The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Displays
Interfacing
Full control of an LCD module requires 8 bidirectional lines for reading and writing data
and 3 outputs for the control signals. To save four lines, you can use the 4-bit data interface
described later. Also, the ability to read the display and the busy flag at D7 are optional. If
you give these up, you can use outputs (such as the 74HC374’s) instead of bidirectional port
bits for D0-D7, and eliminate one of the control lines by tying R/W low.
Figure 8-9 shows an LCD module connected to an 82(C)55, using an 8-bit bidirectional
interface. The interface uses Port A and three bits of Port C on an 8255. You can use any of
the 8255’s port bits, if you write your program to match.
On the LCD module, pins 1-3 connect to ground, +5V, and a contrast potentiometer. For
maximum contrast, connect pin 3 directly to ground. Pins 4-6 are the control signals for the
LCD module. These connect to three outputs on Port C. The eight data bits, pins 7-14 on
the LCD module, connect to Port A.
Listing 8-4 is a BASIC-52 program that initializes a 2-line display and writes LINE 1 and
LINE 2 to the matching lines.
Initializing the module. On power up, the LCD module must initialize properly. If power-up
is clean, with the supply voltage rising from 0.2V to 4.5V in 10 milliseconds or less, the
module initializes automatically. But, if power-up doesn’t meet this requirement, your
program has to provide the initialization routine. It’s a good idea to always include an
initialization routine in your program, since it does no harm, and if the module doesn’t
initialize properly, it won’t respond correctly or at all.
Table 8-3 summarizes the initialization procedure. In short, the module must first receive
three identical commands selecting an 8-bit interface. BASIC-52 easily provides the
necessary delays between the commands. To begin the initializing, you must send the
instruction to select an 8-bit interface, even if your interface is four bits.
Once this is done, the instructions for Function Set, Display On, Display Clear, and Entry
Mode Set tell the controller the configuration you desire. The automatic power-on initialization
routine turns the display off, so if you use it, you have to turn the display on by writing
0Ch to the instruction register. When initializing is complete, you can control the display as
you wish, though you can’t change the number of display lines unless you reinitialize from
the beginning.
Listing 8-4 has two subroutines, one for writing characters to the display and one for writing
instructions. To write a character, set D equal to the character’s code, and call subroutine
800, which sets RS, writes the character to the display, and toggles E. To write an instruction,
The Microcontroller Idea Book 145