- Hacker Embedded STM32: Interrupt-Driven ADC to PWM Control

STM32 HAL Tutorial: Using Timer Interrupts for Responsive Analog-to-PWM Applications

Abstract

Learn how to use STM32 timer interrupts to read ADC values and update PWM output efficiently. Step-by-step guide for responsive sensor-to-actuator control using HAL.

1. Introduction

In previous episodes, we combined ADC readings with PWM to control LEDs or servos.
Now, we’ll make the system interrupt-driven, which:

Reduces CPU blocking from polling loops
Provides precise, periodic updates of PWM based on sensor input
Improves responsiveness in multitasking applications

By the end of this episode, you’ll be able to:

Configure TIMER interrupt.
Read ADC values inside its own ISR.

Update PWM output in real-time without blocking the main loop.

2. Prerequisites

STM32 board with ADC sensor (e.g., potentiometer)
PWM output pin connected to LED
STM32CubeIDE installed

Knowledge of ADC, PWM, and timers from Episodes 5, 9, and 10

This image was made using Fritizing

3. Configuring ADC & TIM in CubeMX

Step 1 – Open Project

Create a new project in STM32CubeMX.

Step 2 – Enable ADC Peripheral

Go to Pinout & Configuration.
Select ADC1.
Assign the analog pin (e.g., PA0) for your sensor.
Click ADC1 → Parameter Settings:
- Resolution: 12-bit (0–4095 digital values)
- Data Alignment: Right
- Scan Conversion Mode: Disabled (single channel)
- Continuous Conversion Mode: Disabled
- Trigger the conversion in ISR

Step 3 – Configure Timer with Interrupt

Enable TIM3 (or any general-purpose timer).
Set period to desired ADC sampling interval (e.g., 10 ms for 100 Hz).
Enable Update Interrupt to trigger HAL_TIM_PeriodElapsedCallback().

Step 3 – Configure PWM Channel

TIM3_CH3 → PWM output pin (e.g., PB0).
Set PWM mode and period as in Episode 10.

Step 4 – Configure NVIC Priority

ADC priority level 2
TIM3 priority level 0

Step 5 – Generate Code

Click Project → Generate Code to initialize HAL ADC structures.

4. Writing the ISRs callback

STM32CubeMX generates a callback function as weak, so the user needs to re-create them:

				
					/* USER CODE BEGIN 4 */
void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
	adcValue = HAL_ADC_GetValue(hadc);
}
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
	// Map ADC value to PWM duty cycle
	pwmValue = (adcValue * 9999) / 4095;
	// Update PWM output
	__HAL_TIM_SET_COMPARE(&htim3, TIM_CHANNEL_3, pwmValue);
	HAL_ADC_Start_IT(&hadc1);
}
/* USER CODE END 4 */

Code Explanation

Timer ISR triggers at fixed intervals (e.g., every 10 ms).
ADC conversion happens inside its own ISR.
PWM duty cycle updates immediately, providing smooth control.
Main loop can remain free for other tasks.

5. Hands-On main code

The main loop is non-blocking and free.
PWM output responds to ADC input automatically via interrupt.

				
					/* USER CODE BEGIN PV */
uint32_t adcValue = 0;
uint32_t pwmValue = 0;
/* USER CODE END PV */

				
					 /* USER CODE BEGIN 2 */
 HAL_ADCEx_Calibration_Start(&hadc1);
 HAL_ADC_Start_IT(&hadc1);
 HAL_TIM_Base_Start_IT(&htim3);
 HAL_TIM_PWM_Start(&htim3, TIM_CHANNEL_3);
 /* USER CODE END 2 */

Full Source Code: hackerembedded/STM32_EP11

6. Compiling and Running

Build Project → Click hammer icon.
Flash Project → Connect STM32 and run (Ctrl + F11).
Test PWM Control:
- Rotate potentiometer → LED brightness changes.
- Main loop remains free to handle other tasks (e.g., UART communication, SPI, I²C sensors)

7. Advantages of Interrupt-Driven Approach

Non-blocking operation → MCU can multitask efficiently
Precise timing → PWM updates at exact intervals
Smooth sensor-to-actuator mapping → No lag or flickering
Scalable → Multiple ADC channels and PWM outputs can be handled in same ISR

8. Common Issues & Fixes

Issue	Cause	Solution
LED not responding	Timer interrupt not started	Call HAL_TIM_Base_Start_IT()
ADC reading incorrect	Reading inside the wrong ISR	Adjust HAL_ADC_PollForConversion() timeout
Main loop blocked	ADC or PWM code outside ISR	Keep main loop free; ISR handles ADC→PWM
Main loop blocked	ADC ISR happening too often	Adjust the sampling time and triggering source

STM32: Interrupt-Driven ADC to PWM Control