1. Article purpose[edit | edit source]
The purpose of this article is to:
- briefly introduce the RTC 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[edit | edit source]
The RTC peripheral is used to provide the date and clock to the application. It supports programmable alarms and wake up capabilities.
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[edit | edit source]
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[edit | edit source]
3.1.1. On STM32MP1 series[edit | edit source]
By default after a backup domain power-on reset (performed at boot time), all RTC registers can be read or written in both secure and non-secure modes.
In OpenSTLinux distribution, the first stage bootloader (FSBL, running in secure mode) keeps this default configuration, leaving full control to Linux® at runtime.
The RTC peripheral is able to generate two interrupts:
- A secure interrupt, connected to the Arm® Cortex®-A7 GIC, not used in OpenSTLinux distribution.
- A non-secure interrupt, connected both to Arm® Cortex®-A7 GIC and Cortex-M4 NVIC (STM32MP15x lines
only): this interrupt is used on Linux® and by default in OpenSTLinux distribution.
The RTC peripheral is part of the backup domain which reset and clock are controlled via the RCC by the first stage bootloader (FSBL, running in secure mode) at boot time.
The RTC reset occurs when the backup domain is reset. To avoid clearing the TAMP register contents, this is only done on cold boot, not on wake up.
Click on 
to expand or collapse 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, 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 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) | |||
| Core | RTC | RTC | ☐ | |||
3.1.2. On STM32MP2 series[edit | edit source]
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) | |||
| Core | RTC 
|
RTC | ⬚ | ☐ | 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 RTC instance.
| Feature | Boot time allocation
|
Comment | ||
|---|---|---|---|---|
| Cortex-A35 secure (ROM code) |
Cortex-A35 secure (TF-A BL2) |
Cortex-A35 non-secure (U-Boot) | ||
| Alarm A | ⬚ | ☐ | ||
| Alarm B | ⬚ | ⬚ | ||
| Wakeup timer | ⬚ | ☐ | ||
| Timestamp | ⬚ | ☐ | ||
| Calibration | ⬚ | ☐ | ||
| Initialization | ⬚ | ☐ | ||
3.2. Runtime assignment[edit | edit source]
3.2.1. On STM32MP13x lines [edit | edit source]
Click on to expand or collapse 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 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.
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) | |||
| Core | RTC | RTC | ☑ | ☐ | RTC is mandatory to resynchronize STGEN after exiting low-power modes. |
3.2.2. On STM32MP15x lines [edit | edit source]
Click on to expand or collapse 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 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.
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) | |||
| Core | RTC | RTC | ☑ | ☐ | ☐ | RTC is mandatory to resynchronize STGEN after exiting low-power modes. |
3.2.3. On STM32MP25x lines [edit | edit source]
Click on to expand or collapse 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 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) | |||
| Core | RTC
|
RTC | ☐OP-TEE | ☐ | ☐ | ☐ | ☐ | 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 RTC 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) | ||
| Alarm A | ⬚OP-TEE | ☐ | ⬚ | ☐ | ||
| Alarm B | ⬚OP-TEE | ⬚ | ☐ | ☐ | ||
| Wakeup timer | ⬚OP-TEE | ☐ | ⬚ | ☐ | ||
| Timestamp | ⬚OP-TEE | ☐ | ⬚ | ☐ | ||
| Calibration | ⬚OP-TEE | ☐ | ⬚ | ☐ | ||
| Initialization | ⬚OP-TEE | ☐ | ⬚ | ☐ | ||
4. Software frameworks and drivers[edit | edit source]
Below are listed the software frameworks and drivers managing the RTC peripheral for the embedded software components listed in the above tables.
- Linux®: RTC framework
- U-Boot:
5. How to assign and configure the peripheral[edit | edit source]
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 Linux kernel configuration, please refer to RTC device tree configuration.
Arm® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere. 
Arm® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.