APPS Sensor — Dual-Channel Plausibility Check

Accelerator Pedal Position Sensor (APPS) firmware for STM32, implementing a dual-channel plausibility check as required in Formula Student vehicle safety systems.

Simulated on Wokwi

Overview

APPS sensors in Formula Student cars must be redundant — two independent sensors read the pedal position simultaneously. If their readings diverge significantly, it indicates a sensor fault or wiring issue and the system must respond safely.

This firmware reads both ADC channels continuously and compares their values. If the two signals differ by more than 10% for longer than 3 seconds, the system:

1. Prints a WARNING: OUT OF BOUNDS message over UART
2. Pulls the fault output pin (PA6) LOW to signal a fault condition
3. Enters a locked fault state (infinite loop) — preventing further operation

How It Works

ADC CH0 ──┐
           ├──► diff() check ──► >10% for 3s? ──► FAULT (PA6 LOW, halt)
ADC CH1 ──┘

The plausibility function computes divergence as a percentage of the average:

int diff(int a, int b) {
    int diff = abs(a - b);
    int avg  = (a + b) / 2;
    if (avg == 0) return diff != 0;
    return (diff * 10) > avg;  // true if divergence > 10%
}

Hardware

Component	Detail
MCU	STM32 (STM32C0 series)
ADC	ADC1, 12-bit resolution
Sensor inputs	PA0 (CH0), PA1 (CH1) — two potentiometers
Fault output	PA6 — pulled LOW on fault
Simulation	Wokwi

Files

File	Description
main.c	Main firmware — ADC init, plausibility loop, fault handling
main.h	HAL includes and pin definitions
diagram.json	Wokwi circuit diagram
wokwi-project.txt	Wokwi project link
Setup.png	Circuit setup screenshot

Author

Luka Sičić — @Luka-Sicic