STM32 MPU: A Simple Explanation with Code Examples

The Memory Protection Unit (MPU) is a crucial hardware component found in many modern microcontrollers, including the STM32 family. It enhances system robustness and security by enforcing access restrictions to different memory regions. This allows you to isolate critical code and data, preventing unintended modification or access by errant code, thereby increasing the reliability and safety of your embedded system. This article provides a comprehensive guide to the STM32 MPU, explaining its functionality, configuration, and usage with detailed code examples.

Understanding the STM32 MPU

The STM32 MPU operates on the principle of defining memory regions and assigning specific access permissions to each region. These permissions dictate which parts of the system (e.g., the processor, DMA controllers) can read, write, or execute code from a given region. By carefully configuring these regions and permissions, you can create a protected memory environment.

Key Concepts

• Regions: The MPU divides the memory space into multiple regions. Each region has a defined start address, size, and access permissions. The number of available regions varies depending on the specific STM32 microcontroller.
• Subregions: Each region can be further divided into smaller subregions, providing finer-grained control over memory access.
• Access Permissions: These permissions define which operations are allowed on a specific region. Typical permissions include:
  • No Access: Prevents any access to the region.
  • Read-Only: Allows read access but prohibits writing.
  • Read-Write: Permits both reading and writing.
  • Execute: Allows code execution from the region.
• Privileged and Unprivileged Modes: The STM32 operates in either privileged or unprivileged mode. The MPU enforces access permissions differently based on the current mode. Privileged mode typically has full access, while unprivileged mode is restricted by the MPU settings.
• Background Region: A special region that defines the default access permissions for any memory area not explicitly covered by other defined regions.

Configuring the STM32 MPU

The STM32 MPU is configured using the Core Peripheral Access Layer (CPAL) and the HAL (Hardware Abstraction Layer) library. Here’s a step-by-step guide to configuring the MPU:

1. Enable the MPU: The first step is to enable the MPU hardware using the HAL_MPU_Enable() function.

“`c

include “stm32h7xx_hal.h” // Or appropriate header for your STM32 family

void MPU_Config(void)
{
MPU_Region_InitTypeDef MPU_InitStruct;

/ Disable the MPU /
HAL_MPU_Disable();

/ Configure region 0 for flash memory (read-only execution) /
MPU_InitStruct.Enable = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress = FLASH_BASE; // Start address of Flash memory
MPU_InitStruct.Size = MPU_REGION_SIZE_512MB; // Adjust size according to your flash size
MPU_InitStruct.SubRegionDisable = 0x00; // Enable all subregions
MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL0;
MPU_InitStruct.AccessPermission = MPU_REGION_PRIV_RO_URO; // Privileged Read-Only, Unprivileged Read-Only
MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_ENABLE; // Enable execution
MPU_InitStruct.IsShareable = MPU_ACCESS_NOT_SHAREABLE;
MPU_InitStruct.IsCacheable = MPU_ACCESS_CACHEABLE;
MPU_InitStruct.IsBufferable = MPU_ACCESS_NOT_BUFFERABLE;
HAL_MPU_ConfigRegion(&MPU_InitStruct);

/ Configure region 1 for SRAM (read-write) /
MPU_InitStruct.Enable = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress = SRAM1_BASE; // Start address of SRAM
MPU_InitStruct.Size = MPU_REGION_SIZE_128KB; // Adjust size according to your SRAM size
MPU_InitStruct.SubRegionDisable = 0x00;
MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL0;
MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS; // Full access
MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_DISABLE; // Disable execution
MPU_InitStruct.IsShareable = MPU_ACCESS_NOT_SHAREABLE;
MPU_InitStruct.IsCacheable = MPU_ACCESS_CACHEABLE;
MPU_InitStruct.IsBufferable = MPU_ACCESS_BUFFERABLE;
HAL_MPU_ConfigRegion(&MPU_InitStruct);

/ Enable the MPU /
HAL_MPU_Enable(MPU_PRIVILEGED_DEFAULT);
}
“`

1. Define Memory Regions: Use the MPU_Region_InitTypeDef structure to define each memory region. You’ll need to specify the start address, size, and access permissions for each region.

2. Configure Subregions (Optional): If you need finer-grained control, you can configure subregions within each region.

3. Configure the Background Region (Optional): Define the default access permissions for memory areas not covered by explicitly defined regions.

4. Enable the MPU: After configuring the regions, enable the MPU using HAL_MPU_Enable().

Code Examples

Example 1: Protecting Flash Memory as Read-Only

The following code snippet demonstrates how to configure the MPU to protect the Flash memory as read-only, preventing any unintended modifications.

Example 2: Creating a Read-Write Data Region in SRAM

This example shows how to create a read-write data region in SRAM, allowing the processor to access and modify data within this region. It complements the first example, ensuring data manipulation occurs only within designated SRAM areas.

Example 3: Isolating a Peripheral’s Memory Region

This example illustrates how to isolate a peripheral’s memory region. This can be useful for preventing accidental access or modification of peripheral registers by errant code.

Example 4: Using Subregions for Fine-Grained Control

This example demonstrates the use of subregions to define different access permissions within a single region.

Debugging and Troubleshooting

When working with the MPU, it’s important to have a robust debugging strategy. Here are some tips:

• Use a Debugger: A debugger allows you to step through your code and examine the MPU registers to verify that the configuration is correct.
• Check for MPU Violations: The STM32 generates exceptions when an MPU violation occurs. Use your debugger to catch these exceptions and identify the cause of the violation.
• Review the Datasheet: The datasheet for your specific STM32 microcontroller provides detailed information about the MPU, including the number of available regions and the supported access permissions.

Conclusion

The STM32 MPU is a powerful tool for enhancing the security and reliability of embedded systems. By understanding its functionality and utilizing the HAL library, you can configure the MPU to protect critical code and data, preventing unauthorized access and ensuring system stability. The provided code examples offer a starting point for implementing MPU functionality in your STM32 projects, enabling you to build more robust and secure embedded applications. Remember to always consult the datasheet for your specific microcontroller and adapt the examples to your specific needs. Careful planning and testing are crucial for ensuring the effectiveness of your MPU configuration.