Which low power MCU is really the lowest power?

With energy efficiency having replaced processing power as the key selection criterion in many embedded applications, most microcontroller manufacturers have at one time or another claimed to be producing the ‘world’s lowest power’ device. But in the real world, with widely varying applications, meaningful direct comparisons between low power microcontrollers are elusive.

The reason that claims to be the world’s lowest power are often greatly overstated is that different devices have different strengths. Some have really low power sleep modes, while some reduce the active power consumption. Some might have multiple sleep modes with different levels of power consumption that use different subsets of the device’s features, switching the rest off to save power.

Devices with different strengths will of course suit different applications. A successful comparison will consider how long the microcontroller will be in each of the states, that is, what is the duty cycle like. Something like a metering application might spend the vast majority of its time in the standby mode, only waking up once an hour to take a reading. This will be very different to an industrial process sensor system which spends most of its time awake.

Comparisons between devices should therefore be made by examining exactly how devices will be used in the particular application being considered. It’s essential to examine the standby power, the active power, and any power that is used while the device is transitioning between the two states. If there is more than one standby mode, you’ll need to know how much power will be consumed in all these states.

 

Active mode

In active mode, the power consumed by the device is the product of the supply voltage and the current consumed. The current consumed depends on switching frequency of the CMOS gates and the supply voltage, so it’s usually quoted as a per-MHz value, and at a certain supply voltage to normalise it for better comparison. For example, TI’s popular MSP430 series of low power MCUsoffers several active modes. For the MSP430FR6879/68791/6877, these modes consume between 375 and 100µA/MHz at 3.0Vcc, depending on whether FRAM or SRAM needs to be used.

Since the power consumed is proportional to the square of the supply voltage, pay particular attention to the supply voltage conditions quoted on the datasheet. Devices will appear to consume less power at the lower end of their range – for battery powered portable devices this may be 1.8V, but when the batteries are fully charged, they supply the full 3V and the current consumption may not be as attractive. Some devices, such as Silicon Labs’ EFM8 Busy Bee series, have a built-in LDO (low drop-out) voltage regulator which maintains the voltage supplied to the rest of the device at a lower level, even when the batteries are fully charged.

 

Sleep mode

Manufacturers will typically quote the current consumption of their lowest power mode as ‘sleep mode’, but for different devices that can mean many different things. Many have several different modes (perhaps ‘sleep’, ‘deep sleep’, ‘standby’, etc.), which switch off more and more parts of the device. So what needs to be checked is that the sleep modes of different devices being compared all offer enough functionality for your application while in that mode (for example, is memory retention required? What about the real time clock?).

For example, Microchip’s nanoWatt XLP series boasts very low 9nA power consumption in its deepest sleep mode, but if brown-out reset is required, it consumes 45nA. If a watchdog timer is needed, that jumps to 200nA, while a mode that runs a real time clock/calendar consumes 400nA. This series of devices is designed for coin-cell battery powered sensor nodes and other ultra low power devices.

The STM8L101 series of 8-bit low power MCUs from STMicroelectronics is designed for medical devices, alarms and door locks, metering applications and portable electronic devices. It consumes 350nA in its lowest power mode. However, there are several modes: run, wait, active-halt and halt. Run is the active power mode. In wait mode, the CPU is stopped but the peripherals are kept running while the device is waiting for an external event. In active-halt mode, the CPU is stopped, but auto-wakeup and the independent watchdog are kept running (if they are enabled). In halt mode, the CPU and peripheral clocks are completely stopped. See the table below for a full explanation of what is switched off in which mode for ST8L and ST8AL devices.

 

Wakeup time

Aside from the power consumed in the different modes, the power consumed when the device is in neither state, that is, during transition, may also be significant. Energy expended while the device is waking up is wasted. The time it takes to transition between the modes should therefore be noted; devices with a fast wake-up time, which spend minimal time in transition, are desirable. For example, for the ST8L101 series mentioned above, the supply current during wake-up time from active-halt mode is 2µA, and the wake-up time from active-halt mode to run mode is 4µs.

In summary, the power consumption of the device must be examined for all the operating modes you will be using, as well as how much power is used when transitioning between these modes. To compare devices for a real-world application, consider which of the device’s low power modes meets the required functionality and how long the device will spend in each mode.