounted. A mode command atomically stores the selection, resets synthetic input, reboots, follows the same physical USB port through re-enumeration, and verifies requested versus active mode. auto uses the verified Switch probe → Windows XInput transition; manual modes bypass probing. The controller chord L + R + Select + Start + System held for three seconds cycles auto → switch → xinput → dinput → mac → auto. The destructive ten-second BOOTSEL pairing reset also restores auto before reboot, providing physical recovery.

Development USB identities are CAFE:4010 (XInput), CAFE:4020 (DInput), and CAFE:4021 (Mac). DInput and Mac expose four input-only generic HID interfaces and no rumble. Mac uses X/Y/Z/Rx sticks plus Simulation Brake/Accelerator triggers. Switch reports input, rumble, and motion capability; XInput reports input and rumble.

profiles edit starts a local-only browser editor at http://127.0.0.1:8765/. It exposes every profile field: all 16 buttons plus the L2/R2 analog triggers can be remapped to any button or trigger output; both sticks and triggers retain independent deadzone/saturation/curve settings; and rumble, confirmation, Turbo/Auto Burst, built-in action chords, and four custom macro sequences are editable. Its live playtest compares current raw stick and trigger input with the unsaved draft, shows deadzone/saturation boundaries and digital thresholds, and highlights pressed physical controls. Select a controller identity and one of its eight profile slots, use Start from defaults for a new draft, then Save to Pico. The backend validates the complete profile before using the existing chunked atomic transaction; invalid drafts never reach flash. Use profiles edit --no-browser for a printed URL or profiles edit --port PORT to choose another local port.

The editor selects Switch Pro, DualSense, or Xbox artwork from the connected controller's USB VID/PID and places each remappable control directly over the matching physical button. Controller artwork is from AL2009man/Gamepad-Asset-Pack under its MIT license; the bundled license and source revision are recorded beside the assets.

Profile names and controller aliases are stored as independently checksummed catalog metadata, so naming does not change the 256-byte profile format. The editor can rename and copy profiles across controllers and slots, import or export JSON backups, and reset one section without discarding the rest of the draft. Its response-curve cards provide named presets, exact Q8.8 fine adjustment, live curve markers, and one-click application to the opposite stick or trigger. Connected-controller details include transport, battery, and supported feedback/motion capabilities; Identify sends one bounded rumble/light pulse only to the selected live controller.

profiles list prints identity index 0 for the global fallback plus each stable Bluetooth identity observed by the firmware. Each identity owns eight persistent profiles and one active index. The JSON export/import commands remain available for version-controlled or scripted profiles. Profile numbers shown to users are 1 through 8; --identity uses the zero-based index from profiles list.

pairings list refreshes and prints stored Bluetooth Classic and BLE addresses. pairings clear --yes deletes all bonds, disconnects active controllers, closes new authentication, and resumes discovery because no controllers remain. Destructive commands require --yes. If multiple compatible Picos are attached, select one with --bus N --address N; the error lists their locations. USB access errors require permission to the matching /dev/bus/usb device.

diagnostics reports Bluetooth initialization stage, real BTstack timer callbacks, controller report traffic, host/local rumble requests and dispatches, active/rumble-capable slot counts, and pending feedback. The AIO build bounds each CYW43 HCI drain to 16 packets so continuous multi-controller traffic returns to BTstack timers instead of starving rumble stop/refresh, configuration, pairing, and profile work.

Per-controller profiles

The profile editor lists Cycle active profile, Toggle motion, and Run custom macro as separate editable actions. Every action chord can contain any combination of the 16 buttons and the L2/R2 analog triggers. The default profile-switching chord is L + R + Select + Start; on DualSense, use L1 + R1 + Create + Options. A stored empty chord selects that default.

• The chord cycles persistent profiles 1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → 1.
• Chord buttons are consumed locally and are not forwarded to the host.
• The new profile applies only after its atomic flash commit completes.
• Confirmation uses one to eight 75 ms pulses matching the active profile number.
• The profile policy independently enables rumble and LED feedback.
• On connection and profile changes, RGB/player LEDs briefly show the active profile color/count, then return to the persistent USB slot color/player number.
• Each controller identity and each of the four active USB slots remain isolated.

Profile input processing is deterministic: physical buttons and analog triggers are mapped and tuned first, Turbo or Auto Burst gates configured buttons second, and active macro overrides apply last. Each profile has four independently triggered macros. A macro supports up to eight state steps, while all four share a fixed sixteen-step decoded pool and a 136-byte sparse wire stream. A wait-only step uses 3 bytes, a button-only step 5 bytes, and a full-precision all-field step 17 bytes; therefore the stream holds 16 typical button steps or exactly 8 all-field steps. Stick coordinates remain signed 16-bit values. Macro triggers can be multi-control chords, partial chords remain ordinary input, and each macro has its own cancel control.

Per-controller motion toggle

The default motion action is D-pad Up + R + Start; on DualSense, use D-pad Up + R1 + Options. Each profile can replace it with any button/trigger chord from the graphical editor; a stored empty chord selects the default.

• A longer rumble confirms motion disabled.
• A shorter rumble confirms motion enabled.
• The chord is consumed locally and is not forwarded to the Switch.
• Other controller slots are unaffected.
• Motion returns to enabled after disconnect or reboot.

Edit src/firmware/input/controller_hotkey_config.h only to change the default motion-enabled state or its feedback patterns.

Per-slot controller colors

Each AIO slot has one color shared by its emulated Switch Pro grips and its physical Bluetooth controller:

1. Blue #0089EB
2. Red #E63946
3. Yellow #F6C945
4. Green #2ECC71

When a controller becomes ready, RGB-capable devices such as DualSense and DualShock 4 receive a darker, more saturated RGB value derived automatically from the slot's Switch grip color. Controllers without an RGB light use player indicator 1, 2, 3, or 4 when Bluepad32 exposes player-LED control. Devices without either capability are left unchanged. Edit only the four grip colors in src/firmware/platform/pico/controller_color_config.h; rebuilding automatically recalibrates their lightbar colors.

Controller capabilities

Controller	Buttons/sticks	Rumble	Motion
DualSense / DualShock 4	Yes	Yes	Yes
Switch Pro / Joy-Con	Yes	Yes	Yes
PS Move ZCM1/ZCM2	Buttons/trigger	Yes	Yes, after calibration
Wii Remote	Mode-dependent	Yes	Accelerometer
8BitDo in Switch-compatible Bluetooth mode	Yes	Model-dependent	Yes when the mode exposes IMU
Xbox Bluetooth controller	Yes	Yes	No hardware IMU

Motion-producing Bluepad32 parsers normalize to 1024 units per degree/second and 8192 units per g in SDL-oriented axes before conversion to Nintendo samples. PS Move motion remains neutral until all model-specific calibration blocks have been received and validated; buttons and rumble remain available while calibration is pending or unavailable. The latest normalized sample is duplicated across the report's three nominal 5 ms slots and remains pending until a regular 0x30 USB report successfully consumes it.

Rumble per controller

Rumble effects are per-slot and independent. The Switch sends rumble commands to a specific USB interface, and the Pico routes each command to the Bluetooth controller in the matching slot. Each slot has a critical-section-protected latest-value mailbox tagged with its connection generation; a newer pending command replaces the older one, and disconnect invalidates commands from the prior controller.

Hardware validation

The four-interface AIO build has been verified on a real Switch with two DualSense controllers: the Switch assigned independent controller slots, and buttons, sticks, calibrated motion, rumble, and disconnect isolation worked per controller. Fresh DualSense pairing through the BOOTSEL-open window has also been verified on hardware.

To reproduce the validation:

1. Verify USB enumeration: Connect the Pico 2 W to a USB host or analyzer. Confirm that four HID interfaces are present, using IN/OUT endpoint pairs 0x81/0x01 through 0x84/0x04.
2. Verify Bluetooth pairing: Hold BOOTSEL until the LED double-blinks, put a controller into explicit pairing mode, and confirm its player light settles.
3. Verify input on one controller: Move sticks and press buttons; confirm only its assigned Switch slot changes.
4. Verify input on two controllers: Move the second controller independently and confirm the first controller's slot is unaffected.
5. Verify the pairing gate: Power-cycle the Pico and confirm a paired controller reconnects with its normal Home/PS/Xbox button without BOOTSEL. Put an unpaired controller into explicit pairing mode and confirm it remains blocked until the BOOTSEL window opens.
6. Verify rumble per slot: Send rumble to interface 0 and confirm only the slot 0 controller vibrates. Send rumble to interface 1 and confirm only the slot 1 controller vibrates.
7. Verify motion: Enable gyro/accel on both controllers. Rotate each controller independently and confirm that motion is per-slot (rotating controller 0 does not affect controller 1's IMU output).

On macOS, inspect the firmware's raw Game Pad values before GameController or browser remapping with:

swift tools/Test-SwitchPicoMac.swift

The diagnostic matches only CAFE:4021, identifies each of the four interfaces by interface and location, and prints changed axes, hats, and buttons with their HID usage and logical range. The four signed stick axes remain X/Y/Z/Rx. Move each analog trigger slowly and confirm output such as LeftBrake page=0x02 usage=0xC5 logical=0...65535 value=32768 and RightAccelerator page=0x02 usage=0xC4 logical=0...65535 value=32768; each trigger should traverse intermediate values across 0...65535, not only the endpoints. The diagnostic continues through hot-plug events until Ctrl-C. If opening a device fails, allow the terminal (or the app launching Swift) under System Settings → Privacy & Security → Input Monitoring, then rerun it.

On the tested Linux host, all four HID interfaces enumerate immediately at the USB layer, but auto initially presents them as a composite Nintendo Pro Controller while probing the host. Linux binds hid-nintendo to each interface and performs synchronous handshake and calibration requests with retries; incomplete composite interoperability causes -110 timeouts and can accumulate into a 15–30 second user-visible delay before the transient hidraw nodes are removed. The timeout is not observed on the Switch. For a Linux laptop, persist dinput for immediate generic-HID enumeration (uv run switch-pico-config mode dinput) or xinput when rumble is required, then restore auto or switch before console use. Profile management uses endpoint-zero vendor transfers and does not depend on hid-nintendo.

Bluepad32 is Apache-2.0. BTstack use on Pico W/Pico 2 W is covered by Raspberry Pi's BTstack license.

Planned features

Limitations

• No NFC/amiibo/IR support.
• Rumble is best-effort: the UART build depends on SDL3 haptics; the AIO build depends on the connected controller's Bluepad32 rumble implementation.
• The UART firmware requires a host computer running the bridge. The Pico 2 W AIO firmware does not; it hosts controllers over Bluetooth, not USB.
• In XInput output mode, Home/System is carried in the raw XUSB Guide bit 0x0400, and Capture is carried in the de-facto Share/reserved bit 0x0800 used by modern open XUSB stacks. The standard Microsoft XInput headers define neither Guide nor Share for XINPUT_GAMEPAD.wButtons, so XInputGetState does not expose either button portably. Guide may be reserved or intercepted by the OS, while Share/Capture support depends on the installed driver or consumers such as GameInput and Steam; qualify the intended controller, driver, and application on real Windows hardware.

Uses

• Remote couch co-op: friends connect via Parsec while the host streams the Switch via a low-latency capture device (e.g., Magewell Pro Capture) and runs the bridge (see setup below).
• Switch automation (Python): write scripts/bots that drive the Pico directly using switch_pico_bridge.switch_pico_uart (see examples/example_switch_macro.py).
• Twitch chat plays: translate chat messages into controller actions on the host, then forward them over UART to the Pico.

Remote couch co-op setup (example)

1. Connect the Switch to a low-latency capture device on the host PC; view it in OBS (or your preferred viewer).
2. Run controller-uart-bridge on the host PC and connect the Pico to the Switch for input.
3. Have friends connect to the host PC using Parsec; they use their controllers on their end, which Parsec forwards to the host (SDL3 sees them).
4. Optional audio routing: Voicemeeter Potato + a virtual audio cable can help manage capture/voice/game audio mixing:
   • Voicemeeter Potato: https://vb-audio.com/Voicemeeter/potato.htm
   • VB-CABLE: https://vb-audio.com/Cable/index.htm

End-to-end data flow (input + rumble)

INPUT (buttons/sticks)
[Any controller] -> [Host OS HID] -> [SDL3 Gamepad] -> [controller-uart-bridge]
                 -> [USB↔UART adapter + UART serial] -> [Pico firmware] -> [USB (Switch Pro)]
                 -> [Nintendo Switch]

RUMBLE (force feedback)
[Nintendo Switch] -> [USB rumble output report] -> [Pico firmware]
                 -> [UART serial + USB↔UART adapter] -> [controller-uart-bridge]
                 -> [SDL3 haptics] -> [Any controller motors]

HD rumble translation

Nintendo sends two stateful four-byte HD-rumble actuator words. Each word can carry full or relative high/low frequency and amplitude commands with up to three subsamples; amplitude uses a logarithmic curve. The Pico decodes both words once in SwitchHapticsDecoder, retains actuator state across packets, and reduces the result to conventional low/strong and high/weak motor magnitudes. SDL3 and Bluepad32 cannot reproduce the original linear-actuator frequencies or left/right spatial effects, but they receive the correct nonlinear band amplitudes.

The UART return frame carries the decoded result rather than raw HD-rumble bytes:

0xBB, 0x02, low-frequency magnitude, high-frequency magnitude, checksum

The checksum is the sum of the first four bytes modulo 256. Firmware and Python bridge versions from before this change are not rumble-protocol compatible; controller input framing remains unchanged.

Hardware wiring (Pico)

• UART1 pins (fixed in firmware):
  • TX: GPIO4 (Pico pin 6) → RX of your USB-serial adapter.
  • RX: GPIO5 (Pico pin 7) → TX of your USB-serial adapter.
  • GND: common ground between Pico and adapter.
• Baud rate: 921600 (default). Some adapters only handle 500,000; both bridges accept a --baud flag.
• Keep logic at 3.3V; do not feed 5V UART into the Pico.

Full hookup checklist

1. Gather the hardware

   • Raspberry Pi Pico flashed with the provided firmware.
   • USB-A-to-micro USB cable (or USB-C if you use a Pico W) to connect the Pico to the Switch or a PC for testing.
   • USB-to-UART adapter capable of 3.3 V logic at 921600 baud (FT232, CP2102, CH340, etc.).
   • Three dupont wires (TX, RX, GND). Optionally add heat-shrink or a small proto board if you want something more permanent.

2. Wire the Pico to the USB-to-UART adapter

   • Pico GPIO4 → adapter RX (sometimes labelled RXD, DI, or R).
   • Pico GPIO5 → adapter TX (TXD, DO, or T).
   • Pico GND → adapter GND. Tie grounds even if the adapter is already USB-powered.
   • Leave VBUS/VCC unconnected unless your adapter explicitly supports 3.3 V power output and you intend to power the Pico from it (the bridge expects the Pico to be powered from USB instead).

3. Connect everything to the host and Switch

   • Plug the USB-to-UART adapter into the computer that will run the Python bridge. Note the COM port (Device Manager > Ports) on Windows or /dev/cu.*//dev/ttyUSB* path on macOS/Linux; pass it via --map/--ports.
   • Connect the Pico's micro USB port to the Nintendo Switch (via the dock's USB-A port, a USB-C OTG adapter, or a PC if you are only testing). The Pico enumerates as a Switch Pro Controller over USB.
   • On the Switch, enable System Settings → Controllers and Sensors → Pro Controller Wired Communication.
   • Any SDL-compatible gamepads you want to use should also be plugged into (or paired with) the same host computer that runs the Python bridge; the bridge is the one reading them.

Finding your USB↔UART adapter “description” (port filtering)

If you have multiple serial/COM devices, you can filter which ports the bridge will consider using the port description (or vendor/product text) shown by the OS.

• macOS/Linux (terminal):
  • Quick list with descriptions: python -m serial.tools.list_ports -v
  • Then run the bridge with a filter, for example: controller-uart-bridge --interactive --include-port-desc CP210
• Windows:
  • Device Manager → Ports (COM & LPT) → open your adapter → copy the device name/vendor text.
  • Then run: controller-uart-bridge --interactive --include-port-desc "USB-SERIAL CH340"

Filters you can use:

• --include-port-desc SUBSTR (repeatable): only consider ports whose description contains the substring.
• --ignore-port-desc SUBSTR (repeatable): exclude ports whose description contains the substring.
• --all-ports: include non-USB serial devices in discovery (useful if your adapter isn’t tagged as USB by the OS).

4. Power-on order and sanity checks

   • Power the Switch/dock so the Pico gets 5 V over USB; its USB stack must stay alive while the bridge streams data.
   • On the host computer, run controller-uart-bridge --list-controllers to make sure SDL sees your pads, then start the bridge with --map/--ports (or --interactive) referencing the adapter path you found earlier.
   • Watch the Rich console output: you should see each controller paired with a UART port and the rumble loop logging reconnects if cables are unplugged.

5. Common pitfalls

   • A flipped TX/RX pair results in silence (no button presses); swap them if the Pico never shows input.
   • Some adapters default to 5 V logic—move the jumper to 3.3 V before touching the Pico.
   • If you use multiple adapters, label each cable; COM port numbers can change between boots.

• When testing on a PC before plugging into a Switch, you can verify activity with the lightweight switch_pico_bridge.switch_pico_uart helper or the Windows "Game Controllers" panel.

Building and flashing firmware

Prereqs: Pico SDK, Arm GNU toolchain, CMake, and picotool.

Using build.py

build.py configures CMake, builds the firmware, checks that both output formats were created, copies the release artifacts into firmware/, and flashes the ELF with picotool.

build.py automatically locates the Pico SDK and Arm GNU toolchain from valid existing build/, build-aio/, or build-feasibility/ CMake caches, then from project-local build/_deps/pico_sdk-src and build/toolchain installs, and finally from conventional user and system locations. A compiler already on PATH is used without setting a toolchain override. Explicit PICO_SDK_PATH and PICO_TOOLCHAIN_PATH values always take precedence; an invalid explicit path is reported instead of silently falling back.

Before running it:

1. Install the Pico SDK, CMake toolchain, and picotool.
2. Connect the Pico in BOOTSEL mode.
3. From the repository root, run:

python3 build.py

The generated files are:

• build/switch-pico.elf, which build.py passes to picotool.
• build/switch-pico.uf2, which can also be copied to the Pico manually.
• firmware/switch-pico.elf and firmware/switch-pico.uf2, refreshed from the corresponding build/ artifacts after every successful build.

To assign one color to every emulated controller slot while building, pass one of these mutually exclusive options:

# Use one random color for all slots
python3 build.py --random-grip-color

# Use one specific six-digit RGB color for all slots
python3 build.py --grip-color FF00AA

Both options update all four slot definitions in src/firmware/platform/pico/controller_color_config.h before building. With no color option, the per-slot blue/red/yellow/green palette is left unchanged. Run python3 build.py --help to see the available command-line options.

If the tools or artifacts are in non-default locations, use these environment variables:

PICO_SDK_PATH=/path/to/pico-sdk \
PICO_TOOLCHAIN_PATH=/path/to/arm-none-eabi-toolchain \
PICOTOOL_PATH=/path/to/picotool \
ELF_PATH=/path/to/switch-pico.elf \
UF2_PATH=/path/to/switch-pico.uf2 \
python3 build.py

PICO_SDK_PATH and PICO_TOOLCHAIN_PATH explicitly select the SDK and cross-compiler installations. PICOTOOL_PATH selects the flashing tool, ELF_PATH selects the ELF that is checked and flashed, and UF2_PATH selects the UF2 that is checked after the build. Their defaults are picotool from PATH, build/switch-pico.elf, and build/switch-pico.uf2, respectively.

Manual build

cmake -S . -B build -DSWITCH_PICO_LOG=OFF
cmake --build build -j

This produces both build/switch-pico.elf and a flashable build/switch-pico.uf2.

Manual UF2 flashing (BOOTSEL, no tools)

If you already have a built (or use the pre-built one in firmware/) .uf2, you can flash it without rebuilding:

1. Unplug the Pico.
2. Hold the BOOTSEL button.
3. While holding BOOTSEL, plug the Pico into your computer over USB (not the Switch), then release BOOTSEL.
4. A USB mass-storage drive (usually RPI-RP2) will appear. Copy the .uf2 onto it (drag-and-drop).
5. The Pico will reboot automatically and the RPI-RP2 drive will disappear when flashing completes.

Tip: if you don’t see RPI-RP2, try a different USB cable (some are charge-only) or a different USB port/hub.

When the AIO firmware is already running on a PC, enter ROM BOOTSEL without touching the board:

uv run switch-pico-config reboot bootsel

The firmware acknowledges the endpoint-zero request, waits 50 ms, and then calls the Pico ROM reset_usb_boot() entry point. Physical BOOTSEL remains the fallback if the firmware or USB management path is unavailable.

Flash alternatives: bootsel + drag-drop or picotool load. Flags:

• SWITCH_PICO_LOG: enable/disable UART logging on the Pico.

Python bridge (recommended)

Works on macOS, Windows, Linux. Uses SDL3 + pyserial.

Install dependencies (pyproject-enabled)

The repository now includes a pyproject.toml, so you can install the bridge and helper scripts as an editable package:

# from repo root
uv venv .venv
source .venv/bin/activate  # or .venv\Scripts\activate on Windows
uv pip install -e .

Prefer stock pip?

python -m venv .venv
source .venv/bin/activate  # or .venv\Scripts\activate on Windows
pip install -e .

• SDL3 runtime: install via your OS package manager (macOS: brew install sdl3; Windows: place SDL3.dll on PATH or next to the script; Linux: install libsdl3-0 or your distribution's equivalent).

Run

source .venv/bin/activate  # or .venv\Scripts\activate on Windows
controller-uart-bridge --interactive
# or, equivalently
python -m switch_pico_bridge.controller_uart_bridge --interactive

Options:

• --map index:PORT (repeatable) to pin controller index to serial (e.g., --map 0:/dev/cu.usbserial-0001 or --map 0:COM5).
• --ports PORTS... or --interactive for auto/interactive pairing.
• --all-ports to include non-USB serial devices in discovery.
• --ignore-port-desc SUBSTR / --include-port-desc SUBSTR to filter serial ports by description (repeatable).
• --include-controller-name SUBSTR to only open controllers whose name matches (repeatable).
• --list-controllers to print detected controllers and their GUIDs, then exit (useful for GUID-based options).
• --baud 921600 (default 921600; use 500000 if your adapter can’t do 900K).
• --frequency 1000 to send at 1 kHz.
• --deadzone 0.08 to change stick deadzone (0.0-1.0).
• --zero-sticks to sample the current stick positions on connect and treat them as neutral (cancel drift).
• --zero-hotkey z to choose the terminal hotkey that re-zeroes all connected controllers on demand (press z by default; pass an empty string to disable).
• --update-controller-db to download the latest SDL GameController database before launching (defaults to the bundled copy in switch_pico_bridge/controller_db/).
• --controller-db-url URL to override the source URL when updating the controller database (defaults to the official mdqinc repo).
• --trigger-threshold 0.35 to change analog trigger press threshold (0.0-1.0).
• --swap-abxy to flip AB/XY globally.
• --swap-abxy-index N (repeatable) to flip AB/XY for controllers first seen at index N (auto-converts to a stable GUID).
• --swap-abxy-guid GUID (repeatable) to flip AB/XY for a specific physical controller (GUID is stable across runs).
• --swap-hotkey x to pick the runtime hotkey that prompts you to toggle ABXY layout for a specific connected controller (default x; empty string disables).
• --sdl-mapping path/to/gamecontrollerdb.txt to load extra SDL mappings (defaults to switch_pico_bridge/controller_db/gamecontrollerdb.txt).
• --debug-imu to print raw gyroscope and accelerometer readings every ~200ms (useful for verifying sensor data and troubleshooting).
• --no-imu to disable sensor reading entirely (useful for controllers without gyro, or if motion causes issues).
• --gyro-scale FLOAT to adjust gyroscope sensitivity (default 1.0; reduce below 1.0 if camera rotates too fast; increase above 1.0 for more sensitivity).

Runtime hotkeys

• By default, pressing z in the terminal re-samples every connected controller's sticks and re-applies neutral offsets. Change/disable with --zero-hotkey.
• Press x (configurable via --swap-hotkey) to open an in-CLI prompt and toggle the ABXY layout for a specific connected controller. This updates the controller's stable GUID list immediately; press again to revert.
• Hotkeys work only when the bridge is started from a TTY/console that currently has focus. Pass an empty string to either flag to disable that shortcut (useful when running unattended).
• If you launch the bridge with --swap-abxy (global swap), the per-controller toggle hotkey will show that the layout is enforced globally and will not override it.

Updating SDL controller mappings

• The bridge ships with a pinned switch_pico_bridge/controller_db/gamecontrollerdb.txt. Run controller-uart-bridge --update-controller-db ... to download the latest database from the official upstream (mdqinc/SDL_GameControllerDB).
• The download only touches switch_pico_bridge/controller_db/gamecontrollerdb.txt; add --controller-db-url https://.../custom.txt if you maintain your own fork.
• If the file is missing, the bridge will automatically attempt a download on startup.

Hot-plugging: controllers and UARTs can be plugged/unplugged while running; the bridge will auto reconnect when possible.

Using the lightweight UART helper (no SDL needed)

For simple scripts or tests you can skip SDL and drive the Pico directly with switch_pico_bridge.switch_pico_uart:

from switch_pico_bridge import SwitchUARTClient, SwitchButton, SwitchDpad

with SwitchUARTClient("/dev/cu.usbserial-0001") as client:
    client.press(SwitchButton.A)
    client.release(SwitchButton.A)
    client.move_left_stick(0.0, -1.0)  # push up
    client.set_hat(SwitchDpad.UP_RIGHT)
    print(client.poll_rumble())  # returns (left, right) amplitudes 0.0-1.0 or None

• SwitchButton is an IntFlag (bitwise friendly) and SwitchDpad is an IntEnum for the DPAD/hat values (alias SwitchHat remains for older scripts).
• The helper only depends on pyserial; SDL is not required.

macOS tips

• Ensure the USB‑serial adapter shows up (use /dev/cu.usb* for TX).
• Some controllers’ Guide/Home buttons are intercepted by macOS; using XInput/DInput mode or disabling Steam’s controller handling helps.

Windows tips

• Use COMx for ports (e.g., COM5). Auto‑detect lists COM ports.
• Ensure SDL3.dll is on PATH or alongside the script.

Linux tips

• You may need udev permissions for /dev/ttyUSB*//dev/ttyACM* (add user to dialout/uucp or use udev rules).
• For the development XInput/DInput/Mac identities, install udev/99-switch-pico.rules into /etc/udev/rules.d/, reload udev, and reconnect the Pico so switch-pico-config can access endpoint zero without root.

IMU / Motion Controls

The bridge supports gyroscope and accelerometer passthrough from controllers that have motion sensors (e.g. the Nintendo Switch Pro Controller and DualSense). Motion data is forwarded to the Pico as a rolling three-sample window; the Pico emits standard 0x30 reports at 15 ms intervals and supports both raw IMU mode 1 and packed quaternion mode 2.

Requirements

• A controller with gyro/accelerometer support that SDL3 can enable.
• The Switch will automatically use motion data once the controller is recognised as a Pro Controller.

Gyro bias calibration

On startup, the bridge collects the first 200 gyro readings while the controller is stationary and averages them to compute a per-axis bias (zero-rate offset). The bias is subtracted from subsequent readings. Keep the controller still during startup for best results.

CLI flags

• --debug-imu: Print raw sensor values (m/s² and rad/s) and converted Switch integer counts every ~200ms. Useful for verifying the sensor is detected and producing sensible data.
• --no-imu: Disable IMU entirely. The bridge sends zero motion data to the Pico, which sends zero-filled IMU bytes to the Switch. Buttons and sticks are unaffected.
• --gyro-scale FLOAT (default 1.0): Multiply all gyro values by this factor before sending. Reduce below 1.0 if the camera moves too fast; increase above 1.0 for more sensitivity.

Troubleshooting

• Gyro not detected: Run with --debug-imu. If no IMU readings appear, SDL3 cannot see sensors on the controller. On Linux, the hid-nintendo kernel driver may expose Nintendo controller motion differently; DualSense motion is supported by SDL3's PlayStation HID driver.
• Wild camera swinging: Rebuild and flash the current Pico firmware. Older builds acknowledged quaternion IMU mode 2 but emitted raw mode-1 bytes, which Zelda interpreted as random quaternion data. Keep the controller still during startup, then use --gyro-scale only for deliberate sensitivity adjustment.
• Verifying Pico output: Use uv run python tools/read_pro_imu.py --vid 0x057E --pid 0x2009 to read raw IMU bytes directly from the Pico's USB HID output. A stationary controller should show gyro values near zero and three non-empty, non-duplicated samples per report.

Implementation notes for maintainers

The failure

Nintendo subcommand 0x40 is a mode selector, not a Boolean enable:

Value	Meaning	Required bytes 13-48 in report 0x30
0	IMU off	Zero-filled
1	Raw IMU	Three 12-byte accelerometer/gyro samples
2	Quaternion	Nintendo's packed 36-byte mode-2 structure

The previous firmware stored the argument in bool is_imu_enabled. A mode-2 request therefore enabled the raw mode-1 packer. Zelda then decoded raw sensor bytes as mode bits, compressed quaternion components, deltas, and timestamps, producing apparently random camera rotation. The fake also advertised firmware 4.91, while the genuine wired Pro Controller used during diagnosis reported 3.48.

Keep SwitchImuMode as a three-state value. Never acknowledge mode 2 and then emit mode-1 bytes.

Mode-1 implementation

• Emit one 0x30 report every 15 ms.
• Advance the report timer by 3: one timer tick for each nominal 5 ms IMU sample.
• Pack three chronological samples as signed little-endian accel X/Y/Z, then gyro X/Y/Z.
• The host bridge must retain and republish its latest three-sample window. Do not drain it at the faster UART rate; that previously produced empty and duplicated USB reports.
• With the advertised factory calibration, 1g is approximately 4096 counts and 1 rad/s is approximately 818.5 gyro counts.

Mode-2 implementation

src/firmware/usb/switch/switch_pro_driver.cpp implements this in integrate_motion_sample() and fill_quaternion_imu_report_data():

1. Reset quaternion state to (0, 0, 0, 1) when transitioning into mode 2.
2. Integrate each report's three gyro samples at 5 ms per sample. The Nintendo quaternion axes use sensor Y, X, Z, not X, Y, Z.
3. Build a delta quaternion from the angular rotation vector, multiply it into the current orientation, and normalize after every sample.
4. Select the largest absolute quaternion component. Its index and sign represent the omitted component; encode the other three signed components at 21-bit precision.
5. Pack accelerometer data in Y, X, Z order, set the mode field to 2, write the 11-bit millisecond timestamp, and set the timestamp/sample count to 3.
6. Integrate and repack only when transmitting the next 15 ms USB report. Calling the integrator from the unrestricted main loop over-integrates the same UART samples.

The mode-2 wire format is bit-packed and fields cross byte boundaries. Use write_bits_le() rather than C/C++ bitfields so layout does not depend on compiler bitfield rules.

Regression and hardware verification

After changing any IMU conversion, calibration, timing, or report packing:

1. Run uv run --with pytest pytest -q.
2. Build with cmake --build build -j.
3. Capture at least 200 raw 0x30 reports. Stationary gyro should remain near zero; there should be no empty windows, duplicated three-sample windows, or timer-step errors.
4. Send subcommand 0x40 with value 2. Every resulting report must have mode bits 2 and timestamp count 3.
5. Inject a known single-axis gyro rate and decode the packed quaternion. The corresponding component must change smoothly with the expected sign.
6. Perform the decisive end-to-end check: genuine Pro Controller → SDL3 bridge → UART → emulated Pico → Zelda. This path was confirmed correct after the mode-2 fix.

Firmware resource usage

The Pico 2 W AIO build is measured from build-aio/switch-pico.elf and its linked binary, not from the larger debug-bearing ELF or UF2 transport file:

Resource	Used or reserved	Device capacity
Executable flash image	695,592 bytes	4 MiB
Indexed profile arenas	256 KiB	4 MiB flash
Adapter configuration	8 KiB	4 MiB flash
BTstack bonds	8 KiB	4 MiB flash
RP2350 terminal sector	4 KiB	4 MiB flash
Linked SRAM	99,040 bytes	520 KiB

The executable plus persistent reservations consume 978,216 bytes (23.32%) of flash, leaving 3,216,088 bytes (3.07 MiB). Linked SRAM consumes 18.60%, leaving 433,440 bytes of link-time headroom.

Profiles use two 128 KiB append-only arenas. Each independently published record contains one identity/profile key, generation, schema, length, and CRC. The compact in-memory index is 1,556 bytes; only the fallback and active profile for each observed identity are decoded and published. Including the active cache, selected-profile buffer, transaction state, profile runtime contexts, and catalog index, the profile subsystem uses approximately 13 KiB of SRAM instead of retaining every profile in decoded form.

The catalog supports eight profiles for the global fallback and each of 16 stable identities. Missing records resolve to defaults, so profiles 5–8 do not consume flash until changed. When an arena fills, the latest indexed records are compacted into its peer and the new superblock is published last. Interrupted or corrupt appends therefore leave the previous valid record available. On first boot after upgrading, the legacy four-profile banks are read from their old flash addresses and copied into the new catalog before the legacy region can be erased.

References

• GP2040-CE (controller firmware ecosystem): https://github.com/OpenStickCommunity/GP2040-CE
• nxbt (Switch controller research/tools): https://github.com/Brikwerk/nxbt
• Nintendo Switch Reverse Engineering notes: https://github.com/dekuNukem/Nintendo_Switch_Reverse_Engineering
• hid-nintendo driver reference: https://github.com/DanielOgorchock/linux/blob/ogorchock/drivers/hid/hid-nintendo.c

Troubleshooting

• No input on Switch: verify UART wiring (Pico GPIO4/5), baud matches both sides, Pico flashed with current firmware, and Pro Controller Wired Communication is enabled on the Switch.
• Constant buzzing rumble: the bridge filters small rumble payloads; ensure baud isn’t dropping bytes. Try lowering rumble scale in switch_pico_bridge.controller_uart_bridge if needed.
• Guide/Home triggers system menu (macOS): try different controller mode (XInput/DInput), disable Steam overlay/controller support, or connect wired.
• SDL can’t see controller: load switch_pico_bridge/controller_db/gamecontrollerdb.txt (default), add your own mapping, or try a different mode on the pad (e.g., XInput).