Ultrasonic Distance Measurement with PWM Feedback (STM32)

This project implements a custom driver for the HC-SR04 ultrasonic distance sensor on an STM32 microcontroller, using direct HAL configuration and timers for precise microsecond timing.
The system measures distance and dynamically adjusts a PWM signal based on the measured value.

Developed in C using STM32 HAL libraries (CubeIDE environment).
Microcontroller family: STM32F0 / STM32F4 (compatible with HAL).

Project Overview

The application measures distance using an HC-SR04 ultrasonic sensor connected to the STM32 board.
The TRIG pin sends a 10 µs pulse to initiate measurement, while the ECHO pin measures the duration of the reflected signal using a timer-based delay.
The time difference is converted into distance (in cm), and a PWM output (from TIM1) is modulated according to the measured distance.

The PWM value is inversely proportional to distance: pwm = 65535 / distance

This allows the PWM signal to change dynamically depending on the proximity of an object.

Hardware Setup

Component	Function	Connection
STM32 MCU	Main controller	Any STM32 board supported by CubeIDE
HC-SR04 Sensor	Distance measurement	TRIG → GPIO Output (PB), ECHO → GPIO Input (PB)
Timer 3 (TIM3)	Microsecond timing	Used for precise echo measurement
Timer 1 (TIM1)	PWM generation	Controls PWM output based on distance
Power Supply	5V for sensor	VCC (5V), GND

Software Description

Main Components

• GPIO: Configured for TRIG (output) and ECHO (input with pulldown)
• TIM3: Used for microsecond timing (Prescaler = 48 → 1 µs tick)
• TIM1: Used for PWM output (Period = 65535)
• Delay_us(): Custom delay function based on TIM3 counter

Measurement Procedure

1. Set TRIG high for 10 µs.
2. Wait for ECHO pin to go high.
3. Record start time (val1) using TIM3 counter.
4. Wait for ECHO pin to go low.
5. Record end time (val2).
6. Compute distance: distance = (val2 - val1) * (0.034 / 2) (0.034 cm/µs is the speed of sound divided by 2 for round-trip distance)
7. Update PWM duty cycle proportionally: pwm = 65535 / distance

🕹️ Example Output Flow (pseudocode)

HAL_GPIO_WritePin(GPIOB, TRIG_Pin, GPIO_PIN_SET);
Delay_us(10);
HAL_GPIO_WritePin(GPIOB, TRIG_Pin, GPIO_PIN_RESET);

while (!(HAL_GPIO_ReadPin(GPIOB, ECHO_Pin)));
val1 = __HAL_TIM_GET_COUNTER(&htim3);

while (HAL_GPIO_ReadPin(GPIOB, ECHO_Pin));
val2 = __HAL_TIM_GET_COUNTER(&htim3);

distance = (val2 - val1) * (0.034 / 2);
pwm = 65535 / distance;
__HAL_TIM_SET_COMPARE(&htim1, TIM_CHANNEL_1, pwm);

Key Features

✅ Custom-built ultrasonic sensor driver using STM32 HAL ✅ High-resolution microsecond timing with Timer 3 ✅ PWM output based on measured distance ✅ Fully functional without external libraries ✅ Suitable for real-time distance-based control systems

Configuration Summary

Peripheral Purpose Key Settings TIM3 Microsecond counter Prescaler = 48, Period = 65535 TIM1 PWM generator Channel 1, Period = 65535 GPIOB TRIG / ECHO pins TRIG = Output, ECHO = Input (Pulldown)

Dependencies

STM32Cube HAL Drivers STM32CubeIDE 1.xx STM32 HAL_TIM and HAL_GPIO modules

Testing

The driver was tested successfully with an HC-SR04 sensor at distances between 2 cm and 200 cm. PWM duty cycle varied linearly with the detected distance. Accuracy was stable within ±1 cm under normal conditions.