MCU Power Saving Design for Optimized LCD Display

Low-power microcontrollers (MCUs) have become the core of many portable systems, but extending battery life while expanding functionality remains a challenge. An effective method to preserve battery life is to run the processor only when necessary, avoiding the power consumption of user interface components such as displays. However, an increasing number of MCUs can selectively shut down parts of the processor while maintaining the liquid crystal display (LCD) image.

Elements of Low-Power Design

Take a water meter, for instance, which needs to display readings on a built-in LCD. However, keeping the MCU system active around the clock is power-consuming. For such products, the LCD should work without direct control from the processor, allowing selective shutdown of parts of the MCU while keeping the LCD image.

The power consumption of any active CMOS electronic circuit can be calculated using the CV²f formula, where C is the total circuit capacitance that needs to charge and discharge within a cycle, V is the power supply voltage, and f is the operating frequency. Thus, the square relationship between supply voltage and power consumption provides significant energy savings.

Another energy-saving method comes from reducing the operating frequency, although this may only impact power rather than energy. If the required number of cycles for algorithm execution remains the same but is spread over a longer time, the overall energy consumption may increase, which is related to leakage current.

Impact of Leakage Current

Semiconductor processes used for low-power MCUs tend to exhibit low leakage current, but the flow of electrons from each transistor to the silicon substrate still slowly consumes battery power. The only way to reduce leakage power is to cut off the power supply to logic blocks, apart from careful process design.

Therefore, MCU power-saving design requires a delicate trade-off between circuit speed and voltage. There is also a third factor—how frequently the MCU needs to be activated within a given time. To avoid wasting energy, the MCU doesn’t simply loop in an idle cycle but enters a sleep state, shutting down most functional units.

MCU Activity Management

Ideally, for metering and monitoring applications, by maintaining a low duty cycle to maximize battery life, the processor spends almost all of its time in a sleep state. For example, an MCU in a utility meter may be in a sleep state for the majority of its lifecycle, waking up only 1% of the time to collect data from sensor inputs.

MCUs designed for low duty cycle environments offer various sleep modes for fine control of power consumption. For instance, a “doze” mode might disable some peripherals, but the core still runs at a low clock rate in an idle loop. In contrast, in “deep sleep” mode, the MCU consumes even less power, with the processor core and almost all peripherals shut down but able to quickly respond to interrupts.

Independent LCD Control from the Processor

Many traditional MCUs shut down the LCD controller when the device enters deep sleep, resulting in a blank screen. However, modern MCU manufacturers recognize the value of maintaining at least static display and allow the LCD to continue running with almost all other MCU parts disabled. Some can even modify the display while the processor core is in sleep mode.

Allowing independent control of the LCD controller from the processor core. To operate the LCD in sleep mode, users only need to select an oscillator source other than the main oscillator, which will be disabled during sleep, and clear a control bit to determine whether the LCD should power off. The clock source for the LCD can be the internal RC oscillator or the clock of timer 1, often connected to an external 32 kHz crystal.

As the pressure to reduce power consumption in systems increases, it is expected that manufacturers will provide more autonomous peripherals like LCD controllers that can be updated without calling the processor core unless a major change is needed. This contributes to a continuous reduction in duty cycle and power consumption.

Sleep modes are crucial for the power supply life of battery-powered MCUs, and maintaining a clear user interface is equally important even when the processor core is powered off. There are already MCU products on the market that can drive LCD displays even in deep sleep mode.