Project Overview

Summary

This project implements embedded firmware for an electrochemical gas sensor running on an STM32G431KB microcontroller. The firmware is written in Rust using the RTIC 2.1 real-time framework with the Embassy HAL for async peripheral drivers.

The system reads sensor current via a Transimpedance Amplifier (TIA), digitises the signal with a 12-bit differential ADC, applies two-point digital calibration via the AD5545 multiplying DAC, and reports concentration in ppm over a UART JSON interface.

The key design contribution is replacing mechanical calibration potentiometers with a digitally-controlled analogue conditioning stage (AD5545 MDAC), while keeping the calibrated analogue output fully backward-compatible with the downstream host instrument.

Key Specifications

Design Goals

1. Backward compatibility — the calibrated analogue output must remain hardware-compatible with the existing host gas analyser; software-only correction is not viable.
2. Accuracy — VREFINT-compensated ADC readings, 8× hardware oversampling, and a simultaneous closed-form calibration solution to eliminate DAC coupling error.
3. Reliability — RTIC’s static task model prevents stack overflow and priority inversion; an IWDG watchdog guards against firmware lockup.
4. Digital calibration — zero and span points are stored in flash and applied as analogue offsets via the MDAC, replacing mechanical potentiometers.

Signal Chain

Electrochemical sensor (target analyte, current output ~nA–µA)
    │
    ▼
TIA (Transimpedance Amplifier)           0–2 V voltage
    │
    ▼
AD5545 Dual 16-bit MDAC
    ├── ChA: Gain control   (scales sensitivity)
    └── ChB: Offset cancel  (subtracts sensor baseline in hardware)
    │
    ▼
ADC1 (differential PA0⁺ / PA1⁻, 12-bit, 8× oversampling)
    │
    ▼
STM32G431KB firmware
    ├── Calibration state machine
    ├── PPM conversion
    └── UART JSON output → host instrument

Workspace Structure

fw/
├── bsp/          Board Support Package — ADC, DAC, SHT45, board init
├── protocol/     JSON command codec (serde_json)
└── main-app/     RTIC application — tasks, calibration logic, stability monitor