MCU power-saving design optimizes LCD display

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

MCU power-saving design optimizes LCD display

Posted Date: 2024-01-21

Low-power microcontrollers (MCUs) are now at the heart of many portable systems, but extending battery life while broadening their functionality remains a challenge.An effective way to maintain a long battery life is to run the processor only when necessary, thus avoiding power consumption by user interface components such as the display.However, an increasing number of MCUs are capable of selectively shutting down the processor while maintaining the liquid crystal display (LCD) image.

Elements of low energy design

For example, a water meter needs to display the reading on the built-in LCD at all times. However, keeping the MCU system active for viewing at any time around the clock consumes a lot of power. The demand for products such as water meters is to enable the LCD to work without being directly controlled by the processor core, and to selectively turn off parts of the MCU while maintaining the LCD image.

The power consumption of any active CMOS electronic circuit can be calculated using the CV²f formula. Among them, C represents the total capacitance of the circuit that needs to be charged and discharged during the cycle, V is the power supply voltage, and f is the operating frequency. Therefore, one factor that reduces power consumption is the supply voltage - the squared relationship between voltage and energy consumption provides significant energy savings.

Another energy saving comes from reducing the operating frequency, although this may only affect power and not energy.If the algorithm execution requires the same number of cycles, but spread over a longer period of time, the overall energy consumption may increase, related to leakage current.

Effect 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 drains the battery.Leakage current is generally constant, although it increases slightly as operating temperature increases due to an increase in hot carriers within the transistor. The only way to reduce leakage power, other than careful process design, is to cut off the power supply to the logic block.

therefore,Power-saving design of MCUs requires careful trade-offs between circuit speed and voltage. There is a third factor - how often the MCU needs to be activated in a given time.Most MCU-based systems don't always have work to do. They typically complete a series of tasks periodically and then stop. To avoid wasting power, the MCU will not simply cycle in an idle cycle, but will enter a sleep state, shutting down most of its functional units.

MCU activity management

Ideally,In metering and monitoring applications, by maintaining a low duty cycle to maximize battery life, the processor within the system sleeps almost all of the time.For example, a utility meter MCU may spend most of its life in a sleep state. It might be awake only 1% of the time, waking up just to collect data from sensor inputs. After the MCU completes its work, it sets a timer to trigger a wake-up interrupt after a period of time, or simply waits for an external stimulus to trigger an interrupt and then turns off power.Since a device is unused for more than 99% of its life cycle, even a small amount of leakage power can consume a considerable amount of battery power.

MCUs designed for these low duty cycle environments offer multiple sleep modes to provide detailed control over power consumption. For example, "Doze" mode may simply disable some peripherals, but the core is still running in an idle loop at an extremely low clock rate. This consumes more power than the "deep sleep" state, in which the processor core and nearly all peripherals are turned off, but typically responds quickly to interrupts.

When it comes to power, the difference between deep sleep and snooze mode can be huge.An MCU in deep sleep might only draw around a few tens of nanoamps - just enough energy to keep the timers and interrupt controller running - while a less intense sleep mode could draw tens or hundreds of microamps, or even More.Obviously, the choice of sleep mode has a significant impact on battery life.

The choice of sleep mode also has a significant impact on product design, especially the user interface. If the LCD doesn't have a backlight, it doesn't use much energy, so it doesn't necessarily need to be powered off. For applications such as meters, where the user may want to check the reading without waking up the system, it is important to continuously display the reading.

Processor independent LCD control

Many traditional MCUs turn off the LCD controller when the device enters deep sleep, leaving the display blank.However, many manufacturers have now recognized the value of at least maintaining a static display and allowing the LCD to continue to operate while almost all other parts of the MCU are disabled. Some can even modify the display while the processor core is sleeping.

Allows the LCD controller to be driven independently of the processor core. To operate the LCD in sleep mode, the user simply selects an oscillator source other than the main oscillator - which is disabled during sleep - and clears a control bit that determines whether the LCD should be powered off. Depending on the MCU model, the clock source for the LCD can be the internal RC oscillator or the clock from Timer 1, which is usually connected to an external 32 kHz crystal and is typically used to control when the processor core next wakes up.

Since many LCD displays do not operate below 3V, the LCD driver has an integrated voltage boost function. When the supply voltage drops below 3V, the voltage booster can be dynamically enabled to increase the output of the LCD display to above 3V. This allows continued operation even when the battery is nearing the end of its discharge cycle and the voltage frequently drops below 3V.

As pressure increases in systems to reduce energy consumption, vendors are expected to offer more autonomous peripherals such as LCD controllers that can be updated without calling the processor core and are only required for major changes. This will help continuously reduce duty cycle and power consumption.

Sleep mode is critical for battery-powered MCU power life, and it's equally important to maintain a clear user interface even when the processor core is powered off. There are also MCU products on the market that can still drive the LCD display even in deep sleep mode.

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