RCC internal peripheral

Applicable for

STM32MP13x lines, STM32MP15x lines, Unknown MPU

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1. Article purpose

The purpose of this article is to:

• briefly introduce the RCC peripheral and its main features,
• indicate the peripheral instances assignment at boot time and their assignment at runtime (including whether instances can be allocated to secure contexts),
• list the software frameworks and drivers managing the peripheral,
• explain how to configure the peripheral.

2. Peripheral overview

The RCC peripheral is used to manage the clock and reset generation for the complete circuit. The RCC gets several internal and external oscillators. They are used as clock sources for the hardware blocks, either directly or indirectly, via the PLLs that allow to achieve high frequencies.

The different features of RCC could be assigned to the execution contexts of the platform:

• On STM32MP13x lines , each resource/sub-function is allocated either to the secure world or to the non-secure world via bits in the RCC_SECCFGR register.
• On STM32MP15x lines , there are two levels of security, which are controlled via two bits in the RCC_TZCR register (only accessible in secure mode):

• TZEN allows to set some RCC registers in secure mode, in particular registers for configuring PLL1 and PLL2, in order to secure a TrustZone perimeter for the Cortex^®-A7 secure core and its peripherals.
• MCKPROT allows extending the TZEN secure clock control perimeter to PLL3 and to the MCU subsystem, so to the Cortex^®-M4 and its bus clock.

• On STM32MP2 unknown microprocessor device, RCC is a RIF-aware peripheral. That means

• Dedicated RCC features are protected by RIF protection embedded in RCC following peripheral firewall rules described in Resource Isolation Framework overview
• Internal peripheral reset and clock gating control inherit from RIFSC configuration

Note:

Clock gating registers of system resources (Internal RAM, RIF-aware peripherals) are protected by RCC RIF protection (no RIFSC inheritance). This allows to manage automatic clock gating according to either processor low power state or system low power state. Indeed, when a peripheral is shared, it should always be available to any execution context at any time.

Please note that all RCC registers can be read from any execution context.

Refer to the STM32 MPU reference manuals for the complete list of features, and to the software frameworks and drivers, introduced below, to see which features are implemented.

3. Peripheral usage

This chapter is applicable in the scope of the OpenSTLinux BSP running on the Arm^® Cortex^®-A processor(s), and the STM32CubeMPU Package running on the Arm^® Cortex^®-M processor.

3.1. Boot time assignment

3.1.1. On STM32MP1 Series

The RCC is used by all the boot components: the ROM code, the FSBL, the SSBL and up to the Linux^® kernel. Each component is able to reset and gate internal peripherals

FSBL is responsible to make a minimal clock tree initialization for loading the next boot stages. Then the main clock tree initialization step is performed by the OP-TEE secure OS: it consists in configuring all the input clocks, the PLLs and the clock sources that are selected as kernel clocks for all peripherals. The whole clock tree configuration is carried out by the device tree (see STM32MP13 clock tree and STM32MP15 clock tree).

Expand

Click on the right to expand the legend...

Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:

• ☐ means that the peripheral can be assigned (☑) to the given boot time context.
• ⬚ means that the peripheral can be assigned to the given boot time context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are statically connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32 MPU reference manuals.

Domain	Peripheral	Boot time allocation				Comment
		Instance	Cortex-A7 secure (ROM code)	Cortex-A7 secure (TF-A BL2)	Cortex-A7 non-secure (U-Boot)
Power & Thermal	RCC	RCC	✓	✓	✓

3.1.2. On STM32MP2 unknown microprocessor device

In Cortex-A main processor mode, the RCC is used by all the boot components: the ROM code, the Cortex-A FSBL, the Secure OS, the SSBL and the Linux^® kernel. Each SW component is able to reset and gate internal peripherals assigned to it.

ROM code clock tree configuration is fixed, mainly based on HSI internal oscillator. The goal is to configure in a safe mode the platform to load, authenticate and execute the FSBL.

The Cortex-A FSBL is responsible to make a minimal clock tree initialization for loading the next boot stages. Then the main clock tree initialization step is performed by the OP-TEE secure OS: it consists in configuring all the input clocks, the PLLs and the clock sources that are selected as kernel clocks for all peripherals. The whole clock tree configuration is carried out by the device tree (see STM32MP25 clock tree).

Click on to expand or collapse the legend...

• ⬚ means that the peripheral can be assigned to the given boot time context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ☐ means that the peripheral can be assigned to the given boot time context.
• ☑ means that the peripheral is assigned by default to the given boot time context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP25 reference manuals.

Domain	Peripheral	Boot time allocation				Comment
		Instance	Cortex-A35 secure (ROM code)	Cortex-A35 secure (TF-A BL2)	Cortex-A35 non-secure (U-Boot)
Power & Thermal	RCC	RCC	✓	✓	☐	Shareable at internal peripheral level thanks to the RIF: see the boot time allocation per feature

The below table shows the possible boot time allocations for the features of the RCC instance.

Feature	Boot time allocation			Comment
	Cortex-A35 secure (ROM code)	Cortex-A35 secure (TF-A BL2)	Cortex-A35 non-secure (U-Boot)
FCALC			⬚
SYSRST		✓	⬚
BOOT_STDB		✓	⬚
RDCR		✓	⬚
SYSCLK		✓	⬚
CPU1_RES	✓	✓	⬚
CPU2_RES
CPU3_RES			☐
DEBUG_CFGR		☐	⬚
SYSRAM_CFGR	✓	☑
VDERAM_CFGR		⬚	⬚
RETRAM_CFGR		⬚	⬚
BKPSRAM_CFGR		⬚	⬚
SRAM1_CFGR		⬚	⬚
SRAM2_CFGR		⬚	⬚
LPSRAM1_CFGR			⬚
LPSRAM2_CFGR			⬚
LPSRAM3_CFGR			⬚
HPDMA1_CFGR		⬚	⬚
HPDMA2_CFGR		⬚	⬚
HPDMA3_CFGR		⬚	⬚
LPDMA_CFGR		⬚	⬚
IPCC1_CFGR		⬚	☐
IPCC2_CFGR		⬚	☐
HSEM_CFGR		⬚	⬚
GPIOA_CFGR		☑	☐
GPIOB_CFGR		☑	☐
GPIOC_CFGR		☑	☐
GPIOD_CFGR		☑	☐
GPIOE_CFGR		☑	☐
GPIOF_CFGR		☑	☐
GPIOG_CFGR		☑	☐
GPIOH_CFGR		☑	☐
GPIOI_CFGR		☑	☐
GPIOJ_CFGR		☑	☐
GPIOK_CFGR		☑	☐
GPIOZ_CFGR		☑	☐
RTC_CFGR		☐	☐
BSEC_CFGR	✓	☑
DDRC_CFGR		☑
PLL3_CFGR			⬚
SYSCPU1_CFGR		✓
MCO1_CFGR		⬚	⬚
MCO2_CFGR		⬚	⬚
OSPI1_CFGR		☐	☐
OSPI2_CFGR		☐	☐
FMC_CFGR		☐	☐
HSI_MON		⬚	☐

3.2. Runtime assignment

3.2.1. On STM32MP1 Series

The Arm^® Cortex^®-A7 secure core controls all the secure registers through the RCC OP-TEE driver. The access to some secure registers from the Cortex^®-A7 non-secure core can be achieved via SCMI requests.

3.2.1.1. On STM32MP13x lines 

Expand

Click on the right to expand the legend...

STM32MP13 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:

• ☐ means that the peripheral can be assigned (☑) to the given runtime context.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are statically connected in the device.

Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP13 reference manuals.

Domain	Peripheral	Runtime allocation			Comment
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)
Power & Thermal	RCC	RCC	✓	✓

3.2.1.2. On STM32MP15x lines 

Expand

Click on the right to expand the legend...

STM32MP15 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:

• ☐ means that the peripheral can be assigned (☑) to the given runtime context.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are statically connected in the device.

Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possiblities might be described in STM32MP15 reference manuals.

Domain	Peripheral	Runtime allocation				Comment
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)	Cortex-M4 (STM32Cube)
Power & Thermal	RCC	RCC	✓	✓	✓

3.2.2. On STM32MP25 unknown microprocessor device

In Cortex-A main processor mode, the Cortex-A secure OS OP-TEE controls all the RCC system resources, i.e. resources which are shared between execution contexts or between different internal peripherals, like oscillators, PLLs, root clocks or reset and clock gating of RIF-aware peripherals. The Cortex-A secure OS OP-TEE provides SCMI services to access system resources. For RIFSC RISUP protected internal peripheral, each SW component can directly access RCC registers to control reset and clock gating.

Click on to expand or collapse the legend...

STM32MP25 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:

• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ☐ means that the peripheral can be assigned to the given runtime context.
• ☑ means that the peripheral is assigned by default to the given runtime context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP25 reference manuals.

Domain	Peripheral	Runtime allocation						Comment
		Instance	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 non-secure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 non-secure (STM32Cube)	Cortex-M0+ (STM32Cube)
Power & Thermal	RCC	RCC	✓OP-TEE ✓TF-A BL31	✓	✓	✓	✓	Shareable at internal peripheral level thanks to the RIF: see the runtime allocation per feature

The below table shows the possible runtime allocations for the features of the RCC instance.

Feature	Runtime allocation					Comment
	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 non-secure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 non-secure (STM32Cube)	Cortex-M0+ (STM32Cube)
FCALC	☑OP-TEE	⬚	☐	⬚
SYSRST	✓OP-TEE ✓TF-A BL31	⬚
BOOT_STDB	✓OP-TEE ✓TF-A BL31	⬚
RDCR	✓OP-TEE ✓TF-A BL31	⬚
SYSCLK	✓OP-TEE ✓TF-A BL31	⬚
CPU1_RES	✓OP-TEE ✓TF-A BL31	⬚
CPU2_RES			☐ if TZ_EN	☐ if TZ_DIS
CPU3_RES	☐OP-TEE	☐	☐	☐
DEBUG_CFGR	☑OP-TEE	☐	☐	☐
SYSRAM_CFGR	☑OP-TEE ☑TF-A BL31
VDERAM_CFGR	☐OP-TEE	☐	⬚	☐
RETRAM_CFGR	☐OP-TEE ☑TF-A BL31	☐	☐	☐
BKPSRAM_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
SRAM1_CFGR	☐OP-TEE	☐	☐	☐
SRAM2_CFGR	☐OP-TEE	☐	☐	☐
LPSRAM1_CFGR	☐OP-TEE	☐	☐	☐
LPSRAM2_CFGR	☐OP-TEE	☐	☐	☐
LPSRAM3_CFGR	☐OP-TEE	☐	☐	☐
HPDMA1_CFGR	☐OP-TEE	☐	☐	☐
HPDMA2_CFGR	☐OP-TEE	☐	☐	☐
HPDMA3_CFGR	☐OP-TEE	☐	☐	☐
LPDMA_CFGR	☐OP-TEE	☐	☐	☐
IPCC1_CFGR	☑OP-TEE	☐	☐	☐
IPCC2_CFGR	☐OP-TEE	☐	☐	☐
HSEM_CFGR	☑OP-TEE	☐	⬚	☐
GPIOA_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOB_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOC_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOD_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOE_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOF_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOG_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOH_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOI_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOJ_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOK_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
GPIOZ_CFGR	☑OP-TEE ☑TF-A BL31	☐	☐	☐
RTC_CFGR	☐OP-TEE ☐TF-A BL31	☐	☐	☐
BSEC_CFGR	☑OP-TEE ☑TF-A BL31
DDRC_CFGR	☑OP-TEE ☑TF-A BL31
PLL3_CFGR	☑OP-TEE	⬚	☐	⬚
SYSCPU1_CFGR	✓OP-TEE ✓TF-A BL31
MCO1_CFGR	☐OP-TEE	⬚	☐	⬚
MCO2_CFGR	☐OP-TEE	⬚	☐	⬚
OSPI1_CFGR	☐OP-TEE	☐	☐	☐
OSPI2_CFGR	☐OP-TEE	☐	☐	☐
FMC_CFGR	☐OP-TEE	☐	☐	☐
HSI_MON	☑OP-TEE	☐	☐	☐

4. Software frameworks and drivers

Below are listed the software frameworks and drivers managing the RCC peripheral for the embedded software components listed in the above tables.

• Linux^®: Reset framework and Clock framework
• OP-TEE: OP-TEE driver
• TF-A BL2: TF-A driver
• U-Boot: U-Boot driver
• STM32Cube: HAL RCC driver

See also the SCMI overview.

5. How to assign and configure the peripheral

The peripheral assignment can be done via the STM32CubeMX graphical tool (and manually completed if needed).

This tool also helps to configure the peripheral:

• partial device trees (pin control and clock tree) generation for the OpenSTLinux software components,
• HAL initialization code generation for the STM32CubeMPU Package.

The configuration is applied by the firmware running in the context in which the peripheral is assigned.

For configuration of reset part of RCC, refer to Reset device tree configuration.

For configuration of clock part of RCC, the STM32CubeMX tool allows configuring the clock tree and integrates all the information documented in the STM32 MPU reference manuals. It is highly recommended to use it to generate your device tree as explained in:

• STM32MP13 clock tree
• STM32MP25 clock tree
• STM32MP25 clock tree

On STM32MP15x lines , the RCC security level differs for each boards and is defined by RCC compatible in OP-TEE device tree:

• "st,stm32mp1-rcc-secure": sets TZEN to 1 and MCKPROT to 0
• "st,stm32mp1-rcc": sets TZEN to 0 and MCKPROT to 0

The STMicroelectronics STM32MP15x lines  boards use TZEN=1.

6. How to go further

The RCC is interfaced with the HDP internal peripheral, thus offering the flexibility to monitor the main RCC state signals on the debug pins.

Please refer to the STM32MPU reference manuals for the full list of signals that can be monitored.