Power overview

Applicable for STM32MP13x lines, STM32MP15x lines

1 Framework purpose[edit]

The purpose of this article is to explain how to handle the STM32MP15x and STM32MP13x low-power modes:

  • Low-power modes available on the device
  • Linux software overview
  • How to enter and exit the low-power modes on Arm® Cortex®-A7 core
  • How to enter a platform low-power mode

2 Low-power modes available on the device[edit]

Refer to STM32MP13 reference manuals or STM32MP15 reference manuals for the full description of low-power modes.
The modes are handled by the RCC and the PWR peripherals.

2.1 On STM32MP13x lines More info.png[edit]

The table below summarizes the device hardware states corresponding to each low-power mode.

MPU mode Platform mode Vddcore state Vddcpu state Clocks state
CRun Run On On On
CStop Stop/LPLV-Stop On/Retention On/Retention Off/Off
CStandby Stop/LPLV-Stop/LPLVL-STOP2/Standby On/Retention/Retention/Off On/Retention/Off/Off Off/Off/Off/Off

2.2 On STM32MP15x lines More info.png[edit]

The AN5109 low-power application note also gives much more information on these modes, including:

  • the detailed description of the operating modes,
  • the low-power mode entry and exit sequences,
  • the low-power mode control registers.

The table below summarizes the device hardware states corresponding to each low-power mode.

The term "subsystem" either refers to Arm® Cortex®-A7 (also called MPU) or to Arm® Cortex®-M4 (also called MCU). A mode prefixed by 'C' corresponds to a subsystem mode.

Level Mode Vddcore state Subsystem Clocks state
MPU Subsystem MPU CRun on on
MPU CStop on off
MPU CStandby on off
MCU Subsystem MCU CRun on on
MCU CStop on off

A platform mode is the combination of MPU and MCU modes.

MPU mode MCU mode Platform mode Vddcore state Clocks state
CRun/CStop/CStandby CRun Run On On
CStop/CStandby CStop Stop (LPDS = 0) On Off
CStop/CStandby CStop LP-Stop or LPLV-Stop (LPDS = 1) Retention Off
CStandby CStop
Standby Off Off

2.3 Wakeup sources[edit]

The above modes are exited due to a wakeup event. The following tables give the list of wakeup sources available in each mode.

2.3.1 On STM32MP13x lines More info.png[edit]

Platform mode Available wakeup sources
Stop BOR, PVD, AVD, Vbat mon, Temp mon, HSE CSS, LSE CSS, RTC, TAMP, USBH, OTG, ETH, USART, I2C, SPI, DTS, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, USART, I2C, SPI, DTS, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
Standby BOR, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, Wakeup pins (from PWR)

2.3.2 On STM32MP15x lines More info.png[edit]

Again, the AN5109 low-power application note details, among other things, the wakeup sources, the software mechanism that ensures the consistency between the low-power mode and the activated wakeup source, and the low-power mode exit sequence.

Plaform mode Available wakeup sources
Stop BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, USB, CEC, ETH, USART, I²C, SPI, LPTIM, IWDG, GPIO, Wakeup pins (from PWR)
LPLV-Stop BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, GPIO, Wakeup pins (from PWR)
Standby BOR, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, Wakeup pins (from PWR)

This list of Wakeup capability peripheral for each mode is defined in the Table 35. Functionalities depending on system operating mode of the STM32MP15 reference manuals.

3 Software overview[edit]

The Linux® suspend framework is used to trigger a low-power mode entry/exit sequence.

Refer to Documentation/power for more details.

Power mgt.png

The user application issues a suspend request to the kernel. This request is handled by the suspend Framework, which notifies all the device drivers to prepare for low-power entry. It then calls the PSCI service.

In addition to this centralized suspend process, most of the drivers implement the runtime pm feature. It is used to dynamically disable the resources of the peripherals (clocks and power when applicable) in case of inactivity (see Documentation/power/runtime_pm.rst ).

3.1 Component description[edit]

Kernel components:

  • Suspend framework: this framework schedules the overall sequence by stopping all the ongoing tasks
  • GenPD driver: this driver is used for low-power mode selection according to the activated wakeup sources.
  • PSCI library: this is a set of standardized functions to request a low-power service to the secure monitor
  • RCC driver: this driver handles the circuit non-secure clocks

Secure monitor components:

  • PWR driver: this driver is responsible for configuring the low-power mode
  • PSCI library: this is a set of standardized functions handling the low-power services
  • Low power driver: the role of this driver is to choose the low-power mode according to the programmed wakeup source(s)
  • RCC driver: this driver handles the circuit secure clocks

3.2 API description[edit]

The suspend process is triggered from the user space through standard commands.

The system sleep control file is the state file, located under: /sys/power/

Only the 'mem' command is supported:

- The whole system activity is stopped and a low-power mode is entered. The software selects the deepest mode according to the activated wakeup source(s).


  echo mem > /sys/power/state

Further details can be found in Documentation/admin-guide/pm/sleep-states.rst

STMicroelectronics deliveries propose a default mapping of the low-power modes for each type of board.

Note that this default mapping can be changed thanks to the device tree. Refer to paragraph 3.3.2.

3.3 Software configuration[edit]

3.3.1 Menuconfig (Linux® kernel)[edit]

The suspend to RAM feature is activated by default in STMicroelectronics deliveries.

It can be deactivated through the kernel menuconfig using Power management options/Suspend to RAM and standby: Menuconfig or how to configure kernel .

3.3.2 Device tree (secure monitor)[edit]

The default system low-power mode mapping can be modified through the secure monitor device tree, as described OP-TEE page.

Below an example in OP-TEE core/arch/arm/dts/stm32mp135f-dk.dts

 &pwr_regulators {
 	system_suspend_supported_soc_modes = <
 	system_off_soc_mode = <STM32_PM_SHUTDOWN>;

For detailed information on the device tree concept, refer to Device tree.

3.3.3 Example of wakeup source activation[edit]

The activation of a wakeup source is done in the corresponding driver.

For example, activating UART4 as wakeup source is done thanks to the following commands:

 echo enabled > /sys/devices/platform/soc/40010000.serial/tty/ttySTM0/power/wakeup
 echo enabled > /sys/devices/platform/soc/40010000.serial/power/wakeup

It is possible to check the state of each wakeup source (activated or not) by displaying the 'wakeup' attribute.

Note that the software implements a consistency check between the selected wakeup source and the appropriate low-power mode.

Info white.png Information
You must be sure your wakeup device is able to provide the wake up signal during low-power mode.

E.g. supply, clock, reset, etc... must be present, if any

4 How to enter and exit low-power modes[edit]

4.1 Platform low-power[edit]

Select the platform allowed modes depending on the required wakeup source.

Activate the wakeup source(s) (peripheral dependent).

Call the low-power mode on both sides (MPU and MCU).

4.2 MPU side[edit]

Activate the wakeup source(s) (peripheral dependent)

Call the low-power mode by issuing the following command:

echo mem > /sys/power/state

  Note that in Weston configuration the low-power mode is entered upon a 'systemctl suspend' command.

4.3 MCU side on STM32MP15x lines More info.png[edit]

Please refer to Coprocessor power management for Arm® Cortex®-M4 commands.

4.4 Example: entering/exiting MPU CStop mode[edit]

Enable at least one wakeup source from table 2.1 in CStop category, for example USART:

 echo enabled > /sys/devices/platform/soc/40010000.serial/tty/ttySTM0/power/wakeup
 echo enabled > /sys/devices/platform/soc/40010000.serial/power/wakeup

Call the low-power entry:

 echo mem > /sys/power/state

or for the Weston configuration:

 cat /etc/systemd/sleep.conf

This Weston sleep configuration file map Weston power state to the kernel power state. In above example "mem" correspond to "mem" in echo mem > sys/power/state.

  systemctl suspend

The MPU is now in CStop mode, and can be woken up by sending a character to the console.

5 How to trace and debug[edit]

The suspend/resume process execution is logged in the MPU console. It gives useful information on the platform state (sleeping or active).

 root@stm32mp1:~# echo mem > /sys/power/state
 [ 1072.267571] PM: suspend entry (deep)
 [ 1072.269687] PM: Syncing filesystems ... done.
 [ 1072.279114] Freezing user space processes ... (elapsed 0.008 seconds) done.
 [ 1072.292835] OOM killer disabled.
 [ 1072.296046] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
 [ 1072.303431] Suspending console(s) (use no_console_suspend to debug)
 [ 1072.332520] dwc2 49000000.usb-otg: suspending usb gadget configfs-gadget
 [ 1072.332537] dwc2 49000000.usb-otg: dwc2_hsotg_ep_disable: called for ep0
 [ 1072.332546] dwc2 49000000.usb-otg: dwc2_hsotg_ep_disable: called for ep0
 [ 1072.468536] Disabling non-boot CPUs ...
 [ 1072.507876] CPU1 killed.
 [ 1072.509635] Enabling non-boot CPUs ...
 [ 1072.510508] CPU1 is up
 [ 1072.527553] dwmac4: Master AXI performs any burst length
 [ 1072.527583] stm32-dwmac 5800a000.ethernet eth0: No Safety Features support found
 [ 1072.527621] stm32-dwmac 5800a000.ethernet eth0: ERROR failed to create debugfs directory
 [ 1072.527631] stm32-dwmac 5800a000.ethernet eth0: stmmac_hw_setup: failed debugFS registration
 [ 1072.588234] dwc2 49000000.usb-otg: resuming usb gadget configfs-gadget
 [ 1072.738469] OOM killer enabled.
 [ 1072.741575] Restarting tasks ... done.
 [ 1072.752596] PM: suspend exit

Get more debug information from the console with the following commands:

 echo N > /sys/module/printk/parameters/console_suspend 
 echo "func pm_dev_dbg +p" > /sys/kernel/debug/dynamic_debug/control

It is also possible to monitor the hardware signals related to the system low-power modes thanks to the HDP internal peripheral.
Please refer to HDP Linux driver for its configuration.

6 To go further[edit]

Refer to STM32MP13 reference manuals and STM32MP15 reference manuals for a detailed description of low-power modes and peripheral wakeup sources.

The AN5109 low power application note gives additional information on the hardware settings used for low-power management.