Power overview




1 Framework purpose[edit]

The purpose of this article is to explain how to handle the STM32MP15x 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 STM32MP15 reference manuals for the full description of low-power modes.
The AN5109 low-power application note also gives details on these modes.

The modes are handled by the RCC and the PWR peripherals.


The table below explains 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.

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

Level Mode Vddcore state Clocks state
Subsystem MPU CRun on on
MPU CStop on Subsystem off
MPU CStandby on Subsystem off
MCU CRun on on
MCU CStop on Subsystem off
MPU mode MCU mode Platform mode Vddcore state Clocks state
CRun CRun Run On On
CStop CRun Run On On
CStandby CRun Run On On
CRun CStop Run On On
CStop CStop Stop/LPLV-Stop/Standby On/Retention/Off Off/Off/Off
CStandby CStop Stop/LPLV-Stop/Standby On/Retention/Off Off/Off/Off

2.1 Wakeup sources[edit]

The above modes are exited due to a wakeup event.

The following table gives the list of wakeup sources available in each mode.

Mode Available wakeup sources
CStop/CStandby/Stop BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, USB, CEC, ETH, USART, I²C, SPI, LPTIM, IWDG, GPIO, Wakeup pins
LPLV-Stop BOR, PVD, AVD, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, GPIO, Wakeup pins
Standby BOR, Vbat mon, Temp mon, LSE CSS, RTC, TAMP, IWDG, Wakeup pins

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.txt ).

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).

 Example: Board $> echo mem > /sys/power/state

Further details can be found in Documentation/power/interface.txt


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 low-power mode mapping can be modified through the secure monitor device tree.

Below an example:

  &pwr {
      system_suspend_supported_modes = <
          STM32_PM_CSLEEP_RUN
          STM32_PM_CSTOP_ALLOW_STOP
          STM32_PM_CSTOP_ALLOW_LP_STOP
          STM32_PM_CSTOP_ALLOW_LPLV_STOP
          STM32_PM_CSTOP_ALLOW_STANDBY_DDR_SR
      >;
      system_off_soc_mode = <STM32_PM_CSTOP_ALLOW_STANDBY_DDR_OFF>;
  };

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:

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


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

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[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:

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

Call the low-power entry:

Board $> echo mem > /sys/power/state

or for the Weston configuration:

Board $> cat /etc/systemd/sleep.conf
[Sleep]
SuspendMode=
HibernateMode=
HybridSleepMode=
SuspendState=mem
HibernateState=mem
HybridSleepState=mem
 Board $> 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


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 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.

Microprocessor Unit

Microcontroller Unit

Brownout reset

Programmable Voltage Detector

Analog Voltage Detector

Cascading Style Sheets (web standard)

Real Time Clock

Tamper

Consumer Electronics Control (HDMI standard)

Ethernet

Universal Synchronous/Asynchronous Receiver/Transmitter

Serial Peripheral Interface

low-power timer (STM32 specific)

Independent Watchdog

General-Purpose Input/Output

Power State Coordination Interface

Reset and Clock Control

Application programming interface

Random Access Memory

Low Power (MIPI® Alliance DSI standard)

Doubledata rate (memory domain)

Out Of Memory

Configuration File System (See https://en.wikipedia.org/wiki/Configfs for more details)

Debug File System (See https://en.wikipedia.org/wiki/Debugfs for more details)

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