Getting started with PWR

Revision as of 10:43, 3 January 2023 by Registered User

This article explains Low-power modes, and provides code examples.

1. Low-power modes Introduction

By default, the microcontroller is in run mode after a system or power reset. Several low-power modes are available to save power when the CPU does not need to be kept running, for example when waiting for an external event. It is up to the user to select the mode that gives the best compromise between low-power consumption, short startup time and available wake-up sources.

Info white.png Information
This example uses the NUCLEO-L476RG board.

The ultra-low-power STM32L476xx supports 7 low-power modes to achieve the best compromise between low-power consumption, short startup time, available peripherals and available wake-up sources.

  • Sleep mode
  • Low-power run mode
  • Low-power sleep mode
  • Stop 0, Stop1, Stop2 modes
  • Standby mode
  • Shutdown mode

1.1. Voltage Regulators

Two embedded linear voltage regulators supply most of the digital circuitries: the main regulator (MR) and the low-power regulator (LPR).

  • The MR is used in the Run and Sleep modes and in the Stop 0 mode.
  • The LPR is used in Low-Power Run, Low-Power Sleep, Stop1 and Stop2 modes. It is also used to supply the 32 Kbyte SRAM2 in standby with SRAM2 retention.
  • Both regulators are in power-down in standby and shutdown modes: the regulator output is in high impedance, and the kernel circuitry is powered down thus inducing zero consumption.

There are two power consumption ranges:

  • Range 1 with the CPU running at up to 80 MHz.
  • Range 2 with a maximum CPU frequency of 26 MHz. All peripheral clocks are also limited to 26 MHz.

File:voltage regulator.png

1.2. Objectives

  • Create a simple project with each low-power mode and wake up when a button (configured in the EXTI setup) is pressed.​
  • Learn how to set up low-power modes with the HAL​
  • Verify the correct functionality by measuring the current consumption​.

1.3. How to

  • During this article, we will be using the code from EXTI Overview

2. Sleep mode

2.1. Definition

In sleep mode, the CPU clocks are OFF and there is no effect on other clocks or analog clock sources. All peripherals continue to operate and can wake up the CPU when an interrupt/event occurs.

File:sleep mode.png

2.2. Configure the sleep mode

2.2.1. STM32CubeMX configuration

The system clock is set to 80 MHz.
File:clock configuration sleep.png

  • An EXTI line is connected to the user button through PC13
  • LED2 connected to PA5 pin
2.2.2. Code configuration
  • Open the project from EXTI overview
  • Open main.c
  • Add a function to enter Sleep mode.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{
  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */
  HAL_Delay(1000);
  HAL_SuspendTick();
  HAL_PWR_EnterSLEEPMode(PWR_MAINREGULATOR_ON,PWR_SLEEPENTRY_WFI);
  HAL_ResumeTick();
}
/* USER CODE END 3 */
Info white.png Information
Set all GPIO to analog state to reduce power consumption
Warning white.png Warning
Make sure you are working in Range 1

2.3. Compile and flash

  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png

2.4. Measure consumption

  • Stop any debug session and do a full-power cycle
  • Use an amperemeter on the IDD connector (JP6 on NUCLEO-L476RG - for other boards check their user manual)

File:capture.png

  • Check the current consumption while in Sleep mode
  • Press the configured button on the Nucleo board to see the consumption variation.
Info white.png Information
It is possible to measure the current consumption with STM32Cube Monitor-Power

To do so, you will need the X-NUCLEO-LPM01A expansion board for power consumption measurement and its user manual.

  • Referring to the datasheet, in Sleep mode at 25°C, VDD = 3V and fHCLK= 80MHz , the current consumption should be 2.96mA

File:datadheet sleep.png

Info white.png Information
Note that you should always check that the measured value is bounded between the TYP and Max value in the datasheet.
  • Using STM32Cube Monitor-Power, we measured :

File:final sleep.png

3. Low-power run mode

3.1. Definition

This mode is achieved when the system clock frequency is reduced below 2 MHz. The code is executed from the SRAM or the Flash memory. The regulator is in low-power mode to minimize the regulator's operating current.

File:lprun.png

3.2. Configure the low-power run mode

3.2.1. STM32CubeMX configuration

The system clock is limited to a maximum of 2 MHz. The MSI internal RC oscillator can be selected as it supports several frequency ranges clock config.png

Info white.png Information
In low-power run mode, all I/O pins keep the same state as in run mode.
3.2.2. Code configuration
  • Open the project from EXTI overview
  • Open main.c
  • Add a function to enter low-power run mode.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{ 
  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */
  HAL_Delay(2000);
  HAL_SuspendTick();
  HAL_PWREx_EnableLowPowerRunMode();
  HAL_ResumeTick();
}
/* USER CODE END 3 */

3.3. Compile and flash

  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png

3.4. Measure the current consumption

  • Referring to the datasheet, in low-power run mode at 25°C, Fhclk = 100kHz and VDD = 3V, the current consumption should be 42μA.

File:datasheet LPRUN.png

  • Using STM32Cube Monitor-Power, we measured :

File:final lprun.png

4. Low-power sleep mode

4.1. Definition

This mode is entered from the low-power run mode. Only the CPU clock is stopped. When wake-up is triggered by an event or an interrupt, the system reverts to the low-power run mode.

4.2. Configure the low-power sleep mode

  • Open the project from EXTI overview
  • Open main.c
  • Add a function to enter low-power sleep mode.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{ 
  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */
  HAL_Delay(2000);
  HAL_SuspendTick();
  HAL_PWR_EnterSLEEPMode(PWR_LOWPOWERREGULATOR_ON, PWR_SLEEPENTRY_WFI);
  //This function will automatically switch to low-power run mode first, if not already enabled
  HAL_ResumeTick();
}
/* USER CODE END 3 */
Info white.png Information
Unlike Sleep mode, in low-power sleep mode the main regulator is OFF and the low-power regulator is ON.

4.3. Compile and flash

  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png
Warning white.png Warning
This example can not be used in DEBUG mode due to the fact that the Cortex-M4 core is no longer clocked during low-power mode so debugging features are disabled.

4.4. Measure the current consumption

  • Referring to the datasheet, in low-power sleep mode at 25°C and VDD = 3V, the current consumption should be 33mA

datasheet lpsleep.png

5. Stop0, Stop1 and Stop2 modes

5.1. Definition

Stop mode achieves the lowest power consumption while retaining the content of SRAM and registers. All clocks in the VCORE domain are stopped, the PLL, the MSI RC, the HSI16 RC and the HSE crystal oscillators are disabled. The LSE or LSI can be kept running. File:Stop mode.jpg

5.2. Stop0 mode

Open the project from EXTI overview

  • Open main.c
  • Add a function to suspend the SysTick (the SysTick is typically set to raise an interrupt every 1 ms).
  • Add a function to enter Stop0 mode
  • Add a function to resume SysTick on wake-up.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{
/* USER CODE END WHILE */

/* USER CODE BEGIN 3 */
  HAL_Delay(1000);
  HAL_SuspendTick();
  HAL_PWREx_EnterSTOP0Mode(PWR_SLEEPENTRY_WFI);
  HAL_ResumeTick();
}
/* USER CODE END 3 */
Info white.png Information
After using the WFI and entering Stop0, Stop1 or Stop2 mode, the code in the handler will be executed.
5.2.1. Compile and flash
  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png
5.2.2. Measure the current consumption
  • Referring to the datasheet, in Stop0 mode at 25°C and VDD = 3.6V, the current consumption should be 113μA

File:datasheet stop0.png

  • Using STM32Cube Monitor-Power, we measured :

File:final stop0.png

5.3. Stop1 mode

Stop1 offers a larger number of active peripherals and wake-up sources, a smaller wake-up time but a higher consumption than Stop2.

  • Change only this line in the code :
HAL_PWREx_EnterSTOP1Mode(PWR_SLEEPENTRY_WFI);
5.3.1. Compile and flash
  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png
5.3.2. Measure the current consumption
  • Referring to the datasheet, in Stop1 mode at 25°C and VDD = 3.6V, the current consumption should be 6.70μA

File:DATASHEET STOP1.png

  • Using STM32Cube Monitor-Power, we measured :

File:final plot stop1.png

5.4. Stop2 mode

In Stop2 mode, most of the VCORE domain is put in a lower leakage mode.

  • Change only this line in the code :
HAL_PWREx_EnterSTOP2Mode(PWR_SLEEPENTRY_WFI);
5.4.1. Compile and flash
  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png
5.4.2. Measure the current consumption
  • Referring to the datasheet, in Stop2 mode at 25°C and VDD = 3.6V, the current consumption should be 1.26μA

File:datasheet stop2.png

  • Using STM32Cube Monitor-Power, we measured :

File:final slot stop2.png

6. Standby mode

6.1. Definition

The standby mode is used to achieve the lowest power consumption with brown-out reset. The internal regulator is switched off so that the VCORE domain is powered off. The PLL, the MSI RC, the HSI16 RC and the HSE crystal oscillators are also switched off.
RTC can remain active (standby mode with RTC, standby mode without RTC).
Brown-out reset (BOR) always remains active in standby mode.
The state of each I/O during standby mode can be selected by software: I/O with internal pull-up, internal pull-down or floating.

File:standby mode.png

6.2. HAL library workflow summary

When exiting standby mode, all registers in the VCORE domain are set to their reset value. File:StandBy HAL.png

6.3. Configure the standby Mode

  • Open the project from EXTI overview
  • Open main.c
  • Add the wake-up flag clear (without this, standby mode is only entered once)
/* Initialize all configured peripherals */
/* USER CODE BEGIN 2 */
__HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU);
/* USER CODE END 2 */
  • Add a function to enter standby mode.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{ 
  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */
  HAL_Delay(2000);
  HAL_PWR_EnterSTANDBYMode();
}
/* USER CODE END 3 */

6.4. Compile and flash

  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png

6.5. Measure the current consumption

  • Referring to the datasheet, in standby mode at 25°C and VDD = 3V, the current consumption should be 150nA

File:datasheet Standby.png

  • Using STM32Cube Monitor-Power, we measured :

File:final standby.png

Warning white.png Warning
You cannot use the configured EXTI button to wake the system up, but you can use RTC or IWDG. Alternatively, you can press the RESET button on the NUCLEO-L476RG board. In any case, code execution will start from the beginning.
Info white.png Information
For more information refer to the RTC article and the WDOG article.

7. Shutdown mode

7.1. Definition

The shutdown mode allows to achieve the lowest power consumption. The internal regulator is switched off so that the VCORE domain is powered off. The PLL, the HSI16, the MSI, the LSI and the HSE oscillators are also switched off.
File:shutdown mode.png

7.2. Configure the shutdown mode

  • Open the project from EXTI overview
  • Open main.c
  • Add the wake-up flag clear (without this, shutdown mode is only entered once)
/* Initialize all configured peripherals */
/* USER CODE BEGIN 2 */
__HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU);
/* USER CODE END 2 */
  • Add a function to enter shutdown mode.
/* Infinite loop */
/* USER CODE BEGIN WHILE */
while (1)
{ /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
    HAL_Delay(2000);
    HAL_PWREx_EnterSHUTDOWNMode();
}
/* USER CODE END 3 */

7.3. Compile and flash

  • Click on Build button Built.png
  • Click on Run button (to execute) Run.png

7.4. Measure the current consumption

  • Referring to the datasheet, in shutdown mode at 25°C and VDD = 3V, the current consumption should be 64.1nA

File:datasheet shutdown.png

  • Using STM32Cube Monitor-Power, we measured :

File:final shutdown.png

Warning white.png Warning
You cannot use the configured EXTI button to wake the system up, but you can use RTC or IWDG. Alternatively, you can press the RESET button on the NUCLEO-L476RG board. In any case, code execution will start from the beginning.
Info white.png Information
For more information refer to the RTC article and the WDOG article.
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