Introduction
We considered the creation of LED-based user interfaces in Chapter 21. Here we are concerned with the use of liquid crystal displays (LCDs) in such interfaces.
Unlike LEDs, LCDs are based on passive display technology: this means that LCDs control the passage of light rather than emitting light. This fact directly contributes to the low power consumption of these devices: large (5V) panels require up to 5 mA: a total power consumption of up to 25 mW, excluding any backlight. Small panels con- sume around half this power. Since a single LED has a very similar power consumption, use of LCD displays in battery-powered embedded systems is particularly popular.
While various types of LCD display are available, they can be divided into two basic groups: graphics displays and text displays. Notebook (and increasingly desktop) PCs use sophisticated graphics displays, made up of many thousands of individual ‘picture elements’ (pixels). Such displays are expensive in their own right and generally require large amounts of memory (typically several megabytes) and powerful processors for efficient operation. As we have seen, the type of embedded devices we are concerned with in this book do not usually justify this level of expense. Instead, we will be concerned here with small displays intended primarily to display text.
Various types of LCD-based character displays are available. These are typically arranged as one, two or four lines of between 16 and 40 characters. Inevitably, the larger displays are more expensive and consume more power. Each LCD character is usually a 5×8 matrix of dots: less commonly, a 5×11 matrix is also used: note that, in each case, the characters themselves are 5×7 and 5×10 pixels in size, with the bottom line being reserved for the cursor. To generate characters on such displays would tend to consume a large percentage of the available CPU time (and most of the ports or address space) on most embedded processors. As a result, most LCD panels include an on-board controller to deal with this: this is generally a variant on the Hitachi HD44780. Displays based on this popular controller all have very similar hardware interfaces and will display the same mixture of English (or Japanese) characters.
We focus on LCD panels based on this ‘standard’ controller in the pattern LCD CHARACTER P ANEL presented in this chapter.
Please note that much of the background material presented here is adapted from the Hitachi HD44780 data sheet.