32-bit Microcontroller Optimizes LCD Power Consumption
Enhancing Embedded System Energy Efficiency
We are all familiar with small liquid crystal displays (LCDs) – not the 50-inch big-screen LCD TVs on your holiday shopping list, but the much smaller segment displays that are found everywhere. LCDs have been popular for a long time and will continue to be used in countless electronic devices. They are embedded in household appliances, consumer electronics, cars, toys, personal medical devices and many other products. LCDs provide effective user feedback while being cost-effective, power-efficient and easy to design into embedded products.
LCDs are commonly sourced as either modules or raw displays. Modules contain a display, a driver and interconnect between the two components. The driver included with the module typically interfaces with a microcontroller (MCU) through a serial bus. Another option is to design with a raw display and a microcontroller that includes a display driver, eliminating the fixed-function LCD driver from the system and freeing up a serial bus. This approach has numerous advantages over a module such as greater design freedom, reduced cost and lower power. Because many low-power and cost-sensitive applications include an LCD, there is a large selection of MCUs that integrate an LCD driver. An integrated LCD driver gives full access to the controller, allowing a high degree of optimization for the display control and performance.
Custom LCDs provide an excellent way to differentiate an embedded product. Off-the-shelf segment and dot-matrix screens have their place, but displays with custom symbols, shapes and arrangements enhance the user experience and make an embedded product stand out. A custom display has a unique number of LCD segments, but many LCD controllers support a fixed number of LCD pins. The unused LCD pins are useless and increase power consumption due to unnecessary toggling. Silicon Labs’ Precision32™ SiM3L1xx 32-bit microcontroller family is an optimal choice for custom displays because the MCUs’ patented crossbar architecture allows developers to configure unused LCD pins as GPIOs that can be mapped to various analog and digital peripherals.
Figure 1. LCD Construction and Operation
Understanding how LCD displays operate will enhance the developer’s understanding of how to optimize the driver settings. An LCD basically consists of two polarizing lenses, two electrodes and liquid crystal. An electric field applied between the electrodes changes the orientation of the liquid crystal, effectively controlling the transmission or reflection of light and making the segment appear light or dark as seen in Figure 1. One important property is that segments should not be driven by a dc voltage because it will significantly reduce the lifetime of the display. Therefore, LCD segments are driven by an ac signal, typically a square type wave at 60 Hz. For example, to turn on a segment, the electrodes are driven by a 3 V peak square wave centered at 0 V. The segment is driven between +3 V and -3 V by switching the electrodes between 3 V and 0 V. The dc voltage is 0 V, and the ac magnitude is 6 V peak-to-peak, as illustrated in Figure 2.
Figure 2. LCD Segment Switching
Another important part of LCDs are the segment and common pins. The total number of segments in the display can be as high as the product of these two numbers. For example, a 4 x 32 display has 4 common and 32 segment pins and can have up to 128 display segments. Figure 3 shows how the LCD segments are connected between segment and common pins. Because the pins are shared between multiple display segments and must be driven by an ac signal, the waveforms on the pins have an odd appearance. Fortunately, this is all handled by the display driver.
Figure 3. LCD with Segment and Common Pins
One major appeal of using LCDs in embedded applications is their low power consumption. The display does not consume any dc power and does not emit light, making the power consumption orders of magnitude less than other types of displays. All of the power consumed by an LCD is dynamic because the segments are driven by an ac signal. Even if the display is not changing, it still consumes power. Each segment can be viewed as a capacitor, so the average current consumption is CVf where C is the capacitance of the display, V is the ac amplitude and f is the switching frequency.
Power consumption can be optimized by adjusting C, V and f. Capacitance is a fixed property of the display and interconnects between the driver and LCD. Locating the driver close to the display will help minimize parasitic capacitance of interconnections. The voltage level controls the contrast of the LCD, putting a limit on the minimum voltage that can be used. As the voltage is reduced, so does power consumption, but the contrast goes down, making the display difficult to read. The last adjustable parameter is frequency. As the frequency is decreased, the power will decrease, but the frequency can become too low and cause the display to flicker and become difficult to view. Constraints on the operating voltage and frequency of LCDs limit the minimum power required by an LCD.
A system may require an LCD voltage that is higher than the available supply voltage for the display to be easily viewed. For this reason, many LCD controllers include a charge pump that can generate a voltage higher than a supply. The efficiency of the charge pump will directly affect power consumption so it is important to choose a microcontroller solution with a high-efficiency LCD charge pump such as the SiM3L1xx 32-bit microcontroller devices.
In many battery-powered systems, the power required by the LCD is a significant driver of cost and size. Reducing power consumption can result in a smaller battery, lower cost, smaller form factor and a greener solution. The SiM3L1xx MCUs, for example, are ideal for battery-powered systems because the integrated LCD driver reduces LCD display current by approximately 40 percent. Silicon Labs’ patent-pending LCD controller intelligently resets all LCD segments before each switching cycle to minimize the amount of charge required from the battery during each LCD segment transition. All of this is handled automatically by the innovative LCD controller. The total power saved by the SiM3L1xx MCU scales with the display, reducing the power of any LCD. Figure 4 shows actual lab data from a real-world example where the total system current was reduced by more than 30 percent when the new LCD segment resetting mode is enabled.
Figure 4. Real-World Power Savings
LCDs continue to provide an excellent user interface for countless electronic products. They are economical, power efficient and easy to add to any embedded design. Silicon Labs’ Precision32 SiM3L1xx 32-bit microcontrollers are an optimal choice for a battery-powered system with a display because the MCUs’ integrated LCD controller reduces the display power by up to 40 percent, thereby reducing the power consumption, size and cost of embedded designs.