The RTC is designed to minimize the power consumption and can be configured to optimize further its consumption.
Use the right power reduction mode
The RTC can be active in the following low-power modes (from the biggest current consumer to the smallest):
Sleep mode
Stop mode if the RTC clock is provided by LSE or LSI
Standby mode if the RTC clock is provided by LSE or LSI
Shutdown mode if the RTC clock is provided by LSE
The precise list of modes compatible with the product and their consumption are listed in product reference manual and datasheet.
Thanks to the RTC wakeup unit, the user can wake up the STM32 MCU from this mode. Depending on the application needs in term of low-power phases and current consumption target, the adequate low-power mode and wakeup frequency must be selected.
Use internal pull-up resistor on tamper pin
The internal pull-up resistor in the tamper input pad is only applied for a short period of time (1, 2, 4 or 8 RTCCLK cycles). Using this RTC internal pull-up resistor instead of standard I/O pull-up/pull-down or external pull-up/pull-down ensures a lowest power consumption.
The trade-off between the tamper detection latency and the power consumption by the RTC internal pull-up can be optimized using TAMPFREQ[2:0], that determines the frequency of the tamper sampling from 128 Hz to 1 Hz when RTCCLK equals 32768 Hz.
Note:
Refer to the product datasheet/reference manual for the availability/electrical characteristics of the pull‑up resistors.
Set RTC prescalers
The prescalers used for the calendar are divided into an asynchronous and a synchronous prescalers. Increasing the PREDIV_A value of the asynchronous prescaler while reducing the synchronous prescaler accordingly to keep the 1 Hz output, reduces the RTC power consumption.
On RTC3 type, the RTC dynamic consumption is optimized for LPCAL = 1 and PREDIV_A + 1 being a power of two (considering the RTC as the only peripheral using LSE).
External optimization factors
The prescaler linked to the LSI clock can also optimize the consumption. The user needs to select the LSI as RTC clock source. This prescaler divides LSI by 128 to get a rough 250 Hz clock instead of the former 32 kHz. This is activated by LSIPRE in RCC_CSR.
Concerning the LSE, users can select a crystal that allows setting the driving strength to the lowest level. The driving strength is determined by the chosen crystal parameters, such as load capacitance, ESR, and shunt capacitance, which define a critical transconductance (gmcrit). For more information, see AN2867.
Reducing the driving strength to the lowest level can lower consumption to a typical 250 nA, compared to 630 nA for high drive capability.
Set LSEDRV to 00 in RCC_BDCR to select the lowest driving strength.
Low-power management firmware examples
The RTC comes with a set of example projects so that the user can quickly become familiar with this peripheral. Refer to the STM32Cube MCU Package of the product for a complete projects list.
For example, for the small STM32L4 product line, the user can find the following projects concerning low-power management:
Analogous examples are available for STM32F0, STM32F2, STM32F3, STM32F4, STM32F7, STM32G4, STM32H7, STM32L0, STM32L1, STM32U3, STM32U5, STM32WB and STM32WL. If one example is not available in the STM32Cube for a given STM32 MCU, the user can adapt it.
RTC3 secure and privileged protection modes
Some STM32 MCUs can run in secure or nonsecure mode. Each mode having its own environment, allowing a trustful zone, not accessible in nonsecure mode, to be maintained in secure mode (TrustZone® feature).
A second level of restriction and security is available with part of each environment associated to a privileged or unprivileged mode.
On some products (see Table 4 Advanced features for RTC2 type and AN4759_Adv-RTC-features.html#id_33173__table_hqc_shf_rpb), the registers programming allow the user to restrict the read/write operation on RTC/TAMP registers to secure or privileged modes. Some configuration bits establish the restriction only for some RTC registers. These registers/bits to program are product dependent, then not detailed here (refer to the product reference manual).
Note:
The backup registers and the monotonic counter have their own protection mode settings.
STM32L4 API and tamper detection application example
STM32CubeL4 firmware libraries for tamper detection
The RTC comes with:
A firmware driver API abstracting the RTC features for the end user
Refer to stm32l4xx_hal_rtc.c and stm32l4xx_hal_rtc_ex.c files in \STM32Cube_FW_L4_Vx.y.z\Drivers\STM32L4xx_HAL_Driver\
A set of example projects in \STM32Cube_FW_L4_Vx.y.z\Projects\STM32L476RG-Eval\Examples\RTC
X-CUBE-RTC for tamper detection
X-CUBE-RTC shows an example where a real-time clock must be maintained alive while using as low power as possible (STM32CubeExpansion_AN4759_RTC_STM32L_Vx.y.z\Projects\STM32L476G_EVAL\RTC_TamperVBATT).
The firmware implements:
Hints to ensure a low current consumption
A display in the debugger (date and time, timestamp date and time) and on LEDs (power-up events, reset events, tamper events, error)
A display on the TFT screen, using STemWin library
Tampering detection capability, both when the main power supply (VDD) is present and when the RTC is battery powered
The ability to timestamp the tampering event
The ability to erase the backup registers that may contain sensitive data upon tamper detection
The application flowchart is shown in the figure below.Figure 18. Application example flowchart
Tamper detection application example (STM32L4)
Hardware setup
In order to use the application example project, the user needs a STM32L476G evaluation board (STM32L476G‑EVAL).Figure 19. STM32L476G-EVAL board
Supply the board using the USB cable and tie PA0 (tamper pin 2) to 0, with the following actions:
Remove the jumper JP7.
Connect the CN7 pin 37 to GND.
Note:
PC13 (tamper pin 1) connected to the blue push-button is not used in this demonstration firmware as it is connected to an external pull-down resistor preventing the use of the tamper detection on level with internal pull-up (the lowest consumption mode).
For more information, refer to the user manual Evaluation board with STM32L476ZGT6 MCU (UM1855).
Software setup
Open the project under the user favorite IDE (integrated development environment). For IAR Embedded Workbench®, open the project.eww file. For Keil® MDK-‑ARM, open the project.uvprojx file, compile and launch the debug session. For SW4STM32, open the SW4STM32 toolchain, browse to the SW4STM32 workspace directory, select and import the project RTC_TamperVBATT. Launching the debug session loads the program in the internal Flash memory and executes it.
In IAR Embedded Workbench debug session, the user can view the calendar and tamper event timestamp (time and date) by opening a Live watch window (View\Live watch) and observe the following global variables:
aShowTime
aShowDate
aShowTimeStampTime
aShowTimeStampDate
The same observation can be done using MDK-ARM development tools. To open the Watch1 window, do View\Watch Windows\Watch1.
LED meaning
LD1 (green): tamper event detected
TFT displays: the tamper date and tamper time are different from their initialization state.
LD2 (orange): power-on reset occurred
TFT displays: the tamper date and tamper time are at their initialization state.
LD3 (red): error (RTC or RCC configuration error, backup registers not erased)
TFT displays: “error occurred” is displayed.
LD4 (blue): reset occurred
TFT displays: the tamper date and tamper time are at their initialization state.
Tamper detection during normal operation
Disconnect the debugger. A power-on reset can be generated by removing the JP17 jumper and placing it again on ST‑LINK.
Open PA0 (input is now floating), the LD1 lights showing that a tamper event is detected. On TFT, the tampering date and time are updated with the timestamp of the tamper event.
Note:
A long delay (up to 2 seconds) may be observed before the tamper event is detected. This is due to the trade-off in the tamper detection frequency (TAMPFREQ) chosen to ensure the lowest power consumption.
A reset can be applied by pushing the black reset button. The application is now able the detect a new tamper event.
Tamper detection when main power supply is off
If the user supplies the VBAT pin with the external CR1220 battery (JP12 on BAT position), the tamper event can be detected even if the main power supply is off, with the following steps:
Remove the JP17 jumper.
Open PA0.
Tie PA0 to GND (simulating that the end-user takes care to close the box containing the electronics before supplying it again).
Set JP17 on STlk.
The LD1 lights showing that a tamper event has been detected during the power-down. On TFT, the tamper date and tamper time are updated with the timestamp of the tamper event
Note:
If the user supplies the VBAT pin with 3 V (JP12 on VDD position), the RTC is no longer supplied when the main power supply is switched off. The application is no longer able to detect the tampering event while the main power supply is off.
