BKPSRAM internal memory

Applicable for STM32MP13x lines, STM32MP15x lines

1 Article purpose[edit]

The purpose of this article is to:

  • briefly introduce the BKPSRAM internal memory 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]

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 More info.png BKPSRAM is 8 Kbytes wide.
  • STM32MP15x lines More info.png BKPSRAM is 4 Kbytes wide.

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]

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]

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.

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 is assigned by default to the given boot time context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
  • 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 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/RAM BKPSRAM BKPSRAM

3.2 Runtime assignment[edit]

3.2.1 On STM32MP13x lines More info.png[edit]

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 is assigned by default to the given runtime context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
  • 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 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/RAM BKPSRAM BKPSRAM Assignment (single choice)

3.2.2 On STM32MP15x lines More info.png[edit]

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 is assigned by default to the given runtime context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
  • 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 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/RAM BKPSRAM BKPSRAM Assignment (single choice)

4 Software frameworks and drivers[edit]

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 [1] 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 white.png 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

Thus, below are listed the software frameworks and drivers managing the BKPSRAM peripheral for the embedded software components listed in the above tables.

5 How to assign and configure the peripheral[edit]

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.

6 References[edit]