MCU key metric: current consumption in low-power mode

In applications such as wearable devices and IoT terminals, power consumption is a very important and demanding design consideration, which requires engineers to choose low-power MCUs and other electronic devices to improve the device's endurance. So, what is a low-power MCU?

In simple terms, it is an MCU that supports a low-power mode, requiring engineers to balance low power consumption, short start-up time, and multiple wake-up events to choose a suitable MCU for their design solutions.

Low Power Mode

The MM32F003 supports sleep, shutdown, and standby modes, allowing for an optimal balance between the requirement for low power consumption, short start-up times, and multiple wake-up events. After a system or power reset, the MCU remains running. The clock source at this point provides the clock for the CPU, and the core executes program code. Several low-power modes can reduce power consumption when the CPU does not need to continue running, such as when waiting for an external event.

(1) Sleep mode

In sleep mode, only the CPU is stopped, all peripherals are working, and the CPU can be woken up in case of an interrupt/event.

(2) Shutdown mode

The shutdown mode can achieve the lowest power consumption while keeping the SRAM and register contents from being lost. In shutdown mode, all internal 1.8V parts are stopped, PLL, HSI oscillator, and HSE crystal oscillator are turned off, and the regulator can be placed in normal or low power mode.

The MCU can be woken up from the shutdown mode by any signal configured as EXTI. The EXTI signal can be a wake-up signal from one of the 16 external I/O ports, the output of PVD.

(3) Standby mode

Standby mode enables the lowest power consumption of the system. This mode turns off the voltage regulator during the CPU deep sleep mode, all internal 1.5V parts of the power supply area are disconnected, and both the HSI and HSE oscillators are turned off. It is possible to wake up without reset by the rising edge of the WKUP pin, external reset of NRST pin, IWDG reset, or watchdog timer wakes up.

There are two ways to enter standby mode, and the following standby mode functions can be selected by setting separate control bits.

(1) Independent Watchdog (IWDG): IWDG can be started by writing to the watchdog's key register or hardware selection. Once the independent watchdog is started, it cannot be stopped again except for a system reset.

(2) Internal oscillator (LSI oscillator): It is set by the LSION bit in the control/status register (RCC_CSR).

After entering standby mode, all I/Os go to a high resistance state except for being used to wake up I/Os, and waking up from standby mode is equivalent to resetting the MM32 chip, and the program is executed from scratch again. The contents of SRAM and registers will be lost.

Actual measurement of current consumption

Current consumption is a composite indicator of multiple parameters and factors, which include operating voltage, ambient temperature, load on I/O pins, the software configuration of the product, operating frequency, flip-flop rate of I/O pins, and location of the program in memory, and the code executed.

Maximum current consumption in shutdown and standby modes

(1) All I/O pins are in input mode and connected to a static level-VDD, or VSS (no load).

(2) All peripherals are off unless otherwise noted.

(3) Flash memory access time is adjusted to the frequency of IH CLK (0 wait cycles for 0 to 24MHz, 1 wait cycle for 24 to 48MHz).

(4) Instruction prefetch function is turned on. When the peripheral is turned on: fPCLK1 = IH CLK.

Maximum current consumption in operation mode

(1) All I/O pins are in input mode and connected to a static level-VDD or VSS (no load).

(2) All peripherals are off unless otherwise noted.

(3) Flash memory access time is adjusted to the frequency of IH CLK (0 wait cycle at 0∼24MHz, 1 wait cycle at 24∼48MHz).

(4) Instruction prefetch function is turned on. When the peripheral is turned on: fPCLK1 = IH CLK.

Using the HSI oscillator, the MM32F003 wakes up from sleep mode in less than 4.2μs, shutdown mode in <12μs, and standby mode in <230μs.