5. Power controller (PWR)

5.1 Power supplies

There are two main power supply schemes:

• • VDD = 1.7 to 3.6 V: external power supply for I/Os with the internal regulator disabled, provided externally through VDD pins. Requires the use of an external power supply supervisor connected to the VDD and PDR_ON pins.
• • VDD = 1.8 to 3.6 V: external power supply for I/Os and the internal regulator (when enabled), provided externally through VDD pins.
• • \( V_{DD\_USB} \) = 3.0 to 3.6 V
  \( V_{DD\_USB} \) is a dedicated independent USB power supply for full speed transceivers.

Note: \( V_{DD\_USB} \) value does not dependent on \( V_{DD} \) and \( V_{DDA} \) . However, \( V_{DD\_USB} \) must be the last supply to be delivered to the device and the first to be switched off. When the three power supplies are shut down, if \( V_{DD\_USB} \) remains active for a short period of time and \( V_{DDA}/V_{DDIO} \) fall below the functional range, the device is not be damaged.

The device is still functional when \( V_{DD\_USB} \) is switched off.

The real-time clock (RTC), and the RTC backup registers can be powered from the \( V_{BAT} \) voltage when the main \( V_{DD} \) supply is powered off.

Note: Depending on the operating power supply range, some peripheral may be used with limited functionality and performance. For more details refer to section "General operating conditions" in the datasheet.

Figure 8. Power supply overview

1. 1. V _DDA and V _SSA must be connected to V _DD and V _SS , respectively.

5.1.1 Independent A/D converter supply and reference voltage

To improve conversion accuracy, the ADC has an independent power supply which can be separately filtered and shielded from noise on the PCB.

• • The ADC voltage supply input is available on a separate V _DDA pin.
• • An isolated supply ground connection is provided on pin V _SSA .

To ensure a better accuracy of low voltage inputs, the user can connect a separate external reference voltage ADC input on V _REF . The voltage on V _REF ranges from 1.7 V to V _DDA .

5.1.2 Battery backup domain

Backup domain description

To retain the content of the RTC backup registers and supply the RTC when V _DD is turned off, V _BAT pin can be connected to an optional standby voltage supplied by a battery or by another source.

To allow the RTC to operate even when the main digital supply ( \( V_{DD} \) ) is turned off, the \( V_{BAT} \) pin powers the following blocks:

• • The RTC
• • The LSE oscillator
• • PC13 to PC15 I/Os

The switch to the \( V_{BAT} \) supply is controlled by the power-down reset embedded in the Reset block.

Warning: During \( t_{RSTTEMPO} \) (temporization at \( V_{DD} \) startup) or after a PDR is detected, the power switch between \( V_{BAT} \) and \( V_{DD} \) remains connected to \( V_{BAT} \) .
During the startup phase, if \( V_{DD} \) is established in less than \( t_{RSTTEMPO} \) (Refer to the datasheet for the value of \( t_{RSTTEMPO} \) ) and \( V_{DD} > V_{BAT} + 0.6 \) V, a current may be injected into \( V_{BAT} \) through an internal diode connected between \( V_{DD} \) and the power switch ( \( V_{BAT} \) ).
If the power supply/battery connected to the \( V_{BAT} \) pin cannot support this current injection, it is strongly recommended to connect an external low-drop diode between this power supply and the \( V_{BAT} \) pin.

If no external battery is used in the application, it is recommended to connect \( V_{BAT} \) to \( V_{DD} \) supply and add a 100 nF ceramic decoupling capacitor on \( V_{BAT} \) pin.

When the backup domain is supplied by \( V_{DD} \) (analog switch connected to \( V_{DD} \) ), the following functions are available:

• • PC14 and PC15 can be used as either GPIO or LSE pins
• • PC13 can be used as a GPIO or additional functions can be configured (refer to Table 27: RTC additional functions for more details about this pin configuration)

Note: Due to the fact that the switch only sinks a limited amount of current (3 mA), the use of PC13 to PC15 GPIOs in output mode is restricted: the speed has to be limited to 2 MHz with a maximum load of 30 pF and these I/Os must not be used as a current source (e.g. to drive an LED).

When the backup domain is supplied by \( V_{BAT} \) (analog switch connected to \( V_{BAT} \) because \( V_{DD} \) is not present), the following functions are available:

• • PC14 and PC15 can be used as LSE pins only
• • PC13 can be used as the RTC additional function pin (refer to Table 27: RTC additional functions for more details about this pin configuration)

Backup domain access

After reset, the backup domain (RTC registers, and RTC backup register) is protected against possible unwanted write accesses. To enable access to the backup domain, proceed as follows:

1. • • Access to the RTC and RTC backup registers
2. 1. 1. Enable the power interface clock by setting the PWREN bits in the RCC_APB1ENR register (see Section 6.3.12: RCC AHB3 peripheral clock enable register (RCC_AHB3ENR) )
   2. 2. Set the DBP bit in the Section 5.4.1 to enable access to the backup domain
   3. 3. Select the RTC clock source: see Section 6.2.8: RTC/AWU clock
   4. 4. Enable the RTC clock by programming the RTCEN [15] bit in the Section 6.3.20: RCC Backup domain control register (RCC_BDCR)

RTC and RTC backup registers

The real-time clock (RTC) is an independent BCD timer/counter. The RTC provides a time-of-day clock/calendar, two programmable alarm interrupts, and a periodic programmable wakeup flag with interrupt capability. The RTC contains 20 backup data registers (80 bytes) which are reset when a tamper detection event occurs. For more details refer to Section 22: Real-time clock (RTC) .

5.1.3 Voltage regulator

An embedded linear voltage regulator supplies all the digital circuitries except for the backup domain and the Standby circuitry. The regulator output voltage is around 1.2 V.

This voltage regulator requires one or two external capacitors to be connected to one or two dedicated pins, \( V_{CAP\_1} \) and for some packages \( V_{CAP\_2} \) . Specific pins must be connected either to \( V_{SS} \) or \( V_{DD} \) to activate or deactivate the voltage regulator. These pins depend on the package.

When activated by software, the voltage regulator is always enabled after Reset. It works in three different modes depending on the application modes.

• • In Run mode , the regulator supplies full power to the 1.2 V domain (core, memories and digital peripherals). In this mode, the regulator output voltage (around 1.2 V) can be scaled by software to different voltage values, Scale 1, scale 2, or scale 3, that can be configured through the VOS[1:0] bits of the PWR_CR register. After reset the VOS register is set to scale 2. When the PLL is OFF, the voltage regulator is set to scale 3 independently of the VOS register content. The VOS register content is only taken into account once the PLL is activated and the HSI or HSE is selected as clock source.

The voltage scaling allows optimizing the power consumption when the device is clocked below the maximum system frequency.

• • In Stop mode , the main regulator or the low-power regulator supplies low power to the 1.2 V domain, thus preserving the content of registers and internal SRAM. The voltage regulator can be put either in main regulator mode (MR) or in low-power mode (LPR). The programmed voltage scale remains the same during Stop mode:

Voltage scale 3 is automatically selected when the microcontroller enters Stop mode (see Section 5.4.1: PWR power control register (PWR_CR) ).

• • In Standby mode , the regulator is powered down. The content of the registers and SRAM are lost except for the Standby circuitry and the backup domain.

Note: For more details, refer to the voltage regulator section in the STM32F412xx datasheet.

5.2 Power supply supervisor

5.2.1 Power-on reset (POR)/power-down reset (PDR)

The device has an integrated POR/PDR circuitry that allows proper operation starting from 1.8 V.

To use the device below 1.8 V, the internal power supervisor must be switched off using the PDR_ON pin (please refer to section Power supply supervisor of the STM32F412xx datasheet). The device remains in Reset mode when \( V_{DD}/V_{DDA} \) is below a specified threshold, \( V_{POR/PDR} \) , without the need for an external reset circuit. For more details concerning the power on/power-down reset threshold, refer to the electrical characteristics of the datasheet.

Figure 9. Power-on reset/power-down reset waveform

5.2.2 Brownout reset (BOR)

During power on, the Brownout reset (BOR) keeps the device under reset until the supply voltage reaches the specified \( V_{BOR} \) threshold.

\( V_{BOR} \) is configured through device option bytes. By default, BOR is off. 3 programmable \( V_{BOR} \) threshold levels can be selected:

• • BOR Level 3 ( \( V_{BOR3} \) ). Brownout threshold level 3.
• • BOR Level 2 ( \( V_{BOR2} \) ). Brownout threshold level 2.
• • BOR Level 1 ( \( V_{BOR1} \) ). Brownout threshold level 1.

Note: For full details about BOR characteristics, refer to the "Electrical characteristics" section in the device datasheet.

When the supply voltage ( \( V_{DD} \) ) drops below the selected \( V_{BOR} \) threshold, a device reset is generated.

The BOR can be disabled by programming the device option bytes. In this case, the power-on and power-down is then monitored by the POR/ PDR or by an external power supervisor if the PDR is switched off through the PDR_ON pin (see Section 5.2.1: Power-on reset (POR)/power-down reset (PDR) ).

The BOR threshold hysteresis is \( \sim 100 \) mV (between the rising and the falling edge of the supply voltage).

Figure 10. BOR thresholds

5.2.3 Programmable voltage detector (PVD)

You can use the PVD to monitor the \( V_{DD} \) power supply by comparing it to a threshold selected by the PLS[2:0] bits in the PWR power control register (PWR_CR) .

The PVD is enabled by setting the PVDE bit.

A PVDO flag is available, in the PWR power control/status register (PWR_CSR) , to indicate if \( V_{DD} \) is higher or lower than the PVD threshold. This event is internally connected to the EXTI line 16 and can generate an interrupt if enabled through the EXTI registers. The rising/falling edge sensitivity of the EXTI line 16 should be configured according to PVD output behavior. For example, if the EXTI line 16 is configured to rising edge sensitivity, the interrupt will be generated when \( V_{DD} \) drops below the PVD threshold. As an example the service routine could perform emergency shutdown tasks.

Figure 11. PVD thresholds

5.3 Low-power modes

By default, the microcontroller is in Run mode after a system or a power-on reset. In Run mode the CPU is clocked by HCLK and the program code is executed. 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 wakeup sources.

The devices feature four low-power modes:

• • Sleep mode (Cortex ^® -M4 with FPU core stopped, peripherals kept running)
• • Stop mode (all clocks are stopped)
• • Standby mode (1.2 V domain powered off)
• • Batch acquisition mode (BAM): the devices are in Sleep mode, the flash memory is off, needed peripheral are kept running, data transfer are still possible through DMA.

In addition, the power consumption in Run mode can be reduce by one of the following means:

• • Slowing down the system clocks
• • Gating the clocks to the APBx and AHBx peripherals when they are unused.

Entering low-power mode

Low-power modes are entered by the MCU by executing the WFI (Wait For Interrupt), or WFE (Wait for Event) instructions, or when the SLEEPONEXIT bit in the Cortex ^® -M4 with FPU System Control register is set on Return from ISR.

Entering Low-power mode through WFI or WFE will be executed only if no interrupt is pending or no event is pending.

Exiting low-power mode

The MCU exits from Sleep and Stop modes low-power mode depending on the way the low-power mode was entered:

• • If the WFI instruction or Return from ISR was used to enter the low-power mode, any peripheral interrupt acknowledged by the NVIC can wake up the device.
• • If the WFE instruction is used to enter the low-power mode, the MCU exits the low-power mode as soon as an event occurs. The wakeup event can be generated either by:
  • – NVIC IRQ interrupt:

When SEVONPEND = 0 in the Cortex ^® -M4 with FPU System Control register: by enabling an interrupt in the peripheral control register and in the NVIC. When the MCU resumes from WFE, the peripheral interrupt pending bit and the NVIC peripheral IRQ channel pending bit (in the NVIC interrupt clear pending register) have to be cleared. Only NVIC interrupts with sufficient priority will wakeup and interrupt the MCU.

When SEVONPEND = 1 in the Cortex ^® -M4 with FPU System Control register: by enabling an interrupt in the peripheral control register and optionally in the NVIC. When the MCU resumes from WFE, the peripheral interrupt pending bit and when enabled the NVIC peripheral IRQ channel pending bit (in the NVIC interrupt clear pending register) have to be cleared. All NVIC interrupts will wakeup the MCU, even the disabled ones. Only enabled NVIC interrupts with sufficient priority will wakeup and interrupt the MCU.

• – Event

This is done by configuring a EXTI line in event mode. When the CPU resumes from WFE, it is not necessary to clear the EXTI peripheral interrupt pending bit or the NVIC IRQ channel pending bit as the pending bits corresponding to the event line is not set. It may be necessary to clear the interrupt flag in the peripheral.

The MCU exits from Standby low-power mode through an external reset (NRST pin), an IWDG reset, a rising edge on one of the enabled WKUPx pins or a RTC event occurs (see Figure 207: RTC block diagram ).

After waking up from Standby mode, program execution restarts in the same way as after a Reset (boot pin sampling, option bytes loading, reset vector is fetched, etc.).

Only enabled NVIC interrupts with sufficient priority will wakeup and interrupt the MCU.

Table 15. Low-power mode summary

1. Refer to Section 5.3.4: Batch acquisition mode for specific BAM entry and exit requirements.

5.3.1 Slowing down system clocks

In Run mode the speed of the system clocks (SYSCLK, HCLK, PCLK1, PCLK2) can be reduced by programming the prescaler registers. These prescalers can also be used to slow down peripherals before entering Sleep mode.

For more details refer to Section 6.3.3: RCC clock configuration register (RCC_CFGR) .

5.3.2 Peripheral clock gating

In Run mode, the HCLKx and PCLKx for individual peripherals and memories can be stopped at any time to reduce power consumption.

To further reduce power consumption in Sleep mode the peripheral clocks can be disabled prior to executing the WFI or WFE instructions.

Peripheral clock gating is controlled by the AHB1 peripheral clock enable register (RCC_AHB1ENR), AHB2 peripheral clock enable register (RCC_AHB2ENR) (see Section 6.3.10: RCC AHB1 peripheral clock enable register (RCC_AHB1ENR) , Section 6.3.11: RCC AHB2 peripheral clock enable register (RCC_AHB2ENR) and Section 6.3.12: RCC AHB3 peripheral clock enable register (RCC_AHB3ENR) ).

Disabling the peripherals clocks in Sleep mode can be performed automatically by resetting the corresponding bit in RCC_AHBxLPENR and RCC_APBxLPENR registers.

5.3.3 Sleep mode

Entering Sleep mode

The Sleep mode is entered according to Entering low-power mode , when the SLEEPDEEP bit in the Cortex ^® -M4 with FPU System Control register is cleared.

Refer to Table 16 and Table 17 for details on how to enter Sleep mode.

Note: All interrupt pending bits must be cleared before the sleep mode entry.

Exiting Sleep mode

The Sleep mode is exited according to Exiting low-power mode .

Refer to Table 16 and Table 17 for more details on how to exit Sleep mode.

Table 16. Sleep-now entry and exit

Table 17. Sleep-on-exit entry and exit

Table 17. Sleep-on-exit entry and exit (continued)

5.3.4 Batch acquisition mode

Entering BAM

The BAM is entered according to Section : Entering low-power mode , when the SLEEPDEEP bit in the Cortex ^® -M4 with FPU System Control register is cleared.

Refer to Table 18 and Table 19 for details on how to enter Sleep mode.

Before entering Sleep mode, the flash memory must be configured by software to operate in the required low-power mode. If data need to be transferred from peripheral to RAM during BAM, the DMA must be enabled before entering Sleep mode.

Exiting BAM

The BAM is exited according to Section : Exiting low-power mode .

Refer to Table 18 and Table 19 for more details on how to exit Sleep mode.

After waking up from BAM, the flash memory must first to be waked up if code execution restarts from flash memory.

This wakeup time must be managed by software running from the internal SRAM.

Table 18. BAM-now entry and exit

Table 19. BAM-on-exit entry and exit

5.3.5 Stop mode

The Stop mode is based on the Cortex®-M4 with FPU deepsleep mode combined with peripheral clock gating. The voltage regulator can be configured either in normal or low-power mode. In Stop mode, all clocks in the 1.2 V domain are stopped, the PLLs, the HSI and the HSE RC oscillators are disabled. Internal SRAM and register contents are preserved.

Some settings in the PWR_CR register allow to further reduce the power consumption. When the flash memory is in power-down mode, an additional startup delay is incurred when waking up from Stop mode (see Table 20: Stop operating modes and Section 5.4.1: PWR power control register (PWR_CR) ).

Table 20. Stop operating modes

Entering Stop mode

The Stop mode is entered according to Section : Entering low-power mode , when the SLEEPDEEP bit in the Cortex ^® -M4 with FPU System Control register is set.

Refer to Table 21 for details on how to enter the Stop mode.

To further reduce power consumption in Stop mode, the internal voltage regulator can be put in low-power mode. This is configured by the LPDS bit of the PWR power control register (PWR_CR) .

If flash memory programming is ongoing, the Stop mode entry is delayed until the memory access is finished.

If an access to the APB domain is ongoing, The Stop mode entry is delayed until the APB access is finished.

In Stop mode, the following features can be selected by programming individual control bits:

• • Independent watchdog (IWDG): the IWDG is started by writing to its Key register or by hardware option. Once started it cannot be stopped except by a Reset. See Section 21.3 in Section 21: Window watchdog (WWDG) .
• • Real-time clock (RTC): this is configured by the RTCEN bit in the Section 6.3.20: RCC Backup domain control register (RCC_BDCR)
• • Internal RC oscillator (LSI RC): this is configured by the LSION bit in the Section 6.3.21: RCC clock control & status register (RCC_CSR) .
• • External 32.768 kHz oscillator (LSE OSC): this is configured by the LSEON bit in the Section 6.3.20: RCC Backup domain control register (RCC_BDCR) .

The ADC can also consume power during the Stop mode, unless it is disabled before entering it. To disable it, the ADON bit in the ADC_CR2 register must be written to 0.

Note: If the application needs to disable the external clock before entering Stop mode, the HSEON bit must first be disabled and the system clock switched to HSI.

Otherwise, if the HSEON bit is kept enabled while the external clock (external oscillator) can be removed before entering stop mode, the clock security system (CSS) feature must be enabled to detect any external oscillator failure and avoid a malfunction behavior when entering stop mode.

Exiting Stop mode

The Stop mode is exited according to Section : Exiting low-power mode .

Refer to Table 21 for more details on how to exit Stop mode.

When exiting Stop mode by issuing an interrupt or a wakeup event, the HSI RC oscillator is selected as system clock.

When the voltage regulator operates in low-power mode, an additional startup delay is incurred when waking up from Stop mode. By keeping the internal regulator ON during Stop mode, the consumption is higher although the startup time is reduced.

Table 21. Stop mode entry and exit

5.3.6 Standby mode

The Standby mode allows to achieve the lowest power consumption. It is based on the Cortex ^® -M4 with FPU deepsleep mode, with the voltage regulator disabled. The 1.2 V domain is consequently powered off. The PLLs, the HSI oscillator and the HSE oscillator are also switched off. SRAM and register contents are lost except for registers in the backup domain (RTC registers and RTC backup register), and Standby circuitry (see Figure 8 ).

Entering Standby mode

The Standby mode is entered according to Section : Entering low-power mode , when the SLEEPDEEP bit in the Cortex ^® -M4 with FPU System Control register is set.

Refer to Table 22 for more details on how to enter Standby mode.

In Standby mode, the following features can be selected by programming individual control bits:

• • Independent watchdog (IWDG): the IWDG is started by writing to its Key register or by hardware option. Once started it cannot be stopped except by a reset. See Section 20.3 in Section 20: Independent watchdog (IWDG) .
• • Real-time clock (RTC): this is configured by the RTCEN bit in the backup domain control register (RCC_BDCR)
• • Internal RC oscillator (LSI RC): this is configured by the LSION bit in the Control/status register (RCC_CSR).
• • External 32.768 kHz oscillator (LSE OSC): this is configured by the LSEON bit in the backup domain control register (RCC_BDCR)

Exiting Standby mode

The Standby mode is exited according to Section : Exiting low-power mode . The SBF status flag in PWR_CR (see Section 5.4.2: PWR power control/status register (PWR_CSR) ) indicates that the MCU was in Standby mode. All registers are reset after wakeup from Standby except for PWR_CR.

Refer to Table 22 for more details on how to exit Standby mode.

Table 22. Standby mode entry and exit

I/O states in Standby mode

In Standby mode, all I/O pins are high impedance except for:

• • Reset pad (still available)
• • RTC_AF1 pin (PC13) if configured for tamper, time stamp, RTC Alarm out, or RTC clock calibration out
• • WKUP pin (PA0/PC0/PC1), if enabled

Debug mode

By default, the debug connection is lost if the application puts the MCU in Stop or Standby mode while the debug features are used. This is due to the fact that the Cortex ^® -M4 with FPU core is no longer clocked.

However, by setting some configuration bits in the DBGMCU_CR register, the software can be debugged even when using the low-power modes extensively. For more details, refer to Section 30.16.1: Debug support for low-power modes .

5.3.7 Programming the RTC alternate functions to wake up the device from the Stop and Standby modes

The MCU can be woken up from a low-power mode by an RTC alternate function.

The RTC alternate functions are the RTC alarms (Alarm A and Alarm B), RTC wakeup, RTC tamper event detection and RTC time stamp event detection.

These RTC alternate functions can wake up the system from the Stop and Standby low-power modes.

The system can also wake up from low-power modes without depending on an external interrupt (Auto-wakeup mode), by using the RTC alarm or the RTC wakeup events.

The RTC provides a programmable time base for waking up from the Stop or Standby mode at regular intervals.

For this purpose, two of the three alternate RTC clock sources can be selected by programming the RTCSEL[1:0] bits in the Section 6.3.20: RCC Backup domain control register (RCC_BDCR) :

• • Low-power 32.768 kHz external crystal oscillator (LSE OSC)
  This clock source provides a precise time base with a very low-power consumption (additional consumption of less than 1 µA under typical conditions)
• • Low-power internal RC oscillator (LSI RC)
  This clock source has the advantage of saving the cost of the 32.768 kHz crystal. This internal RC oscillator is designed to use minimum power.

RTC alternate functions to wake up the device from the Stop mode

• • To wake up the device from the Stop mode with an RTC alarm event, it is necessary to:
  1. a) Configure the EXTI Line 17 to be sensitive to rising edges (Interrupt or Event modes)
  2. b) Enable the RTC Alarm Interrupt in the RTC_CR register
  3. c) Configure the RTC to generate the RTC alarm
• • To wake up the device from the Stop mode with an RTC tamper or time stamp event, it is necessary to:
  1. a) Configure the EXTI Line 21 to be sensitive to rising edges (Interrupt or Event modes)
  2. b) Enable the RTC time stamp Interrupt in the RTC_CR register or the RTC tamper interrupt in the RTC_TAFCR register
  3. c) Configure the RTC to detect the tamper or time stamp event
• • To wake up the device from the Stop mode with an RTC wakeup event, it is necessary to:
  1. a) Configure the EXTI Line 22 to be sensitive to rising edges (Interrupt or Event modes)
  2. b) Enable the RTC wakeup interrupt in the RTC_CR register
  3. c) Configure the RTC to generate the RTC Wakeup event

RTC alternate functions to wake up the device from the Standby mode

• • To wake up the device from the Standby mode with an RTC alarm event, it is necessary to:
  1. a) Enable the RTC alarm interrupt in the RTC_CR register
  2. b) Configure the RTC to generate the RTC alarm
• • To wake up the device from the Standby mode with an RTC tamper or time stamp event, it is necessary to:
  1. a) Enable the RTC time stamp interrupt in the RTC_CR register or the RTC tamper interrupt in the RTC_TAFCR register

• b) Configure the RTC to detect the tamper or time stamp event
• • To wake up the device from the Standby mode with an RTC wakeup event, it is necessary to:
  • a) Enable the RTC wakeup interrupt in the RTC_CR register
  • b) Configure the RTC to generate the RTC wakeup event

Safe RTC alternate function wakeup flag clearing sequence

If the selected RTC alternate function is set before the PWR wakeup flag (WUTF) is cleared, it will not be detected on the next event as detection is made once on the rising edge.

To avoid bouncing on the pins onto which the RTC alternate functions are mapped, and exit correctly from the Stop and Standby modes, it is recommended to follow the sequence below before entering the Standby mode:

• • When using RTC alarm to wake up the device from the low-power modes:
  • a) Disable the RTC alarm interrupt (ALRAIE or ALRBIE bits in the RTC_CR register)
  • b) Clear the RTC alarm (ALRAF/ALRBF) flag
  • c) Clear the PWR Wakeup (WUF) flag
  • d) Enable the RTC alarm interrupt
  • e) Re-enter the low-power mode
• • When using RTC wakeup to wake up the device from the low-power modes:
  • a) Disable the RTC Wakeup interrupt (WUTIE bit in the RTC_CR register)
  • b) Clear the RTC Wakeup (WUTF) flag
  • c) Clear the PWR Wakeup (WUF) flag
  • d) Enable the RTC Wakeup interrupt
  • e) Re-enter the low-power mode
• • When using RTC tamper to wake up the device from the low-power modes:
  • a) Disable the RTC tamper interrupt (TAMPIE bit in the RTC_TAFCR register)
  • b) Clear the Tamper (TAMP1F/TSF) flag
  • c) Clear the PWR Wakeup (WUF) flag
  • d) Enable the RTC tamper interrupt
  • e) Re-enter the low-power mode
• • When using RTC time stamp to wake up the device from the low-power modes:
  • a) Disable the RTC time stamp interrupt (TSIE bit in RTC_CR)
  • b) Clear the RTC time stamp (TSF) flag
  • c) Clear the PWR Wakeup (WUF) flag
  • d) Enable the RTC TimeStamp interrupt
  • e) Re-enter the low-power mode

5.4 Power control registers

5.4.1 PWR power control register (PWR_CR)

Address offset: 0x00

Reset value: 0x0000 8000 (reset by wakeup from Standby mode)

Bits 31:22 Reserved, must be kept at reset value.

Bit 21 FISSR : Flash Interface Stop while System Run

0: Flash Interface clock run (Default value).

1: Flash Interface clock off.

Note: This bit could not be set while executing with the flash itself. It should be done with specific routine executed from RAM.

Bit 20 FMSSR : Flash Memory Sleep System Run.

0: Flash standard mode (Default value)

1: Flash forced to be in STOP or DeepPower Down mode (depending of FPDS value bit) by hardware.

Note: This bit could not be set while executing with the flash itself. It should be done with specific routine executed from RAM.

Bits 19:16 Reserved, must be kept at reset value.

Bits 15:14 VOS[1:0] : Regulator voltage scaling output selection

These bits control the main internal voltage regulator output voltage to achieve a trade-off between performance and power consumption when the device does not operate at the maximum frequency (refer to the corresponding datasheet for more details).

These bits can be modified only when the PLL is OFF. The new value programmed is active only when the PLL is ON. When the PLL is OFF, the voltage regulator is set to scale 3 independently of the VOS register content.

00: Reserved (Scale 3 mode selected)

01: Scale 3 mode <= 64 MHz

10: Scale 2 mode (reset value) <= 84 MHz

11: Scale 1 mode <= 100 MHz

Bit 13 ADCD1 :

0: No effect.

1: Refer to AN4073 for details on how to use this bit.

Note: This bit can only be set when operating at supply voltage range 2.7 to 3.6V and when the Prefetch is OFF.

Bit 12 Reserved, must be kept at reset value.

Bit 11 MRLVDS : Main regulator Low Voltage in Deep Sleep

0: Main regulator in Voltage scale 3 when the device is in Stop mode.

1: Main regulator in Low Voltage and flash memory in Deep Sleep mode when the device is in Stop mode.

Bit 10 LPLVDS : Low-power regulator Low Voltage in Deep Sleep

0: Low-power regulator on if LPDS bit is set when the device is in Stop mode.

1: Low-power regulator in Low Voltage and flash memory in Deep Sleep mode if LPDS bit is set when device is in Stop mode.

Bit 9 FPDS : Flash power-down in Stop mode

When set, the flash memory enters power-down mode when the device enters Stop mode.

This allows to achieve a lower consumption in stop mode but a longer restart time.

0: Flash memory not in power-down when the device is in Stop mode

1: Flash memory in power-down when the device is in Stop mode

Bit 8 DBP : Disable backup domain write protection

In reset state, the RCC_BDCR register, the RTC registers (including the backup registers), and the BRE bit of the PWR_CSR register, are protected against parasitic write access. This bit must be set to enable write access to these registers.

0: Access to RTC and RTC Backup registers.

1: Access to RTC and RTC Backup registers.

Bits 7:5 PLS[2:0] : PVD level selection

These bits are written by software to select the voltage threshold detected by the programmable voltage detector

000: 2.2 V

001: 2.3 V

010: 2.4 V

011: 2.5 V

100: 2.6 V

101: 2.7 V

110: 2.8 V

111: 2.9 V

Note: Refer to the electrical characteristics of the datasheet for more details.

Bit 4 PVDE : Programmable voltage detector enable

This bit is set and cleared by software.

0: PVD disabled

1: PVD enabled

Bit 3 CSBF : Clear standby flag

This bit is always read as 0.

0: No effect.

1: Clear the SBF Standby Flag (write).

Bit 2 CWUF : Clear wakeup flag

This bit is always read as 0.

0: No effect.

1: Clear the WUF Wakeup Flag after 2 System clock cycles.

Bit 1 PDDS : Power-down deepsleep

This bit is set and cleared by software. It works together with the LPDS bit.

0: Enter Stop mode when the CPU enters deepsleep. The regulator status depends on the LPDS bit.

1: Enter Standby mode when the CPU enters deepsleep.

Bit 0 LPDS : Low-power deepsleep

This bit is set and cleared by software. It works together with the PDDS bit.

0: Voltage regulator on during Stop mode.

1: Low-power Voltage regulator on during Stop mode.

5.4.2 PWR power control/status register (PWR_CSR)

Address offset: 0x04

Reset value: 0x0000 0000 (not reset by wakeup from Standby mode)

Additional APB cycles are needed to read this register versus a standard APB read.

Bits 31:15 Reserved, must be kept at reset value.

Bit 14 VOSRDY : Regulator voltage scaling output selection ready bit

0: Not ready

1: Ready

Bits 13:10 Reserved, must be kept at reset value.

Bit 9 BRE : Backup regulator enable

When set, the Backup regulator (used to maintain the backup domain content) is enabled. If BRE is reset, the backup regulator is switched off. Once set, the application must wait that the Backup Regulator Ready flag (BRR) is set to indicate that the data written into the backup registers will be maintained in the Standby and \( V_{BAT} \) modes. The DBP bit of PWR_CR register must be set to 1 before PWR_CSR.BRE can be written.

0: Backup regulator disabled

1: Backup regulator enabled

Note: This bit is not reset when the device wakes up from Standby mode, by a system reset, or by a power reset. This bit is reset by a backup domain reset.

Bit 8 EWUP1 : Enable WKUP1 pin (PA0)

This bit is set and cleared by software.

0: WKUP1 pin is used for general purpose I/O. An event on the WKUP1 pin does not wakeup the device from Standby mode.

1: WKUP1 pin is used for wakeup from Standby mode and forced in input pull down configuration (rising edge on WKUP1 pin wakes-up the system from Standby mode).

Note: This bit is reset by a system reset.

This bit is set and cleared by software.

0: WKUP2 pin is used for general purpose I/O. An event on the WKUP2 pin does not wakeup the device from Standby mode.

1: WKUP2 pin is used for wakeup from Standby mode and forced in input pull down configuration (rising edge on WKUP2 pin wakes-up the system from Standby mode).

Note: This bit is reset by a system reset.

This bit is set and cleared by software.

0: WKUP3 pin is used for general purpose I/O. An event on the WKUP3 pin does not wakeup the device from Standby mode.

1: WKUP3 pin is used for wakeup from Standby mode and forced in input pull down configuration (rising edge on WKUP3 pin wakes-up the system from Standby mode).

Note: This bit is reset by a system reset.

Bits 5:4 Reserved, must be kept at reset value.

Set by hardware to indicate that the Backup Regulator is ready.

0: Backup Regulator not ready

1: Backup Regulator ready

Note: This bit is not reset when the device wakes up from Standby mode or by a system reset or power reset. This bit is reset by a backup domain reset.

This bit is set and cleared by hardware. It is valid only if PVD is enabled by the PVDE bit.

0: V _DD is higher than the PVD threshold selected with the PLS[2:0] bits.

1: V _DD is lower than the PVD threshold selected with the PLS[2:0] bits.

Note: The PVD is stopped by Standby mode. For this reason, this bit is equal to 0 after Standby or reset until the PVDE bit is set.

This bit is set by hardware and cleared only by a POR/PDR (power-on reset/power-down reset) or by setting the CSBF bit in the PWR_CR register.

0: Device has not been in Standby mode

1: Device has been in Standby mode

This bit is set by hardware and cleared either by a system reset or by setting the CWUF bit in the PWR_CR register.

0: No wakeup event occurred

1: A wakeup event was received from the WKUP pin or from the RTC alarm (Alarm A or Alarm B), RTC Tamper event, RTC TimeStamp event or RTC Wakeup).

Note: An additional wakeup event is detected if the WKUP pin is enabled (by setting the EWUP bit) when the WKUP pin level is already high.

5.5 PWR register map

The following table summarizes the PWR registers.

Table 23. PWR - register map and reset values

Refer to Section 2.2 on page 49 for the register boundary addresses.