USBPods Lite — Pico 2 W USB to Bluetooth Audio

Open-source firmware that turns a Raspberry Pi Pico 2 W (or the Waveshare RP2350B-Plus-W USB dongle — one universal binary covers both) into a driver-free USB sound card that streams to Bluetooth headphones with the hi-res codecs ordinary dongles never ship.

Plug it into any computer, console or handheld with a USB port. No app, no driver, no pairing dance on the host — the dongle owns the Bluetooth link, so the host just sees a sound card.

This is the RP2350 successor of the original PicoW-usb2bt-audio project that lived in this repo (video demo of the original Pico W version). The RP2040 Pico W is not supported by this firmware — see the older tags for that board.

• Driver-free: enumerates as a standard USB sound card — USB Audio Class 2, plus a legacy UAC 1, at a 48 kHz or 44.1 kHz sample rate.
• Codecs: SBC, AAC-LC, AAC-ELD (the low-latency Apple mode that makes AirPods sound their best), LDAC and LHDC V5 — see the table below. Each one can be switched off so you can force the negotiation down a rung.
• Media keys: play/pause/skip pressed on the headset reach the computer as USB consumer-control keys, and absolute volume is synced both ways.
• Two device slots with instant switching, plus auto-connect at power-on.
• Serial console on the USB CDC port: the whole product UI in one keystroke-per-action screen, no software to install.

The dongle offers every enabled codec and takes the best one the headset accepts. What actually got negotiated — and the rate it is running at right now — is on the console's status line.

AAC-ELD is Apple's undocumented vendor codec; the wire format we reverse engineered for it is written up in aac-eld-apple.md — capability bytes, the millisecond-based RTP timestamps, and the per-frame size headers.

This repo is the free Lite firmware. The Full firmware at Under Development adds:

The web UI recognizes a Lite dongle and offers the upgrade path.

1. Download USBPods_Pico2W_Lite.uf2 from the releases page (or build it yourself, below).
2. Hold BOOTSEL while plugging the board in — it appears as an RP2350 drive.
3. Copy the UF2 onto the drive; the board reboots into the firmware.

1. Select the USBPods Lite audio output on your computer.
2. Put your headphones in pairing mode — the dongle scans and connects on its own. Afterwards it reconnects at power-on.
3. For anything else, open the serial console: any terminal on the USB CDC port, e.g. screen /dev/tty.usbmodem* (macOS/Linux) or PuTTY on the COM port (Windows). Press Enter for the menu.

Settings are stored in flash and survive reboots. Codec toggles apply at the next connection — a running stream is never torn down under you. LDAC and LHDC rate changes apply immediately, mid-stream.

In Lite each slot owns its headset: pairing a new device into a slot forgets the one it replaces, link key included.

Requirements:

• pico-sdk 2.1.1 — the VS Code Raspberry Pi Pico extension layout under ~/.pico-sdk works out of the box (or set PICO_SDK_PATH).

• sdk patches — the build refuses to configure until the local pico-sdk patches are applied; see sdk-patches/README.md. They fix BTstack/TinyUSB behaviour the firmware depends on, and a stock SDK silently changes wire behaviour rather than failing to compile.

• Rust for the LHDC V5 encoder crate, with the soft-float Cortex-M33 target:

  rustup target add thumbv8m.main-none-eabi
  

Then:

cmake -B build -G Ninja
ninja -C build

The result is build/USBPods_Pico2W_Lite.uf2.

Adding a codec is one file in src/btstack/codec/ plus one line in codec_registry.c; nothing else in the firmware changes.

Bug reports, test results with your headsets, and pull requests are welcome — see CONTRIBUTING.md (note the relicensing grant, needed because most files are shared with the commercial edition).

This project wouldn't have been possible without the foundational work of:

1. tinyusb uac2_headset: TinyUSB UAC2 headset demo
2. a2dp_source_demo: BTstack's A2DP source demo
3. avdtp_source_test.c: BTstack audio tests (SBC, AAC, aptX, LDAC)

Copyright (C) 2023-2026 wasdwasd0105

The USBPods firmware in this repository is licensed under GPL-3.0-only (see LICENSE.txt), with an additional permission under GPLv3 section 7 that allows building and distributing binaries linked against the bundled codec libraries and the Pico SDK stack. Without that exception a GPL firmware could not legally ship with FDK-AAC inside — read it before redistributing binaries.

The commercial USBPods Full firmware is a separate product by the same copyright holder and is not covered by this license.

Bundled third-party code keeps its own licenses:

• BTstack — comes with the pico-sdk under BlueKitchen's license terms for Raspberry Pi silicon
• TinyUSB — MIT
• FDK-AAC — Fraunhofer FDK AAC Codec Library for Android license (3rd-party/fdk-aac/NOTICE; GPL-incompatible on its own, covered by the linking exception)
• LHDC V5 encoder — Apache-2.0 (AOSP-derived Rust reimplementation, 3rd-party/lhdcv5)

 Copyright (C) 2013 - 2016 Sony Corporation

  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

NOTICE

---------------
 Certification
---------------
   Taking the certification process is required to use LDAC in your products.
   For the detail of certification process, see the following URL:
      https://www.sony.net/Products/LDAC/aosp/

 Copyright 2018-2019 Huang-Huang Bao

  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

See 3rd-party/fdk-aac/NOTICE for the Fraunhofer FDK AAC license.