How to handle multiple interrupts connected on same interrupt line in STM32?

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28-08-2024

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Demystifying Multiple Interrupts on a Single Line in STM32: A Practical Guide

The STM32 microcontroller offers versatile interrupt handling capabilities, but managing multiple interrupt sources sharing the same interrupt line can be tricky. This article will address the common challenges and provide a clear approach to handling such scenarios effectively.

The Challenge: STM32 microcontrollers often multiplex multiple GPIO pins onto a single EXTI line, making it difficult to determine the source of an interrupt. This is particularly relevant when you have several sensors or devices connected to the same interrupt line, as in the example raised by Sachin Sharma.

The Solution: Fortunately, STM32 provides mechanisms to overcome this challenge. Let's break down the key concepts:

1. Distinguishing Interrupt Sources:

Question: How can I identify which GPIO pin triggered the interrupt when multiple pins share an EXTI line?

Answer: The EXTI_Line_TypeDef structure and associated macros are your friends! This structure allows you to map the interrupt lines to their corresponding GPIO pins.

Here's a breakdown:

1. EXTI Line Structure: The EXTI_Line_TypeDef defines the interrupt line numbers (e.g., EXTI_Line0, EXTI_Line1, etc.)

2. GPIO Pin Mapping: Each interrupt line corresponds to a specific GPIO pin or a group of pins. The GPIO_TypeDef structure defines the GPIO pins (e.g., GPIOA, GPIOB, etc.), and the EXTI_Line_TypeDef structure provides the mapping. For instance, EXTI_Line0 might be mapped to GPIOA_Pin0, GPIOB_Pin0, or even multiple pins depending on your configuration.

3. Utilizing the EXTI_Line_TypeDef: When an interrupt occurs on the shared line, the EXTI_Line_TypeDef structure will help you identify the specific GPIO pin that caused it. By examining the EXTI_Line structure, you can pinpoint the exact source of the interrupt.

Example: Imagine you have two buttons connected to GPIOA_Pin0 and GPIOB_Pin0, both sharing the same EXTI0 line.

// Assuming you have configured EXTI0 interrupt correctly
void EXTI0_IRQHandler(void) {
  // Check which pin triggered the interrupt
  if (HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_0) == GPIO_PIN_RESET) {
    // Interrupt triggered by GPIOA_Pin0
    // Perform specific actions for this interrupt source
  } else if (HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_0) == GPIO_PIN_RESET) {
    // Interrupt triggered by GPIOB_Pin0
    // Perform specific actions for this interrupt source
  }
}

This code checks the state of the connected GPIO pins to determine which triggered the interrupt.

2. Handling Multiple Interrupts on a Single Line:

Question: What if I want to ensure that only one interrupt source triggers at a time?

Answer: This requires a slightly different approach and involves controlling the interrupt sources:

1. Software-Based Control: You can use software to disable interrupts on all but one source before enabling the interrupt on the desired source. This method involves managing the EXTI_Line structure and the EXTI_Line_TypeDef structure.

2. Hardware-Based Control: In some cases, you can use external hardware logic, like a priority encoder or a multiplexer, to filter the interrupt sources and ensure that only one interrupt is sent to the microcontroller at a time.

Example: Consider having two push buttons connected to the same interrupt line, but only wanting to process one button press at a time.

void Button1_PressHandler(void) {
  // Disable interrupt on Button2
  HAL_GPIO_EXTI_Callback(GPIO_PIN_1, DISABLE);
  // Perform actions for Button1 press
  // Re-enable Button2 interrupt after handling Button1
  HAL_GPIO_EXTI_Callback(GPIO_PIN_1, ENABLE);
}

void Button2_PressHandler(void) {
  // Disable interrupt on Button1
  HAL_GPIO_EXTI_Callback(GPIO_PIN_0, DISABLE);
  // Perform actions for Button2 press
  // Re-enable Button1 interrupt after handling Button2
  HAL_GPIO_EXTI_Callback(GPIO_PIN_0, ENABLE);
}

This code disables the interrupt on the other button while handling the active button press, effectively ensuring only one button triggers at a time.

Key Considerations:

• Latency: Software-based control can introduce a small delay in handling interrupts. If you require fast and deterministic interrupt response, hardware-based solutions might be more suitable.
• Synchronization: Proper synchronization between interrupt handlers is essential to avoid race conditions when managing multiple interrupt sources.

Going Beyond the Basics:

• Priority Levels: You can assign priority levels to different interrupt sources to handle higher priority events more promptly.
• Real-Time Operating Systems (RTOS): RTOS provide more sophisticated mechanisms for interrupt handling, especially when dealing with multiple interrupts and complex applications.

Remember: The specific configuration and approach will vary depending on the STM32 microcontroller series and the application requirements. Consult the STM32 documentation for detailed instructions on managing interrupts.

This article is an extension of the question asked by Sachin Sharma on Stack Overflow. We encourage you to explore the original thread and other related resources for more detailed information and discussions.

By understanding the underlying principles and applying appropriate techniques, you can confidently manage multiple interrupt sources sharing the same interrupt line in your STM32 projects.