LCD with a 32 bit MCU for low power applicationsFollow article
Having a screen as an interactive feedback system in your design can add greatly to the user experience. However, this can be tricky in low power applications as screens and displays consume power. One of the best types of displays to use in these applications is a liquid crystal display or LCD. Also if your system already uses an MCU then pairing the two is a straightforward low power solution.
First let’s look at how the LCD works which will help you realize how to optimize their power consumption.
LCDs can come as an entire module which includes a display, a driver and interconnect between the two components. They can alsocome as a raw displays which gives the user more flexibility as long as they can provide their own driver. Many 32 bit MCUs have built in LCD drivers which save BOM, space and an integrated LCD driver gives full access to the controller, allowing a high degree of optimization for the display control and performance. The STM32L151C6, Silicon Labs EFM32 family and MSP432 are all 32 bit MCUs or MCU families that have a built in LCD driver.
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. 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.
Another important part of LCDs are the segment and common pins. 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.
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. To reduce the power consumption you can adjust these but they have drawbacks. The capacitance C is a built in physical factor of the LCD chosen but putting the LCD and driver/MCU close together can reduce parasitic capacitance thus reducing power. The AC amplitude adjusts the contrast so lowing it will reduce the difference between the light and dark segments, this can make it harder to read as it is lowered. The switching frequency can be adjusted but will eventually lead to flickering if too low. Finding the optimum balance here is key.
A thing to note when looking at an MCU with an LCD combination is that the LCD voltage may need to be higher than the available supply voltage. Because of this 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. Check the data sheet of the MCU to see if the option is available, for example figure 4 is from a Silicon Labs EFM32 device.
I hope you all have learned about LCDs and how they are a good solution in low power applications especially when paired with an MCU that has a built in driver. Let me know if you have any questions!