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In general, this chapter is about interfacing an external device to the Propeller using a liquid crystal display (LCD) as the target device. Specifically, this chapter is about attaching various LCDs to your projects. Almost all the LCDs on the market with built-in controllers use the scheme we are about to investigate to communicate with them. Displays that go from 8 characters on one line to 40 characters on each of four lines are available. All these displays can be addressed with the techniques discussed in this chapter. We need some way to see what is going on in our programs as we develop them. We will focus our efforts on a 16-character-by-2-line LCD (see Figure 21-1). These LCDs can be purchased for between $4.50 and $6 on the Internet. They provide an inexpensive

way of adding a display to our projects, and the 32 characters they display provide enough information for most simple experiments. We will develop the software to control this LCD in the following experiments, and we will use this LCD as the display for all our experiments. This being the case, we need a comprehensive set of routines to allow us to have complete control of this device. The basic routines will be developed as we proceed. You can add to them as your proficiency and needs increase.

Our goal in this experiment is to get the LCD up and running from its most primitive first instructions. A number of very specific instructions have to be sent to the LCD to start it up. These are specified in the startup instruction set for the LCD specified in the manual for the Hitachi 44780 controller. This Integrated Circuit controls the internal workings of the LCD and almost all similar LCDs on the market. Our particular LCD can be controlled by sending data to it either 4 bits at a time or 8 bits at a time. We will cover both techniques, with the 8-bit protocol first. It is useful to have the full 40+ page data sheet on hand when you are doing anything more than sending text to the LCD. The Hitachi 44780 data sheet can be downloaded from the Internet by searching Hitachi 44780 and looking for a suitable PDF version of the datasheet. Datasheets can also be downloaded from the websites of most electronic component suppliers like Digikey, Mouser, and Avnet.

The circuitry needed to connect the LCD to the Propeller chip is shown in Figure 21-2. Although the LCD can be written to 4 bits at a time or all 8 bits at one time, the diagram shows all eight lines connected to the data bus so that we can experiment with either 4 or 8 bits. After all is said and done, we will use the 4-bit mode so that we can free up four lines on the Propeller. It is possible to write to the LCD by sending it the characters we want it to display as well as to read from the LCD to determine what is in the current LCD memory. These 16 2 LCDs can display only 32 characters, but they have enough memory to store 80 characters. The display can be scrolled left and right to see the hidden characters. If you are never going to read from the LCD (and reading is seldom necessary, except to read the Busy bit), you can tie the R/W line low and save one line on the Propeller for other uses. (The busy bit is then handled with a short delay to allow it to turn Figure 21-2 Connecting the LCD off.) Because using 4 bits for communications to the Propeller chip (8-bit mode)