dfsdm-audio

Real-time MEMS microphone audio capture on the STM32H725, streamed over UART and visualised on a host PC.

Hardware

Item	Value
MCU	STM32H725 (Cortex-M7, 550 MHz)
Microphone	IM69D120 (PDM, 1.8 V I/O)
Interface	DFSDM (digital filter), DMA → UART4
Baud rate	921 600
Sample rate	32 kHz (DFSDM clock 3.2 MHz / OSR 50 / IntOSR 2)
Sample format	int16 little-endian, mono

Wire format per DMA half/full callback:

[0xAA 0x55 0xAA 0x55]  4-byte sync word
[1024 × int16 LE]      2048 bytes of audio samples

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1 — Firmware (STM32H725)

Prerequisites

Tool	Version	Notes
ARM GCC	10.3-2021.10	bundled in toolchain/ (Linux) and toolchain_win/ (Windows)
CMake	≥ 3.22
Ninja	any

The bundled toolchains are excluded from git (.gitignore). Download from developer.arm.com and unpack into toolchain/ (Linux) or toolchain_win/ (Windows) at the repo root.

Expected layout:

toolchain/
  bin/
    arm-none-eabi-gcc
    arm-none-eabi-objcopy
    ...

Build

# Add the toolchain to PATH for this shell session
export PATH="$(pwd)/toolchain/bin:$PATH"

# Configure (generates build/Debug/)
cmake --preset Debug

# Compile (36 translation units, ~4 seconds)
cmake --build --preset Debug

Outputs in build/Debug/:

File	Description
dfsdm-audio.elf	ELF with full debug info (for GDB / STM32CubeIDE)
dfsdm-audio.hex	Intel HEX — flash this to the board

Release build (size-optimised, -Os):

cmake --preset Release && cmake --build --preset Release

Flash

Use STM32CubeProgrammer (GUI or CLI):

STM32_Programmer_CLI -c port=SWD -w build/Debug/dfsdm-audio.hex -v -rst

Or drag-and-drop the .hex onto the ST-LINK virtual mass-storage drive if your board supports it.

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2 — Host visualiser (tools/dfsdm-viz)

A Rust/egui desktop app that connects to the board over UART and shows:

• Live scrolling waveform — Raw (cyan) and/or HPF-filtered (amber)
• WAV recording with timestamped filenames
• Signal toolbox: Welch PSD, STFT spectrogram, statistics, autocorrelation, RMS envelope + ZCR

Prerequisites

Tool	Notes
Docker	used for cross-compilation; no local Rust install required

Build (cross-compile for Linux + Windows)

cd tools/dfsdm-viz
./build_cross.sh

The script builds a Docker image on first run (~30 s), then compiles both targets in release mode.

Outputs:

File	Platform
target/x86_64-unknown-linux-gnu/release/dfsdm-viz	Linux x86-64
target/x86_64-pc-windows-gnu/release/dfsdm-viz.exe	Windows x86-64

Run

Linux:

./target/x86_64-unknown-linux-gnu/release/dfsdm-viz

Windows — double-click dfsdm-viz.exe or run from a terminal:

dfsdm-viz.exe

The port can be selected from the toolbar ComboBox after launch, or passed on the command line:

dfsdm-viz --port /dev/ttyUSB0          # Linux
dfsdm-viz.exe --port COM3              # Windows

All CLI options:

Options:
  -p, --port <PORT>              Serial port (optional; pick in GUI if omitted)
  -b, --baud <BAUD>              Baud rate [default: 921600]
  -s, --sample-rate <HZ>         Audio sample rate [default: 32000]
      --width <PX>               Window width  [default: 1400]
      --height <PX>              Window height [default: 650]

Local dev build (requires libudev-dev on Linux)

cd tools/dfsdm-viz

# Ubuntu / Debian
sudo apt install libudev-dev libdbus-1-dev

cargo build          # debug
cargo build --release