Low Power Modes in STM32
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. STM32 have 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. Today in this tutorial we are going to discuss these modes.
There are 3 Low Poer Modes available in STM32, and they are as follows
- SLEEP MODE -> FPU core stopped, peripherals kept running
- STOP MODE -> all clocks are stopped
- STANDBY MODE -> 1.2 V domain powered off
I will first start with the simplest one, which is SLEEP MODE. In this mode, CPU CLK is turned OFF and there is no effect on other clocks or analog clock sources. The current consumption is HIGHEST in this mode, compared to other Low Power Modes.
In order to enter the SLEEP MODE, we must disable the systick interrupt first, or else this interrupt will wake the MCU every time the interrupt gets triggered.
Next, we will enter the sleep mode by executing the WFI (Wait For Interrupt), or WFE (Wait For Event) instructions. If the WFI instruction was used to enter the SLEEP MODE, any peripheral interrupt acknowledged by the NVIC can wake up the device.
As mentioned above, I have entered the SLEEP MODE using the WFI instruction, so the device will wakeup whenever any interrupt is triggered.
Inside the callback function we can resume the systick, so that we can use the delay function again for the rest of the code.
This is another feature available in SLEEP MODE, where the MCU will wake up when the interrupt is triggered, it will process the ISR, and go back to sleep when the ISR exits. This is useful when we want the controller to run only in the interrupt mode.
The above function must be called before going into the SLEEP MODE to activate the sleeponexit feature. We can disable it by calling
The entire code for the SLEEP MODE, with SLEEPONEXIT feature is shown below
Below is the result of the above code.
Basically, when the MCU wakes up because of the UART interrupt, it goes back to sleep after processing the ISR (i.e after printing the string), but when it wakes up due to the EXTI, the SLEEPONEXIT is disabled, and rest of the main function is executed as usual.
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. STOP MODE have may different categories depending on what should be turned off. Below is the picture from the STM32F446RE reference manual
Just like sleep mode, In order to enter the STOP MODE, we must disable the systick interrupt, or else this interrupt will wake the MCU every time the interrupt gets triggered.
Next, we will enter the sleep mode by executing the WFI (Wait For Interrupt), or WFE (Wait For Event) instructions. If the WFI instruction was used to enter the STOP MODE, EXTI, Independent watchdog (IWDG), or RTC can wake up the device. Also I am using the first mode as shown in the picture above i.e. The main Regulator will be turned off and only the LOW Power Regulator will be running.
As mentioned above, I have entered the STOP MODE using the WFI instruction, so the device will wakeup whenever any interrupt is triggered by an EXTI, Independent watchdog (IWDG), or RTC.
We must reconfigure the SYSTEM CLOCKS after wakeup, as they were disabled when entering the STOP MODE. Also don’t forget to resume the systick.
SleeponExit works the same way as it does for the SLEEP mode. You can check the implementation above under the SLEEP mode.
The entire code for STOP MODE is as shown below
The RTC Setup is very long, and it is explained in the VIDEO. DO check the video below to understand it.
When the MCU enters the STOP MODE, it can be woken up by either RTC periodic trigger, or by EXTI line. When the former wakes the MCU, the interrupt is processed, where it will print the string, and than goes back in the STOP MODE. Whereas, when the EXTI line wakes the device, the sleeponexit will be disabled, and the main loop will run.
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.
Before entering the STANDBY MODE, we must disable the wakeup flags as shown below
Next, enable the wakeup pin, or the RTC periodic wakeup (if you want to use RTC)
and finally enter the STANDBY MODE
The standby wakeup is same as a system RESET. The entire code runs from the beginning just as if it was a RESET. The only difference between a reset and a STANDBY wakeup is that, when the MCU wakesup, The SBF status flag in the PWR power control/status register (PWR_CSR) is set. The wakeup can be triggered by WKUP pin rising edge, RTC alarm (Alarm A and Alarm B), RTC wakeup, tamper event, time stamp event, external reset in NRST pin, IWDG reset.
The entire code for the STANDBY MODE is as shown below
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