BKPSRAM internal memory

Applicable for

STM32MP13x lines, STM32MP15x lines

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1. Peripheral overview

The BKPSRAM internal memory is located in the VSW power domain, allowing it to be supplied during Standby low power mode, or to be switched off.

• STM32MP13x lines  BKPSRAM is 8 Kbytes wide.
  
• STM32MP15x lines  BKPSRAM is 4 Kbytes wide.

1.1. Features

Refer to STM32MP13 reference manuals or STM32MP15 reference manuals for the complete feature list, and to the software components introduced below, to see which features are currently implemented.

1.2. Security support

The BKPSRAM is a secure peripheral (under ETZPC control).

2. Peripheral usage and associated software

2.1. Boot time

The BKPSRAM internal memory is not used during a cold boot or a wake up from Standby with DDR OFF.

The BKPSRAM internal memory is used by the runtime secure monitor (from the FSBL or the OP-TEE secure OS) during wake-up from Standby low power mode with the DDR in Self-Refresh mode. In that case, the BKPSRAM internal memory contains the secure context that has to be restored before jumping back to Linux execution, in DDR.

2.2. Runtime

2.2.1. Overview

The BKPSRAM peripheral can be allocated to either:

• the Arm^® Cortex^®-A7 secure to be used by the runtime secure monitor (from the FSBL or the OP-TEE secure OS) to save/restore the secure context before entering/after exiting Standby low power mode with DDR in Self-Refresh mode. Standby low power mode is reached thanks to PSCI secure services (from the FSBL or OP-TEE secure monitor). This is the default assignment.

or

• the Cortex-A7 non-secure to be used under Linux^® as reserved memory, for instance.

Warning

Default OpenSTLinux delivery prevents to define BKPSRAM as non-secure. This requires to modify TF-A source code with one of the following strategies: set BKPSRAM as non-secure and degrade low power modes support, removing Standby mode or manage on-the-fly secure/non-secure switch of the BKPSRAM in the secure monitor for sequential usage for Standby management and Linux kernel reserved memory

2.2.2. Software frameworks

2.2.2.1. On STM32MP13x lines 

Domain	Peripheral	Software components		Comment
		OP-TEE	Linux
Core/RAM	BKPSRAM	FSBL or OP-TEE secure monitor	Linux reserved memory

2.2.2.2. On STM32MP15x lines 

Domain	Peripheral	Software components			Comment
		OP-TEE	Linux	STM32Cube
Core/RAM	BKPSRAM	FSBL or OP-TEE secure monitor	Linux reserved memory

2.2.3. Peripheral configuration

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

2.2.4. Peripheral assignment

2.2.4.1. 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 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 STM32MP13 reference manuals.

Domain	Peripheral	Runtime allocation			Comment
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)
Core/RAM	BKPSRAM	BKPSRAM	☐	⬚	Assignment (single choice)

2.2.4.2. 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 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
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)	Cortex-M4 (STM32Cube)
Core/RAM	BKPSRAM	BKPSRAM	☐	☐		Assignment (single choice)