het68 — 6-channel I2S USB sound card for Raspberry Pi Pico 2 (RP2350)

This firmware turns a Raspberry Pi Pico 2 (RP2350) into a 6-channel USB audio input device (microphone). It captures six independent I2S data lines from six ICS-43434 MEMS microphones (SEL hardwired GND, left channel only), moves the samples into memory with DMA, and streams them to the host over USB Audio Class 2.0 (UAC2) using TinyUSB.

Audio format

Property	Value
Class	USB Audio Class 2.0 (UAC2), isochronous IN
Channels	6
Sample rate	48 kHz
Sample format	24-bit signed, packed little-endian (S24_3LE, 3 bytes/sample)
USB packet	1 ms = 48 × 6 × 3 = 864 bytes (within the 1023-byte full-speed iso limit)
Product string	Pico 6ch Microphone 48k/24

The host sees a standard 6-channel 48 kHz / 24-bit capture device and can record all six microphones simultaneously (e.g. with arecord, Audacity, Reaper, OBS).

arecord -D hw:<card>,0 -c 6 -r 48000 -f S24_3LE -d 3 capture.wav

How capture works

• The Pico is the I2S master. One PIO state machine generates the shared WS/SCK clocks for all six microphones.
• Six PIO RX state machines (one per mic SD line on GP16/18/20/26/27/28) capture serial data. Mics 1–3 use PIO0; mics 4–6 use PIO1 (same clkdiv, started together). Each line reads the left I2S slot (SEL hardwired GND on modules).
• DMA moves the captured 32-bit slots into a double-buffered memory region; the USB device task packs them into 24-bit S24_3LE frames.

Hardware wiring (Grove Shield for Pi Pico)

Board: Seeed Grove Shield for Pi Pico v1.0. Shield power switch → 3.3 V (never 5 V — destroys ICS-43434).

Each microphone needs two 4-pin Grove cables (clock + data). Cable colours are the same on every cable — only the shield socket and mic socket change.

Grove 4-pin cable — wire colours

Pin	Wire colour	Usual name
1	white	signal (lower-number pin)
2	yellow	signal (higher-number pin)
3	red	VCC (3.3 V from shield)
4	black	GND

On the mic breakout, strap SEL → GND (left channel only). The yellow wire (pin 2) on the data cable is not connected on the module.

Wiring overview

Shield                          Each mic module (×6)
────────                        ────────────────────
[UART0]─────── Debug Probe      (no mic cable)
[UART1]──clock──► Mic1 CLK IN ──► CLK OUT ──► Mic2 ──► Mic3
[I2C0]──clock───► Mic4 CLK IN ──► CLK OUT ──► Mic5 ──► Mic6
[D16]──data─────► Mic1 DATA
[D18]──data─────► Mic2 DATA
[D20]──data─────► Mic3 DATA
[A0]──data──────► Mic4 DATA
[A1]──data──────► Mic5 DATA
[A2]──data──────► Mic6 DATA
[I2C1]────────── PS1240 piezo (no mic)

Clock cable (7× Grove cable — same colours on every clock cable)

From shield into CLK IN on mic 1 (branch A) and mic 4 (branch B); daisy-chain CLK OUT → CLK IN along each branch. Do not use red (pin 3) on clock cables.

Pin	Wire	Shield / mic signal	GPIO
1	white	SCK	GP9
2	yellow	WS	GP8
3	red	not used	—
4	black	GND	GND

Branch	Plug shield clock socket into	Then daisy-chain
Mics 1–3	UART1	Mic1 → Mic2 → Mic3
Mics 4–6	I2C0	Mic4 → Mic5 → Mic6

(UART1 and I2C0 are the same GP8/GP9 on the PCB — two sockets, one clock bus.)

Data cable (6× Grove cable — identical colours on every mic)

Plug shield end into the port in the Mic column; plug mic end into DATA. Use pin 1 (white) for SD on analog ports A0–A2 (do not use pin 2 there).

Pin	Wire	Mic module (ICS-43434)	Shield port
1	white	SD (serial data)	see table below
2	yellow	not used (SEL = GND on PCB)	—
3	red	VDD (3.3 V)	same socket
4	black	GND	same socket

Mic	USB ch	Shield data socket	SD GPIO (pin 1 white)
1	1	D16	GP16
2	2	D18	GP18
3	3	D20	GP20
4	4	A0	GP26
5	5	A1	GP27
6	6	A2	GP28

Analog ports A0–A2: the shield ties A0 pin 2 to A1 pin 1 (GP27) and A1 pin 2 to A2 pin 1 (GP28). Use only pin 1 (white) on each port — one mic per socket.

Debug UART — Grove UART0 → Raspberry Pi Debug Probe

Pin	Wire	GPIO	Signal	→ Probe connector U
1	white	GP1	UART RX	← Probe TX (orange)
2	yellow	GP0	UART TX	→ Probe RX (yellow)
3	red	—	not used	—
4	black	GND	ground	GND

Baud rate: 115200 (./serial.sh).

Sync beacon — Grove I2C1 → PS1240 piezo

Passive piezo between the two signal wires. Do not use red or black on this cable.

Pin	Wire	GPIO	Signal
1	white	GP7	Piezo B
2	yellow	GP6	Piezo A
3	red	—	not used
4	black	—	not used

GPIO quick reference

Grove port	Function	GPIO
UART0	Debug UART	GP0 TX, GP1 RX
UART1	Clock branch mics 1–3	GP9 SCK, GP8 WS
I2C0	Clock branch mics 4–6	GP9 SCK, GP8 WS
I2C1	Piezo beacon	GP6, GP7
D16 / D18 / D20	Mic 1–3 SD	GP16 / GP18 / GP20
A0 / A1 / A2	Mic 4–6 SD (pin 1)	GP26 / GP27 / GP28
(header)	Grove DPS310 barometer (optional)	GP2 SDA / GP3 SCL (I2C1)

Barometer — Grove DPS310 (optional, v1.2.1+)

Seeed 101020812 Infineon DPS310 (300–1200 hPa, −40–85 °C). Wire to free header pins GP2 (SDA) / GP3 (SCL) + 3V3/GND — not to shield I2C0/I2C1 (those are mic clocks / piezo). Firmware probes 0x77 then 0x76; if the pins are busy or no device answers, baro is skipped. CLI: BARO (aliases DPS, PRESSURE); also in STATUS, boot dump, and heartbeat (baro=…hPa). v1.2.3: moist-air speed of sound (Cramer/NPL-style) from DPS310 T+P and assumed RH (default 50 %, set with BARO RH <0-100>): Arden-Buck (p_{sat}), (x_w=RH\cdot p_{sat}/p), then (c=331.36\sqrt{T_K/273.15},(1+0.314 x_w+0.037 x_w^2)). Shown as SOUND c=… / BARO … c=…; fed into DOA TDOA (default 343 m/s without baro). Details: wiring_and_bom.md, Seeed wiki.

BOM and module layout: wiring_and_bom.md.

Drone detection (DOA + sync beacon)

Alongside the USB sound card the firmware runs an autonomous acoustic front-end:

• Multi-source DOA (drone + vehicle + bird + walker + wind) with CLI + DET log. core1 runs parallel front-ends:

  Class	Band	Cue	Tracks
  drone	~800 Hz–6 kHz	continuous, wind gate kept, crest limit	up to 2
  wind	LPF ~250 Hz	same energy as the gate → intensity + steered az/el	status
  vehicle	~80 Hz–2.5 kHz	pass-by → ICE / EV	up to 2
  bird	~2–8 kHz	chirp bout → songbird / corvid / bird	up to 2
  walker	~150 Hz–600 Hz	footstep bout (≥3 steps) → human/cat/dog	1

  Entity gallery (entity_store) = classification templates (ACID dual-slot, last two flash sectors). Detection log (detection_log) = timed events in a separate flash region (two sectors below the gallery) with first_seen, last_seen, occurrence, max_gap_ms. DET timestamps are written only after TIME SYNC <unix> since boot. DET EXPORT emits JSON Lines (NVREVT) for smart-NVR event correlation.

  UART CLI (help on boot / first byte / USB mount): HELP, STATUS, TIME / TIME INFO / TIME SYNC, LOG ON|OFF, DET LIST|EXPORT|BACKUP|IMPORT|DEL|CLEAR, ENT LIST|EXPORT|IMPORT, DRONE LIST|CLEAR, BARO, RID LIST|ON|OFF. See TEST_SCENARIOS_1.0.6.md.

• OpenDroneID / EU Direct Remote ID (v1.1.0+). On CYW43 boards (PICO_BOARD=pico2_w, pimoroni_pico_plus2_w_rp2350, or other Pico W variants) the firmware scans BLE advertisements for service UUID 0xFFFA and parses ASTM F3411 Basic ID + Location + System messages (e.g. Dronetag / built-in RID). Tracks show up in the heartbeat as rid=N, via RID LIST, and as DET class remoteid. v1.1.1: System-message timestamp (ODID 2019 epoch → Unix) auto-applies TIME SYNC when unsynced or when drift ≥ 2 s (rate-limited), so DET timestamps work without a UART TIME SYNC. v1.1.2: TIME INFO reports sync source / age / quality; known drones (+ RID fields) persist in a flash ACID store (DRONE LIST); STATUS dumps all stored data and statistics (also printed on boot). v1.2.1: optional Grove DPS310 barometer on GP2/GP3 (BARO). Non-wireless pico2 builds compile a stub for RID. Flash end layout: DRONE / BT TLV / DET / ENT (two sectors each).

  SRC class=wind az=180.0 el=10.0 inten=-22.5dB
  SRC class=drone id=0 az=137.4 el=22.8 conf=0.7 lvl=-31.2dB
  DET id=3 class=wind first=1720000000 last=1720000012 occ=8 max_gap_ms=400 az=180.0 el=10.0 inten=-22.5dB
  TRACKS drone=1 vehicle=0 bird=0 walker=0 wind=1 entity=0
  

  • Species / ICE·EV / wind direction / re-ID are best-effort heuristics (v1.0.7: soft score bands, gait regularity, ambiguity→generic bird, tighter cross-class gates). Still not a trained model.
  • Walker tracking assumes one walking entity at a time.

• Array geometry. A cube standing on a vertex, 512 mm edge. Mics 1–3 are the three upper faces (mic 1 = north, then +120°, +240° azimuth, all at +35.26° elevation); mics 4–6 are the opposite lower faces (−35.26° elevation, azimuths interleaved by 60°). Select the edge at build time with any integer size — HET68_DOA_EDGE_MM=150 ./build.sh (default 512 mm); DOA_MAXLAG and the comparison window derive from it automatically. How to physically build the cube, what to build it from, and a full edge-length trade-off analysis (accuracy, speed, compute, memory) are in array_cube_design.md.

• Synchronisation beacon. The PS1240 piezo emits a periodic BPSK-modulated m-sequence burst on a ~4 kHz carrier (its resonance), driven differentially via the GP6/GP7 software H-bridge. This pseudo-noise code is what neighbouring nodes will cross-correlate (matched filter) for acoustic ranging / clock sync; the receive/ranging side is a later phase. The beacon count appears in the heartbeat (bcn=).

Downloads

Pre-built firmware for each tagged release is published on GitHub Releases:

https://github.com/fedurca/het68/releases

Create a GitHub Release whose tag is SemVer core (semver.org) — x.y.z or vx.y.z (e.g. 1.2.4 / v1.2.4) — and CI builds firmware for pico2, pico2_w (RP2350 + CYW43439 Wi‑Fi/BT), and Pico Plus 2 W, then attaches .uf2 / .elf assets to that release.

Build

The Pico SDK is cloned locally into ./pico-sdk (gitignored); no environment variables are required. On a fresh checkout:

./scripts/fetch_pico_sdk.sh   # Pico SDK 2.2.0 + TinyUSB submodule
./build.sh

See BUILDING.md for details.

Target board

The default board is pico2 (Raspberry Pi Pico 2, RP2350A, 4 MB flash, no PSRAM). Select a different board with PICO_BOARD:

./build.sh                                          # default: pico2
PICO_BOARD=pico2_w ./build.sh                       # Pico 2 W: RP2350A + CYW43439 Wi-Fi/BT
PICO_BOARD=pimoroni_pico_plus2_w_rp2350 ./build.sh  # 16 MB flash, 8 MB PSRAM, Wi-Fi/BT

Board	PICO_BOARD	Notes
Pico 2	pico2	RP2350A, 4 MB flash, no wireless
Pico 2 W	pico2_w	RP2350A + CYW43439 Wi‑Fi/BT (OpenDroneID)
Pico Plus 2 W	pimoroni_pico_plus2_w_rp2350	RP2350B, 16 MB flash, 8 MB PSRAM, Wi‑Fi/BT

The Pimoroni Pico Plus 2 W (RP2350B) is the recommended upgrade for multi-node work: 16 MB flash, 8 MB PSRAM (headroom for recording / on-device detection), and 2.4 GHz Wi-Fi + Bluetooth for networking nodes. Official Pico 2 W is the drop-in wireless Pico 2 (same 4 MB flash, CYW43439). Boards whose LED lives on a wireless module do not define PICO_DEFAULT_LED_PIN, so the heartbeat LED is skipped there (the UART heartbeat still runs). Rationale, PSRAM sizing, power (all these boards are 3.3 V designs; USB needs 3.3 V USB_OTP_VDD, so a fully-1.8 V node is custom-hardware only), and alternatives are in array_cube_design.md.

TinyUSB / pico-sdk patches

Upstream Pico SDK 2.2.0 (commit a1438dff) ships TinyUSB 0.18.0, which does not yet contain all fixes needed for stable RP2350 UAC2 6-channel streaming. This repo carries local patches under patches/ — full rationale and per-fix notes are in PATCHES.md.

What	Where
Patch script	patches/apply_all.py
Detailed fix list	PATCHES.md
Reference .patch (ISO activate, PR 2937)	patches/tinyusb-0.18.0-pr2937-iso-activate.patch

What they fix (summary):

• RP2350 USB device controller quirks (EP_ABORT spin, NULL EP0 control register)
• -O3 undefined behaviour from TU_VERIFY / TU_ASSERT in TinyUSB audio/stack code
• UAC2 enumeration race on Linux (SET_INTERFACE vs. first ISO IN packet, err -110)
• ISO endpoint busy-flag handling after abort

How to apply:

Patches are applied automatically on every ./build.sh run (idempotent: reset patched files to git HEAD inside pico-sdk/lib/tinyusb, then re-apply). To apply manually:

python3 patches/apply_all.py

Expected output ends with ✓ All patches OK. Do not edit files under pico-sdk/ by hand — add changes to patches/apply_all.py or a patch file instead.

Target SDK version: patches are written and tested against the vendored tree Pico SDK 2.2.0 / TinyUSB 0.18.0 (pico-sdk/lib/tinyusb). After upgrading the SDK submodule, re-run apply_all.py and check for [pattern not found] errors; see PATCHES.md for upstream-overlap notes (some fixes may already be merged).

./build.sh

This produces build/pico_6mic_soundcard.uf2 (and .elf). The default board is PICO_BOARD=pico2.

• Bench test without microphones — build the diagnostic firmware, which bypasses I2S and streams a simulated 1 kHz tone on all channels:

HET68_USB_DIAG=ON ./build.sh

Flash

Download a pre-built .uf2 / .elf from GitHub Releases, or build locally (./scripts/fetch_pico_sdk.sh && ./build.sh).

• Via Raspberry Pi Debug Probe (SWD), recommended for the lab setup:

./fixdebugger.sh           # frees the USB bus, then flashes the ELF over SWD
./fixdebugger.sh --test    # also runs a short arecord smoke test

• Via UF2 / BOOTSEL: hold BOOTSEL while plugging in the Pico, then copy build/pico_6mic_soundcard.uf2 (or the versioned asset from the release) to the RP2350 mass-storage volume.

Lab capture

10-second 6-channel reference recording (stops PipeWire so arecord can open the device directly):

./record10s.sh                  # -> /tmp/lab_6ch_10s.wav
./record10s.sh capture.wav      # custom path

Usage

After reset the Pico enumerates as a standard 6-channel 48 kHz / 24-bit USB audio device on Windows, macOS and Linux. Select it as the input device in any audio application and record from all six microphones at once.

License

Copyright (C) 2025-2026 het68 project contributors.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License v3.0 as published by the Free Software Foundation. This program is distributed WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the full license text in LICENSE or https://www.gnu.org/licenses/gpl-3.0.html.

Note: the vendored pico-sdk/ and the TinyUSB patches under patches/ carry their own upstream licenses (BSD-3-Clause / MIT) and are not covered by this project's GPLv3.