STM32CubeWB0: Low-power management

1. Introduction

The STM32CubeWB0 firmware provides a software framework to support all the STM32WB0 devices hardware power save modes.
Once the low-power mode is enabled in the application, the power save software combines the low-power requests coming from the application with the radio operating mode, choosing the best power save mode applicable in the current scenario. The power modes software framework enables the negotiation between the radio module and the application requests, and prevents data loss.

1.1. Supported power modes

STM32WB0 SoC supports four main PWR power modes:

• Shutdown

• Deepstop with no low-speed clock

• Deepstop with low-speed clock

• Sleep (WFI)

The power save software implements a mechanism to save and restore all the peripheral configurations and the application contexts when a power save procedure is called. So, from the application point of view, the exit from a low-power procedure is fully transparent: when a wake-up from power save occurs, the CPU executes the next instruction after the power save function call.

Information

From the application point of view, the exit from a low-power procedure is fully transparent.

1.1.1. Shutdown mode

The shutdown level turns off the device: all voltage regulators, clocks, and the RF interface are not powered.

The only wake-up source is a low pulse on the RSTN pad or a configurable pulse on PB0 (only on STM32WB09xE devices). No smart power management is in place, because there is no memory retention in this power save level configuration.

1.1.2. Deepstop mode without low-speed clock

The Deepstop with no low-speed clock enabled configures the device in deep-sleep. All the peripherals and clock sources are turned off. A wake-up is possible only from GPIOs enabled for this scope. The supported wake-up GPIOs are the following:

• PA0 to PA15 and PB0 to PB11 on STM32WB07xC/STM32WB06xC devices.
• All GPIOs on STM32WB05xZ and STM32WB09xE devices.

1.1.3. Deepstop mode with low-speed clock

The deepstop with low-speed clock enabled configures the device in deep-sleep and the timer clock sources (LSI or LSE) remain running. A wake-up is possible from all GPIOs enabled for this scope, RTC, IWDG, LPUART (only on STM32WB05xZ and STM32WB09xE devices), radio IP, and the radio timer. The supported wake-up GPIOs are the following:

• PA0 to PA15 and PB0 to PB11 on STM32WB07xC/STM32WB06xC devices.
• All GPIOs on STM32WB05xZ and STM32WB09xE devices.

1.1.4. Sleep mode (WFI)

The sleep (WFI) configures the device in CPU halt. Only the CPU is halted. The rest of the chip continues to run normally. The chip wakes up from any interruption.

1.1. Low-power in Connectivity applications

1.1.1. Low-power manager Utility (LPM)

1.1.1.1. LPM Utility

The low-power manager provides a simple interface to receive the input from up to 32 different users and computes the lowest possible power mode the system can use. Each LPM client can request to disable or to enable Deepstop modes with low-speed clock or not when necessary. The following APIs are available:

• UTIL_LPM_SetStopMode for Deepstop mode (with low-speed clock).
• UTIL_LPM_SetOffMode for Deepstop mode (without low-speed clock).

Information

By default, when no LPM user requests to disable Off on Stop modes, the Deepstop without low-speed clock (lowest low-power mode available) is selected.

Users can obtain the low-power mode that is going to be selected at that time withUTIL_LPM_GetMode API.

1.1.1.2. Platform interfaces

Once it is time to enter in low-power mode, the application is responsible to call UTIL_LPM_EnterLowPower. According to the lowest low-power mode available, LPM Utility calls the appropriate interface to enter Sleep or Deepstop mode at SoC level. Those interfaces have to be defined at application level (system side).

Information

UTIL_LPM_EnterLowPower also calls low-power exit function.

The low-power platform interfaces are:

• PWR_EnterSleepMode and PWR_ExitSleepMode for Sleep mode.
• PWR_EnterStopMode and PWR_ExitStopMode for Deepstop mode with low-speed clock.
• PWR_EnterOffMode and PWR_ExitOffMode for Deepstop mode without low-speed clock.

NOTE: On STM32WB0, the low-power platform interfaces are defined on files stm32wb0x_hal_pwr.[ch].

1.1.2. Low-power Stop modes for Connectivity

Both SLEEP and DEEPSTOP (with low-speed clock) are supported. As a reminder, Link Layer (2.4 GHz Radio) requires low-speed clock for Bluetooth Low Energy (BLE) periodic wake-up (adv, connected events, etc.)

1.1.2.1. Save & restore mechanism

1.1.2.1.1. CPU context registers

All the Cortex M0+ registers are saved before entering in Deepstop and restored after wake-up.

Hereafter is the description of the context save mechanism implemented on CS_contextSave() function for the STM32WB0 solution:

• Load the CONTROL register into R2
• Load the process stack pointer into R1
• Switch to Main Stack Pointer
• Store R4-R7 and LR (5 words) onto the stack
• Move thread {r8 - r12} to {r3 - r7}
• Store R8-R12 (5 words) onto the stack
• Load address of struct RAM_VR into R4
• Load the stack pointer into R3
• Store the MSP into RAM_VR.SavedMSP (second word of the structure)
• Store PSP, CONTROL
• Trigger WFI (deep sleep)

Hereafter is the description of the context restore mechanism implemented on CS_contextRestore() function for the STM32WB0 solution:

• Load address of RAM_VR into R4
• Load the MSP from RAM_VR.SavedMSP (second word of the structure)
• Restore the MSP from R4
• Load PSP from the stack in R0, and load CONTROL register from the stack in R1
• Load R8-R12 (5 words) from the stack
• Move {r3 - r7} to {r8 - r12}
• Load R4-R7 (4 words) from the stack
• Load LR from the stack
• Restore PSP from R0
• Restore CONTROL register from R1
• Load PC (1 words) from the stack

At wake-up, a call to CPUcontextRestore reloads all registers allowing to retrieve peripheral configuration and execution context. As a result, the next executed instruction is the one after __WFI in PWR_EnterSleepMode, PWR_EnteStopMode, PWR_EnterOffMode.

2. Interfaces

2.1. LPM Utility

The low-power manager (LPM) utility APIs are available on a files stm32_lpm.[ch]:

UTIL_LPM_Init
Description Initialize the LPM resources.. Syntax void UTIL_LPM_Init( void ); Parameters None Return value None

UTIL_LPM_DeInit
Description Un-initialize the LPM resources. Syntax void UTIL_LPM_DeInit( void ); Parameters None Return value None

UTIL_LPM_GetMode
Description This API returns the low-power mode selected that is applied when the system enters low-power mode. Syntax UTIL_LPM_Mode_t UTIL_LPM_GetMode( void ); Parameters None Return value Type: UTIL_LPM_Mode_t Description: LPM mode.

UTIL_LPM_SetStopMode

Description This API notifies the low-power manager if the specified user allows the Stop mode or not (Deepstop with low-speed clock). Syntax void UTIL_LPM_SetStopMode( UTIL_LPM_bm_t lpm_id_bm, UTIL_LPM_State_t state ); Parameters [in] lpm_id_bm Type: UTIL_LPM_bm_t Description: Identifier of the user ( 1 bit per user ) [in] state Type: UTIL_LPM_State_t Description: Specify whether StopMode is allowed or not by this user. Return value None

UTIL_LPM_SetOffMode

Description This API notifies the low-power manager if the specified user allows the Off mode or not (Deepstop without low-speed clock). Syntax void UTIL_LPM_SetOffMode( UTIL_LPM_bm_t lpm_id_bm, UTIL_LPM_State_t state ); Parameters [in] lpm_id_bm Type: UTIL_LPM_bm_t Description: Identifier of the user ( 1 bit per user ) [in] state Type: UTIL_LPM_State_t Description: Specify whether Off mode is allowed or not by this user. Return value None

UTIL_LPM_EnterLowPower
Description System enters low-power mode. It is called by the low-power manager in a critical section (PRIMASK bit set) to allow the application to implement dedicated code before entering the low-power mode. Syntax void UTIL_LPM_EnterLowPower( void ); Parameters None Return value None

2.2. Low-power platform interfaces

The following low-power functions are available on files stm32wb0x_hal_pwr.[ch]:

PWR_EnterSleepMode
Description Interface to enter Sleep mode.In Sleep mode, all I/O pins keep the same state as in Run mode. CPU clock is off and all peripherals including Cortex-M0+ core such as NVIC and SysTick can run and wake up the CPU when an interrupt or an event occurs. Syntax void PWR_EnterSleepMode(void); Parameters None Return value None

PWR_ExitSleepMode
Description Interface to exit Sleep mode. Syntax void PWR_ExitSleepMode(void); Parameters None Return value None

PWR_EnterStopMode
Description Interface to enter Stop mode (Deepstop with low-speed clock). Syntax void PWR_EnterStopMode(void); Parameters None Return value None

PWR_ExitStopMode
Description Interface to exit Stop mode (Deepstop with low-speed clock). Syntax void PWR_ExitStopMode(void); Parameters None Return value None

PWR_EnterOffMode
Description Interface to enter Off mode (Deepstop without low-speed clock). Syntax void PWR_EnterOffMode(void); Parameters None Return value None

PWR_ExitOffMode
Description Interface to exit Off mode (Deepstop without low-speed clock). Syntax void PWR_ExitOffMode(void); Parameters None Return value None

2.3. CPU context save & restore

The following CPU context save & restore functions are available on file cpu_context_switch.s:

CPUcontextSave
Description It saves the current Cortex® registers context and triggers sleeping through the 'WFI' instruction. Syntax void CPUcontextSave(void); Parameters None Return value None

CPUcontextRestore
Description It restores the Cortex® general purpose registers from stack. Syntax void CPUcontextRestore(void); Parameters None Return value None

3. How to setup low-power modes

3.1. Low-power initialization & configuration

In Connectivity applications, the low-power mode is configured in Core/Inc/app_conf.h file where the user can activate

• CFG_FULL_LOW_POWER to deactivate incompatible features such as debugger and Nucleo LEDs.
• CFG_LPM_SUPPORTED to activate low-power manager Utility.
• CFG_LPM_EMULATED to emulate the Deepstop mode without losing the debugger connection and breakpoints nor watchpoints. It requires CFG_LPM_SUPPORTED to be activated.

UTIL_LPM_Init is responsible for initializing low-power manager Utility (in Core/Src/app_entry.c file). Once this is done, the user can dynamically enable or disable Stop/Off mode with UTIL_LPM_SetStopMode/UTIL_LPM_SetOffMode APIs.

3.2. Low-power entrance when IDLE

The low-power mode can only be entered when the application is in Idle mode.
To do so, UTIL_LPM_EnterLowPower has to be called.