RTC internal peripheral

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1. Article purpose[edit source]

The purpose of this article is to

briefly introduce the RTC peripheral and its main features
indicate the level of security supported by this hardware block
explain how it can be allocated to the three runtime contexts and linked to the corresponding software components
explain, when needed, how to configure the RTC peripheral.

2. Peripheral overview[edit source]

The RTC peripheral is used to provide the date and clock to the application. It supports programmable alarms and wake up capabilities.

2.1. Features[edit source]

Refer to STM32MP15 reference manuals for the complete list of features, and to the software components, introduced below, to know which features are really implemented.

2.2. Security support[edit source]

The RTC peripheral is a secure peripheral.

3. Peripheral usage and associated software[edit source]

3.1. Boot time[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: 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.

3.2. Runtime[edit source]

3.2.1. Overview[edit source]

The RTC peripheral can be allocated to the Arm^® Cortex^®-A7 non-secure core to be used under Linux^® with RTC framework.

3.2.2. Software frameworks[edit source]

Domain	Peripheral	Software frameworks			Comment
Domain	Peripheral	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)	Cortex-M4 (STM32Cube)	Comment
Core	RTC		Linux RTC framework

3.2.3. Peripheral configuration[edit source]

The configuration is applied by the firmware running in the context to which the peripheral is assigned. The configuration can be done alone via STM32CubeMX tool for all internal peripherals, and then manually completed (especially for external peripherals) according to the information given in the corresponding software framework article.

For Linux kernel configuration, please refer to RTC device tree configuration.

3.2.4. Peripheral assignment[edit source]

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.
✓ 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 a runtime 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
Domain	Peripheral	Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)	Cortex-M4 (STM32Cube)	Comment
Core	RTC	RTC	✓	✓		RTC is mandatory to resynchronize STGEN after exiting low-power modes.