Fixing SDMMC1 Clock On STM32L431: A Simple Solution

Introduction

Are you encountering clock issues with the SDMMC1 on your STM32L431? You're not alone! This article dives into a specific problem encountered by a user while trying to get the SD card interface working on an STM32L431RCT6 board. We'll explore the issue, the debugging process, and a straightforward solution that might just save your day. Whether you're a seasoned embedded developer or just starting, understanding these nuances can significantly improve your troubleshooting skills. Let’s delve into the specifics of the STM32L431 SDMMC1 clock issue and how to resolve it.

The Initial Problem: SD Card Not Working

The user, new to STM HAL (Hardware Abstraction Layer), aimed to create an application for dumping SPI flashes to SD cards. Using a board with an STM32L431RCT6 from VCC-GND-Studio, they started with the CardInfo example, a common starting point for SD card interfacing. However, the example failed to work on the L431 board, although it worked perfectly on F405 and L151 boards. This discrepancy pointed to a hardware-specific or configuration issue on the L431. Initial attempts to run the application highlighted a critical problem: the SD card interface was not being properly initialized, preventing the program from accessing the SD card. This is a common issue that can stem from various factors, including incorrect clock configurations, improper pin assignments, or even hardware defects. The first step in diagnosing such problems is to systematically check each component involved in the SD card interface.

Diving Deeper: Debugging the Clock Issue

To diagnose the issue, the user stepped through the code and discovered that HAL_RCCEx_GetPeriphCLKFreq(RCC_PERIPHCLK_SDMMC1) was returning 0 inside the HAL_SD_InitCard() function. This strongly suggested that the clock for the SDMMC1 peripheral was not correctly enabled or configured. The HAL library relies on these clock frequencies to be properly set up for the SD card interface to function. A zero return value indicates that the peripheral clock is either disabled or not correctly initialized, effectively halting any communication with the SD card. The function HAL_RCCEx_GetPeriphCLKFreq is crucial for retrieving the clock frequency of a given peripheral, and a value of 0 is a clear indicator of a misconfiguration. This discovery narrowed the focus to the clock initialization process, particularly within the board support package (BSP) and HAL initialization routines. Understanding the clock tree and how different peripherals are clocked is essential in STM32 development, making this debugging step particularly insightful.

This pointed to a potential problem with __HAL_RCC_SDMMC1_CLK_ENABLE() within the BSP_SD_MspInit() function, which is responsible for the low-level initialization of the SDMMC1 peripheral. The __HAL_RCC_SDMMC1_CLK_ENABLE() macro is intended to enable the clock for the SDMMC1 peripheral, but if it's not configured correctly, the clock might not start, leading to the observed issue. The user's systematic approach to debugging, by tracing the code execution and checking return values, is a best practice in embedded systems development. Identifying the specific function call that returns an unexpected value is a critical step in pinpointing the root cause of the problem. This methodical approach allows developers to focus their attention on the relevant sections of code and hardware configurations.

The Solution: Configuring the Peripheral Clock

Recalling previous issues with the TRNG (True Random Number Generator) on the same board, the user remembered a solution involving the HAL_RCCEx_PeriphCLKConfig() function. In that instance, calling HAL_RCCEx_PeriphCLKConfig() before __HAL_RCC_RNG_CLK_ENABLE resolved the TRNG clock issue. The underlying principle here is that some peripherals require specific clock source configurations before their main clock enable is activated. This is because the STM32 microcontrollers have a complex clock tree, allowing different peripherals to be clocked from various sources (e.g., HSI, HSE, PLL). Incorrectly configuring the clock source can prevent the peripheral from functioning correctly, even if the main clock enable is called. The TRNG issue served as a valuable clue, suggesting a similar clock configuration problem might be affecting the SDMMC1 peripheral.

Based on this experience, the user decided to try the same approach for the SDMMC1 clock. Before calling __HAL_RCC_SDMMC1_CLK_ENABLE(), they added a call to HAL_RCCEx_PeriphCLKConfig() within the BSP_SD_MspInit() function. This function allows you to configure the clock source for various peripherals, including SDMMC1. The specific configuration used was to select the PLL (Phase-Locked Loop) as the clock source for SDMMC1. This is a common practice, as the PLL can provide a higher, more stable clock frequency required for SD card communication. The user's intuition, guided by their previous experience with the TRNG, proved to be correct. This highlights the importance of understanding the underlying hardware and clock system when working with embedded systems.

Here's the code snippet the user added:

#if defined(SDMMC1)
  if (hsd->Instance == SDMMC1) {
    RCC_PeriphCLKInitTypeDef PeriphClkInitStruct;
    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_SDMMC1;
    PeriphClkInitStruct.Sdmmc1ClockSelection = RCC_SDMMC1CLKSOURCE_PLL;
    HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);

    __HAL_RCC_SDMMC1_CLK_ENABLE();
  }
#endif

This code first checks if SDMMC1 is defined and if the current SD handle's instance matches SDMMC1. If both conditions are met, it proceeds to configure the peripheral clock. A RCC_PeriphCLKInitTypeDef structure is used to set the clock selection. The PeriphClkInitStruct.PeriphClockSelection is set to RCC_PERIPHCLK_SDMMC1 to specify that the configuration is for SDMMC1. The crucial part is PeriphClkInitStruct.Sdmmc1ClockSelection, which is set to RCC_SDMMC1CLKSOURCE_PLL, selecting the PLL as the clock source. Finally, HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) applies the configuration. After this, the __HAL_RCC_SDMMC1_CLK_ENABLE() macro is called to enable the SDMMC1 clock. This sequence ensures that the clock source is correctly configured before the peripheral clock is enabled, resolving the issue.

The Result: Success!

Adding this configuration step resolved the issue, and the CardInfo example started working correctly on the STM32L431RCT6 board. The HAL_RCCEx_PeriphCLKConfig function call ensured that the SDMMC1 peripheral received the correct clock source before being enabled. This highlights a critical detail in STM32 clock configuration: some peripherals require a specific clock source to be selected before their clock is enabled. Failing to do so can lead to the peripheral not functioning correctly, even if the main clock enable bit is set. The successful resolution demonstrates the importance of understanding the clock tree and peripheral clocking mechanisms in STM32 microcontrollers. This experience also underscores the value of reusing knowledge gained from solving previous issues, as the TRNG clock problem provided a valuable clue for tackling the SDMMC1 clock problem.

Why This Matters: Clock Configuration in STM32

This scenario underscores the importance of proper clock configuration in STM32 microcontrollers. The clock system in STM32 devices is complex, with multiple clock sources and dividers that can be configured to clock various peripherals. Incorrect clock settings can lead to a variety of issues, including peripherals not working, timing errors, and unexpected behavior. Understanding the clock tree and the specific requirements of each peripheral is crucial for successful STM32 development. The RCC (Reset and Clock Control) peripheral in STM32 devices allows developers to configure various aspects of the clock system, including selecting clock sources, setting clock dividers, and enabling/disabling clocks for individual peripherals. The HAL library provides functions like HAL_RCCEx_PeriphCLKConfig to simplify this process, but it's essential to understand the underlying concepts to effectively troubleshoot clock-related issues.

In this case, the SDMMC1 peripheral required the PLL to be explicitly selected as its clock source before the clock was enabled. This is not always obvious from the HAL documentation or example code, making it a common pitfall for developers new to STM32. By sharing this solution, the user has provided valuable insight into a specific clock configuration requirement for the SDMMC1 peripheral on the STM32L431. This kind of practical knowledge, gained through debugging and experimentation, is invaluable in the embedded development community. The lesson here is to always double-check the clock configuration for any peripheral that is not functioning as expected, and to consult the STM32 reference manual for detailed information on the clock system.

Is This a Library Issue or a Board Variant Issue?

The user raised an important question: Is this a library issue or a board variant issue? The fact that the CardInfo example worked on other STM32 boards (F405 and L151) but not on the L431 suggests that the issue is specific to the L431's configuration or the board design. It might be that the board variant definition for the STM32L431 in the STM32duino library does not include the necessary clock configuration for SDMMC1. In such cases, the library might assume a default clock configuration that is not suitable for the specific board. Alternatively, the issue could stem from the board's hardware design. Some boards might have specific clock routing or external components that require a different initialization sequence.

If the clock configuration is missing from the board variant definition, it would be appropriate to add it there. This would ensure that the SDMMC1 peripheral is correctly initialized for all users of that board. However, if the issue is specific to a particular board revision or hardware configuration, a more targeted solution might be necessary. In any case, sharing the solution with the community is a valuable step, as it allows other users to benefit from the experience and potentially contribute to a more robust solution. Ultimately, determining whether the fix belongs in the library or the board variant requires a deeper understanding of the STM32duino library structure and the specific requirements of the STM32L431RCT6 board. Further investigation and discussion within the community can help to arrive at the most appropriate solution.

Conclusion

In conclusion, the user's experience highlights a specific clock configuration issue with the SDMMC1 peripheral on the STM32L431RCT6 board. By systematically debugging the problem and leveraging previous experience with a similar issue, they were able to identify and implement a solution. This involved configuring the PLL as the clock source for SDMMC1 before enabling the peripheral clock. The solution underscores the importance of understanding the STM32 clock system and the specific clocking requirements of different peripherals. While the user's fix works for their project, the broader question of whether this fix belongs in the STM32duino library or the board variant definition remains open for discussion. Sharing this experience with the community is a valuable contribution, as it can help other developers facing similar issues and potentially lead to a more robust and comprehensive solution.

For more information on STM32 clock configuration, you can visit the STMicroelectronics website for detailed documentation.