STM32H743 USB Mass Storage Device

A minimal, no-std Rust firmware that turns an STM32H743 microcontroller into a USB mass storage device, exposing an SD card as a standard USB drive.

Originally developed for the Copper flight control framework to retrieve telemetry logs from flight controllers, but useful for any STM32H743-based project needing USB storage access.

Features

• Plug-and-play — Works with any OS (Windows, Linux, macOS) without drivers
• Full SCSI support — Implements the complete USB Mass Storage Class (Bulk-Only Transport) protocol
• 4-bit SDMMC — Fast SD card access via hardware SDMMC1 peripheral
• Minimal footprint — ~5KB RAM, optimized for size with LTO
• Safe Rust — No heap allocation, memory-safe embedded code

Hardware Requirements

Component	Details
MCU	STM32H743 (tested on MicoAir H743 flight controller)
USB	OTG2_HS pins: PA11 (DM), PA12 (DP)
SD Card	SDMMC1: PC12 (CLK), PD2 (CMD), PC8-PC11 (D0-D3)
LED	PE6 (optional, blinks during operation)

Building

Prerequisites

# Install Rust embedded toolchain
rustup target add thumbv7em-none-eabihf

# Install probe-rs for flashing (optional)
cargo install probe-rs-tools

# Install just command runner (optional)
cargo install just

Compile

cargo build --release

Flash

Using probe-rs (SWD debugger):

cargo run --release

Using DFU bootloader:

just dfu
# or manually:
llvm-objcopy -O binary target/thumbv7em-none-eabihf/release/sdcard-usb-msc sdcard-usb-msc.bin
dfu-util -a 0 -s 0x08000000:leave -D sdcard-usb-msc.bin

Usage

1. Insert an SD card into the board
2. Flash the firmware
3. Connect USB to your computer
4. The SD card appears as a removable drive

The device identifies as:

• Vendor: COPPER
• Product: SD Card Reader
• VID:PID: 1209:0001

Use Case: Copper-rs Log Retrieval

When running Copper on a flight controller, telemetry and flight logs are stored on the SD card. This firmware provides a simple way to extract those logs:

1. Power down the flight controller
2. Flash this USB MSC firmware
3. Connect to your computer and copy the log files
4. Re-flash your flight control firmware

No need for SD card adapters or removing the card from the board.

Adapting to Your Hardware

To use with a different STM32H743 board, modify:

1. Pin assignments in main.rs:

   • SDMMC1 pins (CLK, CMD, D0-D3)
   • USB OTG pins
   • LED pin (optional)

2. Memory layout in build.rs if your board has different flash/RAM:

   FLASH : ORIGIN = 0x08000000, LENGTH = 2M
   RAM   : ORIGIN = 0x24000000, LENGTH = 512K

3. Clock configuration if using different oscillator setup

Technical Details

• USB Mode: Full-Speed (12 Mbps) via internal PHY
• SD Mode: 4-bit SDMMC at 25 MHz
• Block Size: 512 bytes
• Supported Commands: TestUnitReady, Inquiry, ReadCapacity10/16, RequestSense, ModeSense6/10, Read, Write, ReadFormatCapacities

Dependencies

Crate	Purpose
stm32h7xx-hal	Hardware abstraction layer
usb-device	USB device framework
usbd-storage	Mass Storage Class implementation
synopsys-usb-otg	USB OTG controller driver
defmt + defmt-rtt	Debug logging via RTT

License

MIT OR Apache-2.0