Understanding the MPU Clock Requirements on STM32G4

A detailed guide on clock selection and configuration for MPU operation

stm32 microcontroller board oscilloscope setup

Key Highlights

System Clock Dependency: The MPU does not require a dedicated clock source—it relies on a properly configured and stable system clock.
Clock Sources Options: Designers can select from internal sources like HSI or external sources such as HSE combined with PLL for desired performance.
Integration with Clock Setup: MPU functionality is contingent on overall clock setup, including RCC, PLL configuration, and ensuring peripheral clocks are enabled.

Introduction

In the STM32G4 series microcontrollers, the Memory Protection Unit (MPU) is part of the ARM Cortex-M4 core. Its function is to provide memory region protection by restricting access based on user-configured attributes rather than requiring a dedicated clock source. Instead, the MPU operates in conjunction with the overall system clock configuration which is managed by the Reset and Clock Control (RCC) and, when applicable, the Phase-Locked Loop (PLL).

Clock Configuration for STM32G4

Overview of Clock Sources

The STM32G4 series offers multiple clock sources, such as:

HSI (High-Speed Internal Oscillator): An internally generated signal that offers reasonable stability and frequency precision for many applications.
HSE (High-Speed External Oscillator): Typically an external crystal or resonator providing higher accuracy and stability. This source is preferred for applications where precision timing is critical.
LSI and LSE: While mainly used for low-power or backup operations, these clocks contribute to aspects like low-speed peripherals and real time clock (RTC) modules.
PLL (Phase-Locked Loop): The PLL can multiply a reference clock (from either HSI or HSE) to achieve higher system clock frequencies. It is central to generating the correct operating frequency when the internal oscillators by themselves do not meet the required performance targets.

Operationally, the choice between HSI and HSE is determined primarily by the design requirements regarding overall system performance and clock accuracy. Regardless of the choice, ensuring a stable and correctly configured system clock is critical for the integral functioning of the MPU.

System Clock and MPU Relationship

The MPU’s operation is not conditioned by a separate clock enable directive. Instead, it utilizes the system clock that is generated and distributed throughout the MCU. In the STM32G4 series, the appropriate system clock can be derived from any of the available sources, provided that:

A reliable clock source is chosen and activated. This could be either HSI or HSE, whichever meets the precision and frequency requirements of your application.
If higher clock frequencies are necessary, the chosen clock source may be augmented using the PLL. The PLL then produces the final system clock frequency that drives the entire device.
Once the system clock is running reliably, peripheral clocks and specific modules like the MPU will operate in synchrony with it.

It is important to note that there is no one “magic” clock bit that should be enabled for MPU operations alone. Instead, the overall clock configuration process, involving the selection and stabilization of the desired clock source, indirectly ensures that the MPU is functional.

Detailed Clock Setup Process for STM32G4

Step 1: Selecting the Clock Source

When configuring a system for proper MPU functionality, begin by choosing a clock source that is robust and meets the performance criteria.

Internal versus External Oscillators

Internal Oscillators (HSI): These are built into the microcontroller, allow for quick startup, and generally provide sufficient precision for most applications. They are easier to implement because they do not require external components.
External Oscillators (HSE): These are used when a higher degree of accuracy or stability is required. HSE is particularly beneficial in systems where timing precision is critical and may be used as the reference for PLL to generate high frequencies.

Step 2: Enabling the Clock in the RCC

The Reset and Clock Control (RCC) subsystem in the STM32G4 microcontrollers orchestrates all clock operations. Key registers are made available to control and monitor the state of various oscillators. For example, enabling the HSE oscillator involves configuring the RCC control register, and ensuring that the oscillator stabilizes before proceeding to further system clock configuration.

Enabling the HSE Clock

In typical applications, the HSE clock is enabled by setting specific bits in the RCC control register, such as the bit corresponding to HSE enable. Once set, the system waits until the ready flag confirms that the oscillator has achieved stability. This process is often automated or guided by development tools such as STM32CubeMX.

Step 3: Configuring and Enabling the PLL

For applications needing high operating frequencies, the PLL is configured after the clock source stabilization. The PLL takes the input from the chosen oscillator and multiplies it to yield the desired output frequency. This process involves configuring various parameters such as PLLM (divider), PLLN (multiplier), and PLLP (post-divider) to generate reliable system clock frequencies.

Example PLL Configuration

Suppose the external HSE clock runs at 8 MHz and your application design requires a 168 MHz system clock. The PLL can be tuned by adjusting the parameters to achieve this frequency. These multipliers and dividers must adhere to the limits specified in the STM32G4 reference manual.

Step 4: MPU Configuration in the System Context

After successfully enabling and stabilizing the system clock using the selected clock source and PLL configuration, the MPU may be configured. The MPU setup includes defining memory regions, setting access permissions, and configuring control registers specific to the Cortex-M4 derived MPU. This step ensures that system memory is organized and protected according to your application’s requirements.

Practical Considerations

It is essential to understand that enabling the MPU does not involve a unique clock enable command for the MPU itself. Instead, it is critical that the overall system clock (derived from HSI, HSE, or a combination using the PLL) is stable before enabling the MPU through the appropriate system control registers. The MPU shares the system clock, and its performance mirrors that of your clock configuration.

Integrating a Comprehensive Clock and MPU Configuration Table

Configuration Aspect	Description	Notes
Clock Source Selection	Choosing between HSI (internal) and HSE (external) based on precision required.	HSI is easier to implement; HSE offers better stability for high precision.
RCC Configuration	Enabling the selected clock in the Reset and Clock Control module.	Use STM32CubeMX or direct register configuration; check oscillator stabilization.
PLL Setup	Multiplying the clock input from HSI or HSE to achieve desired system frequencies.	Configure PLLM, PLLN, PLLP parameters carefully to meet timing specifications.
System Clock Implementation	Ensuring that the final, stabilized system clock is used across all system modules.	All peripherals and the MPU depend on a reliable system clock.
MPU Activation	Configure MPU regions and permissions after a stable system clock is ensured.	No dedicated clock for the MPU; it inherits the system clock’s stability.

Further Technical Insights and Recommendations

Generic MPU Configuration Steps

When working with an MPU on the STM32G4, the following steps secure both a proper clock setup and reliable memory protection:

Initialize the selected clock source: Based on your application needs, enable either HSI or HSE. External oscillators (HSE) are generally preferred for higher frequency and better accuracy.
Wait for Clock Stabilization: Utilize RCC status registers to confirm the clock source has reached a stable state. This is crucial before enabling the PLL.
Configure the PLL: If amplifying the clock frequency, adjust the PLL parameters to achieve your target system clock. Maintain adherence to the specifications defined in the reference manual.
Activate the Clock System: Ensure that the complete system clock configuration is successful. Associate peripheral clocks accordingly.
Enable and Configure the MPU: Now, proceed with MPU configuration by setting up memory regions and access control registers. This ensures the security features are implemented only after the clock system is stable.

Addressing Common Misconceptions

A common misunderstanding is that the MPU must have a dedicated clock source activated separately. In reality, since the MPU is an integral part of the Cortex-M4 core, it relies entirely on the system clock. Therefore, there is no additional step for “MPU clock enablement” outside of the overall system clock configuration. The focus should be on ensuring:

That the chosen clock source (HSI or HSE) meets the performance targets.
That the PLL, if utilized, is configured with the correct multiplication and division ratios.
That the RCC properly distributes a stable system clock to all modules of the MCU.

In summary, while there may seem to be a need for a separate clock enabling mechanism for the MPU, it ultimately depends on the integrity of the overall clock system, which is defined by the suitable selection and configuration of the source clock (HSI/HSE) and the PLL in the case of higher frequency requirements.

Practical Implementation

Developers can use tools such as STM32CubeMX to visually initialize and configure the system clock parameters. These tools simplify the steps needed to:

Select a source clock,
Configure the RCC, PLL, and peripheral clocks,
And then enable the MPU using standard software libraries or direct register manipulation.

By ensuring that your clock configuration is robust, you set up a reliable environment for the MPU to work without issues. This integrated approach minimizes potential pitfalls related to clock instability or misconfiguration.