F3-Sensor Hub

This project is an embedded Rust firmware for the STM32F3 Discovery board. It continuously samples the on-board accelerometer, gyroscope, and magnetometer sensors and streams timestamped measurements over USB using either ASCII or binary encoding. Additionally, it generates a 20 usec pulse in GPIO A0 every 10ms (48000 ticks) as a reference.

The firmware is intended for high-rate data logging, robotics, SLAM, and filter development (EKF, complementary filter, AHRS, etc.).

Features

✔ High-rate IMU Sampling

• Accelerometer: 400 Hz
• Gyroscope: 400 Hz
• Magnetometer: 10 Hz
• All rates configurable at compile time.

✔ 48 MHz Timestamping

Samples are timestamped using a 32-bit cycle counter derived from the 48 MHz system clock, providing:

• Sub-microsecond resolution
• Monotonic timing
• No drift relative to sensor sampling tasks

✔ USB Streaming

The device appears to the host as a USB CDC (virtual serial) device. All samples are pushed over USB as soon as they are collected.

You can choose between:

• ASCII mode — Human-readable CSV lines
• Binary mode — Compact and bandwidth-efficient 11-byte packets

Sample Format

Binary Format (11 bytes)

Each sample is represented as:

[0]     kind (u8)
[1..5]  timestamp (u32, little-endian)
[5..11] x, y, z components (3 × u16, little-endian)

Where:

Sensor	kind
Magnetometer	MAGNETOMETER (e.g. 0x01)
Accelerometer	ACCELEROMETER (e.g. 0x02)
Gyroscope	GYROSCOPE (e.g. 0x03)

Rust implementation:

impl RawSample3D {
    pub fn to_bytes(&self) -> [u8; 11] {
        let mut buf = [0u8; 11];

        let (kind, ts, data) = match *self {
            RawSample3D::Magnetometer(ts, d) => (MAGNETOMETER, ts, d),
            RawSample3D::Accelerometer(ts, d) => (ACCELEROMETER, ts, d),
            RawSample3D::Gyroscope(ts, d) => (GYROSCOPE, ts, d),
        };

        buf[0] = kind;
        buf[1..5].copy_from_slice(&ts.to_le_bytes());
        buf[5..11].copy_from_slice(&helpers::u16x3_to_bytes(data));

        buf
    }
}

ASCII Format

The ASCII representation produces a CSV string per sample:

A,12345678,-750,1023,15001
G,12345692,-5,2,0
M,12345710,120,32,900

Rust implementation:

pub fn to_ascii(&self) -> String<64> {
    let mut buf: String<64> = String::new();

    let (tag, ts, data) = match *self {
        RawSample3D::Magnetometer(ts, d) => ('M', ts, d),
        RawSample3D::Accelerometer(ts, d) => ('A', ts, d),
        RawSample3D::Gyroscope(ts, d) => ('G', ts, d),
    };

    write!(buf, "{},{}", tag, ts).ok();
    for x in data {
        write!(buf, ",{}", x).ok();
    }

    buf
}

Building and Flashing

You need:

• rustup
• thumbv7em-none-eabihf target

Example:

rustup target add thumbv7em-none-eabihf
cargo build --release

TODOs

• Clean code. Currently it is a mess.
• Samples are currently written to a buffer during the ISR. I think this is likely the cause of high jitter (40-60 usec). Investigate alternative ways, such as writing samples in main loop provided I can guarantee that sample timestamp remains acurate, or maybe using DMA.