RP2354A microphone array — Zephyr

中文说明：README-CN.md

Four synchronized IM72D128V01 PDM microphones on the custom RP2354A board. Zephyr, its modules, and the SDK all live in zephyrproject/ inside this repository. The schematic and open-source hardware project are hosted on EasyEDA Pro. The build target is rpi_pico2/rp2350a/m33; app.overlay sets this chip's 2 MiB flash, pin wiring, and USB functions. See RP2354A board.

USB

One full-speed composite device, VID:PID 2fe3:0007 (legacy microphone-only firmware: 2fe3:0006), product Pico 2 Mic Array UAC2, manufacturer Pico Array Prototype, configuration Audio, shell and telemetry:

Function	USB interfaces	Windows label
Diagnostics shell	CDC 0 / data 1	Mic Array Shell
MIC/2 telemetry and controls	CDC 2 / data 3	Mic Array Telemetry
Four-channel 48 kHz PCM16 capture	UAC2 microphone	Pico 2 Mic Array UAC2
Stereo 48 kHz PCM16 playback	UAC2 speaker	Pico 2 Mic Array UAC2

COM numbers come from the PC. mic_app picks the telemetry port from the VID, PID, and interface, then checks the MIC/2 handshake. UAC2 is independent of both serial ports. Record the four raw channels with WASAPI exclusive; the Windows shared mixer presents stereo. See CDC telemetry.

With no saved audio record, the USB stream is the four microphones. Beamforming, noise reduction, and direction estimation are off. A saved record restores noise mode, hum filter, input channel, beam mode, azimuth, assumed elevation, and the learned 129-bin noise spectrum before capture starts. Direction estimation is separate and stays off until enabled. Saving is mic_app Save & restart, or the MIC/2 command SETTINGS_SAVE, and it runs only while UAC is idle. The last 8 KiB of flash holds two alternating 4 KiB records with a generation count and CRC32. A normal UF2 update leaves that partition in place; erasing the whole flash clears it. See persistent audio settings.

The linked image in build/zephyr/zephyr_final.map uses 147,228 bytes of the 2,088,960-byte firmware partition and 513,760 of 532,480 bytes of RAM, leaving 18,720 bytes. CONFIG_DMA=n because this application owns the DMA controller. CONFIG_ASSERT=n and CONFIG_SPIN_VALIDATE=n keep debug kernel checks out of the realtime build. Capture sequence checks, USB health counters, configuration bounds, and the settings CRC stay on.

Select Pico 2 Mic Array UAC2 as the Windows output device to play through the MAX98357A and connected speaker while recording from the array. USB accepts stereo; the amplifier module selects left, right or a mono mix for the single speaker. Acoustic echo cancellation is not implemented. See UAC2 microphone and speaker for playback, diagnostics and validation results.

Signal path

One PIO0 state machine clocks GP6 and samples GP7–GP10 together. Every microphone SELECT pin is grounded, so the data pins are driven while CLOCK is low. The program samples on the fifth of ten PIO cycles, late in that low half, with GPIO input synchronizers enabled. Each FIFO word holds eight chronological 4-bit samples, earliest in bits 0..3, in order U1, U2, U3, U4.

At the nominal 150 MHz system clock the PIO divider is 1250/256: 3.072 MHz PDM, then 48,000 PCM frames/s after decimation by 64. The rate follows the 12 MHz crystal. The fractional divider adds cycle-to-cycle jitter.

Two hardware-chained DMA channels fill 24 buffers. Each destination is a 1024-word, 4 KiB area, aligned to 4 KiB, and the write address wraps inside it. A late rearm cannot run past that buffer before the fault path stops the stream. One block is 128 PCM frames, 2.667 ms. The pool holds 64 ms. A missing block, a missed rearm, a DMA bus error, or a stalled PIO RX FIFO stops capture, increments faults, and invalidates an in-progress RAM recording. The shell remains available. Reboot to capture again.

Conversion:

1. CIC4, decimation 16, as the exact 61-tap convolution through byte lookup tables. Core 1 runs this on the incoming block.
2. 31-tap halfband FIR, decimation 2, to 96 kHz. Core 0.
3. 63-tap halfband FIR, decimation 2, to 48 kHz. Core 0.
4. Three-tap CIC compensation at 48 kHz. Core 0.

Core 1 is a bare RP2350 worker started through the ROM FIFO handshake. It does not run Zephyr, take interrupts, or touch DMA or USB. Queued beamforming and GCC-PHAT windows also run there. Core 0 owns scheduling, the FIR stages, noise reduction, USB, and DMA block lifetime. The two cores overlap by one block, which adds 2.667 ms. The raw block stays owned until both cores finish. Hot filter code is in SRAM.

Capture discards the first 48 blocks, 128 ms, while the microphones and the CIC settle. Each later block is used as follows.

• Direction estimation reads the raw four-channel PCM, before the beamformer. It is off at boot. An analysis frame is four 2048-point windows, 8192 frames (170.7 ms). Manual beamforming skips the next frame. Rejected frames carry no angle, and the estimate does not move the beam. See onboard direction.
• The beamformer runs on core 1. Off passes the four channels through. Broadside sums the face-normal direction. Manual applies a 21-tap fractional delay on the 25 mm cross below, with a common delay of 14 samples and a 20 ms crossfade. Changing the beam clears the learned noise profile. See manual beamforming.
• Noise reduction, when enabled, runs on core 0 after the beamformer and copies its mono result into all four UAC slots. Off leaves the four channels distinct. Filter mode is an 80 Hz high-pass and an optional 50 or 60 Hz notch on one selected channel. Suppress mode is a 20 Hz high-pass, the same notch, and learned stationary-noise suppression on the 129 bins of a 256-point FFT. Suppress adds one 128-sample hop, another 2.667 ms. Learning takes two seconds; keep the sound you want to preserve out of that interval. See noise reduction.

UAC2 uses a 2048-frame ring and holds a 32 ms reserve (1536 frames) before streaming. Full-speed packets are 47, 48, or 49 frames so the device follows the host clock. UAC carries the beamformed and noise-reduced stream. CDC snapshots are the PCM from before the beamformer.

nr config off|filter|suppress [hum=0|50|60] [channel=1..4]
nr learn
nr stats
bf config off|broadside|manual <azimuth-deg> [elevation-deg=0]
bf stats
doa config on|off
doa stats

Close UAC before nr config or uacpattern. Beam changes are accepted during a UAC recording. bf stats prints mode, tenth-degree azimuth and elevation, revision, pending and fade counts, and each channel's delay in milliseconds.

Array geometry

Sound ports sit on a 25 mm cross. In the exposed-port view, USB-C upper right:

Channel	Microphone	GPIO	X, Y (mm)	Position
1	U1	GP7	(0, 25)	top
2	U2	GP8	(−25, 0)	left
3	U3	GP9	(0, −25)	bottom
4	U4	GP10	(25, 0)	right

Azimuth is 0° right, 90° top, 180° left, and 270° bottom. Elevation 0 lies in the board plane; 90 points straight out of the port face and is the broadside direction. Elevation is an assumed steering angle. The same frame is used by the beamformer, the direction estimator, and mic_app. Coordinates and the diagram are in array geometry, array-geometry.svg, and array-geometry.json.

python tools/extract_geometry.py

Wiring

GPIO numbers are not QFN package pins. Source: src/board_pins.h and the EasyEDA Pro hardware project.

Net	GPIO	U5 QFN-60 pin	Goes to
SCLK	GP6	9	U1–U4 CLOCK
DATA1	GP7	10	U1, PCM channel 1
DATA2	GP8	12	U2, PCM channel 2
DATA3	GP9	13	U3, PCM channel 3
DATA4	GP10	14	U4, PCM channel 4
DIN	GP11	15	U7 pin 3, external amplifier
BCLK	GP12	16	U7 pin 4
LRC	GP13	17	U7 pin 5

U7 pins 1 and 2 are VBUS and GND for that amplifier. U6 is the ME6217C33M5G 3.3 V regulator. USB D−/D+ connect to U5 package pins 51 and 52. The crystal is 12 MHz. GP0–GP5 are free; the overlay disables uart0, i2c0, i2c1, and adc.

Flash is the in-package 2 MiB. Firmware occupies 0x000000–0x1FDFFF. The audio-settings partition is 0x1FE000–0x1FFFFF.

Build and flash

Zephyr, its modules, and the SDK stay in zephyrproject/ inside this repository. setup.ps1 and setup.sh do the same work: check out Zephyr 27150c9dd348690a6b41d3992dd5d3d1458fdc66, update the modules named by that manifest, apply patches/zephyr-udc-rpi-pico.patch, and install Zephyr SDK 1.0.1 with the Arm toolchain. The patch acknowledges USB start-of-frame and keeps a queued control transfer from being reported as an endpoint error. zephyrproject/ is gitignored. The image is build/zephyr/zephyr.uf2, and the CMake cache stays in build/.cache.

Windows

Install Git, Python 3, CMake, Ninja, and 7-Zip. Then install west:

python -m pip install west

setup.ps1 downloads zephyr-sdk-1.0.1_windows-x86_64_minimal.7z and runs setup.cmd /t arm-zephyr-eabi /h /c.

.\setup.ps1
.\build.ps1
.\build.ps1 -Pristine

Hold BOOTSEL while connecting the board, then copy the UF2 to the mass-storage volume that appears. Copy it only to that volume. Once this firmware is running, boot in the shell restarts into BOOTSEL with mass storage and picoboot both enabled. The serial ports disconnect until the new image enumerates.

python tools/flash.py <shell-port>

The script checks the UF2 header, sends boot, and copies onto the single drive whose INFO_UF2.TXT contains Board-ID: RP2350. More than one such drive stops the script. It then waits for the same USB serial number on the shell interface, Windows MI_00. The COM number may change.

Ubuntu

Install the host packages, then west. ~/.local/bin has to be on PATH.

sudo apt install --no-install-recommends git cmake ninja-build gperf ccache dfu-util device-tree-compiler wget curl python3-dev python3-pip python3-setuptools python3-wheel xz-utils file make gcc g++
python3 -m pip install --user west
bash setup.sh
bash build.sh
bash build.sh --pristine

setup.sh reads uname -m and downloads the matching minimal archive, zephyr-sdk-1.0.1_linux-x86_64_minimal.tar.xz or zephyr-sdk-1.0.1_linux-aarch64_minimal.tar.xz. It then runs zephyrproject/zephyr-sdk-1.0.1/setup.sh -t arm-zephyr-eabi -h -c.

Hold BOOTSEL while connecting the board and copy build/zephyr/zephyr.uf2 onto the RP2350 mass-storage volume. tools/flash.py is the Windows helper. The shell is a CDC ACM device; host tools that take a COM port take the corresponding /dev/ttyACM* node instead.

Shell and recording

CDC ignores the baud rate; 115200 is a fine terminal setting. Capture starts without a terminal. After warmup, mic stats shows ready=1, an increasing blocks count, and dropped=0 faults=0. max_process_us stays within the 2.667 ms block. Peaks are the latest block's per-channel absolute PCM16 values, not SPL. Close the terminal before a host script opens the port.

mic stats
mic record 1000
mic status
mic dump
mic raw
uacstats
uacpattern on
doa capture
doa dump

mic record accepts 1..333 ms, up to 16,000 frames in the shared diagnostic buffer. The host tool records, then reads the hex dump, and checks the header and length. The text transfer has no extra checksum. A second record is rejected while one is active. A dropped block marks that take INVALID.

python -m pip install pyserial
python tools/record.py <shell-port> capture.wav --ms 250
python tools/raw_pdm.py <shell-port> --output build-tests/raw-pdm

tools/raw_pdm.py counts GP6 edges, exports 10.67 ms of all four PDM lanes plus the firmware PCM, and rebuilds PCM with a separate bitwise CIC and integer FIR. uacpattern replaces the microphones with a diagnostic ramp; turn it off before treating a recording as microphone audio. doa capture holds one raw frame for doa dump.

Continuous four-channel recording uses UAC2:

python tools/uac_wasapi.py --list
python tools/uac_wasapi.py build-tests/continuous-48k.wav --seconds 60

The WAV is exclusive-mode PCM16 at 48 kHz. A JSON sidecar records Windows packet flags and position gaps. An unfinished recording is left as .partial.wav.

Host checks, without the board:

New-Item -ItemType Directory -Force build-tests | Out-Null
gcc -std=c11 -O2 -Wall -Wextra -Werror -Isrc tests/filter_test.c src/pdm_filter.c -lm -o build-tests/filter_test.exe
.\build-tests\filter_test.exe
python -m unittest discover -s tests -p "test_*.py" -v

The C test compares the lookup CIC with a scalar reference, including overflow and block edges, then checks channel isolation, polarity, DC gain, and a synthetic sigma-delta sine. The Python tests cover the host tools. They do not measure pin timing or on-device throughput.

Desktop viewer

mic_app is a Tauri, Vue, and TypeScript application. Build it with npm run desktop:build inside mic_app; the resulting mic_app/release/mic_app.exe is not part of the source tree. It opens Mic Array Telemetry only. From there you can enable direction estimation, choose Off, Broadside, or Manual, set azimuth and assumed elevation, apply the beam, and relearn noise. Save & restart writes the audio record and reconnects. Simulation draws the same interface and does not open a port. Audacity or tools/uac_wasapi.py can record UAC while the app is connected. Leave the shell port closed while the app is using the board.

Microphone: Infineon IM72D128. Chip: Raspberry Pi RP2350.

License

The firmware, host tools, desktop application, hardware drawings, and documentation in this repository are under the MIT license. See LICENSE.

Zephyr and the Zephyr SDK are Apache-2.0. The setup scripts download them into zephyrproject/, which is not part of this repository. patches/zephyr-udc-rpi-pico.patch modifies Zephyr's RP2350 USB device driver; that file remains under the Apache-2.0 terms. The Pico SDK used by sdk-benchmark is the workspace's hal_rpi_pico module (BSD-3-Clause). Packages installed for mic_app keep their own licenses.