The Microcontroller Idea Book (Jan Axelson, 1994)

Displays

Table 8-1. LCD modules containing the HD44780 controller often use this 14-line interface.

Memory Areas

The HD44780’s on-chip memory includes a CG (character-generator) ROM, CG RAM, DD (display data) RAM, an instruction register, and a data register.

The CG ROM stores the segment patterns for generating 192 different characters, including the Roman (English) alphabet in upper and lower case, numbers, some math and other special symbols, and Japanese kana characters. These are fixed in ROM and can’t be altered.

The CG RAM stores segment patterns for up to 16 user-designed characters such as logos, special symbols, or other simple graphics characters that you design on the 5 X 8 matrix. To create a custom character, you write a series of 5-bit words to the CG RAM. Each word represents the segment pattern for one row in the desired character. The patterns stored in CG RAM disappear on powering down, so you must reload them on each time you power up.

Chapter 8

X=don’t care

Displays

Each character in the CG ROM and CG RAM has an 8-bit address, or character code. Conveniently, the codes for the upper and lower-case Roman alphabet and common punctuation are same as the ASCII codes for those characters (21h through 7Dh). For example, the pattern for A is stored at address 41h, B is stored at at 42h, and so on.

An 8-bit instruction register (IR) stores instruction codes and addresses, and an 8-bit data register (DR) stores character codes. When you read or write to the chip, you must select the appropriate register.

The DD RAM stores up to eighty 8-bit character codes. Each character position on the display corresponds to an address in the DD RAM, and the character codes stored in the DD RAM determine what is displayed at each position.

On power up, on a 2-line display, the leftmost position on the top line has an address of 0, with the rest of the positions in the line addressed in sequence. The second line begins at 40h, even if the top line has fewer than 40h positions.

The instructions allow you to configure a module so that the DD RAM’s address increments each time a character is written to the display. This way, the characters automatically appear in sequence on the display without your having to specify an address each time.

Because the second line begins at 40h, however, the display will not wrap around automatically to this line. For example, on a 2-line, 16-position display, line 1 ends at 0Fh and line 2 begins at 40h. To move from the rightmost position of line 1 to the leftmost position of line 2, you have to change the address counter to 40h. In addition, some displays with a single physical line of characters have two logical lines. In a 16-character display of this type, the first 8 characters are addressed from 0 to 7, and the second 8 are addressed from 40h to 47h. With this type of display, you must set the address counter to 40h before you write to the second half of the line.

On a small display where all 80 bytes of DD RAM aren’t needed, you can use the spare DD RAM as general-purpose RAM.

Reading and Writing

Writing to the LCD module involves the following steps:

Bring RS high to write data, or low to write an instruction.

Bring R/W low.

Bring D0-D7 to their desired states.

Wait at least 140 nanoseconds.

Bring E high for at least 450 nanoseconds.

Bring E low.

Chapter 8

Figure 8-9. Using an 8255 to control a character-based LCD module.

Read operations are similar to writes, with R/W high instead of low. The data appears on D0-D7 in 320 nanoseconds or less after E goes high.

The HD44780 cannot accept a new instruction until it has finished executing its previous instruction. Table 8-2 shows the maximum time each instruction requires.

BASIC-52 is slow enough that you don’t have to worry about the required delays. If you’re using an assembly-language routine, your program must include delays after each instruction, or you can use the instruction that reads the HD44780’s busy flag at D7 to determine when the module is ready to accept a new instruction.

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,

Chapter 8

Listing 8-4 (page 1 of 2). Initializes a 2-line LCD module and displays a message on each line.

Displays

Listing 8-4 (page 2 0f 2).

390 REM display “LINE 1", ”LINE 2"

400 D=ASC(L):GOSUB 800

410 D=ASC(I):GOSUB 800

420 D=ASC(N):GOSUB 800

430 D=ASC(E):GOSUB 800

440 D=20H:GOSUB 800

450 D=ASC(1):GOSUB 800

460 I=0C0H

470 GOSUB 900

480 D=ASC(L):GOSUB 800

490 D=ASC(I):GOSUB 800

500 D=ASC(N):GOSUB 800

510 D=ASC(E):GOSUB 800

520 D=20H:GOSUB 800

530 D=ASC(2):GOSUB 800

600 END

790 REM write data to the display

800 XBY(X)=RS+1

810 XBY(X)=RW

820 XBY(A)=D

830 XBY(X)=E+1:XBY(X)=E

840 RETURN

890 REM write an instruction to the display

900 XBY(X)=RS

910 XBY(A)=I

920 XBY(X)=E+1:XBY(X)=E

930 RETURN

Chapter 8

Table 8-3. Initialization procedure for LCD modules using HD44780 controller.

Power on

Wait 15 milliseconds after V+ = 4.5V

Function set = 30h

Wait 4.1 milliseconds

Function set = 30h

Wait 100 microseconds

Function set = 30h

Function set to match display module

Display on

Display clear

Entry mode set

set I equal to the instruction and call subroutine 900, which clears RS, writes the instruction to the display, and toggles E.

Using the example program as a model, you can experiment with your own messages by adapting the code in lines 400-530.

Listing 8-5 is another test program that displays a prompt on the host’s screen and then displays the character you type at the keyboard both on the host’s screen and on the LCD module. To use this program, you must add lines 10 through 380 of Listing 8-4 to initialize the module and variables.

The 4-bit Interface

The HD44780’s 4-bit data interface can be convenient if you don’t have a lot of port bits to spare. The minimum interface requires just 6 outputs, to D4-D7, RS, and E. The drawback is that the 4-bit interface is slower in operation and more complicated to program.

To send an instruction using a 4-bit interface, you send half at a time over D4-D7, along with the appropriate RS and R/W signals. D0-D3 are unused. For example, with an 8-bit interface, writing Z (5Ah) to the display requires the following operations:

clear R/W set RS

write 5Ah to D0-D7 bring E high, then low

Displays

Listing 8-5. Displays key presses on the host computer’s screen and on an LCD module.

1REM reserve space for 1 string variable,

2REM 1 character in length

3STRING 3,1

4REM You must add lines 10 through 380 of listing 8-4

5REM to this program

400 Z=0

410 DO

420 INPUT “Press a key: ”,$(0)

430 PRINT $(0)

440 REM reset display after 8 characters

450 IF Z=8 THEN Z=0:RW=0:I=1:GOSUB 900

460 REM keep track of how many characters are displayed 470 Z=Z+1

480 REM display the character matching the key press

490 D=ASC($(0),1):GOSUB 800

500 WHILE 1=1

600 END

790 REM write data to the display

800 XBY(X)=RS+1

810 XBY(X)=RW

820 XBY(A)=D

830 XBY(X)=E+1:XBY(X)=E

840 RETURN

890 REM write an instruction to the display

900 XBY(X)=RS

910 XBY(A)=I

920 XBY(X)=E+1:XBY(X)=E

930 RETURN

Chapter 8

Listing 8-6 (page 1 of 2). Creates and displays a custom character (upside-down question mark) on an LCD module.

1REM You must add lines 5-380 from Listing 8-4 to this

2REM program.

400 REM R1-R8 store row patterns for custom character

401R1=4

402R2=0

403R3=4

404R4=8

405R5=10H

406R6=11H

407R7=0BH

408R8=0

410 REM custom character number

420 CC=0

430 XBY(X)=RS

440 XBY(X)=RW

450 REM display clear

460 XBY(A)=1

470 XBY(X)=E+1:XBY(X)=E

480 REM set CG RAM address to 0

490 XBY(A)=40H

500 XBY(X)=E+1:XBY(X)=E

510 REM store R1-R8 in CG RAM

520 D=(CC)+R1

530 GOSUB 800

540 D=(CC)+R2

550 GOSUB 800

560 D=(CC)+R3

570 GOSUB 800

580 D=(CC)+R4

590 GOSUB 800

600 D=(CC)+R5

610 GOSUB 800

620 D=(CC)+R6

630 GOSUB 800

640 D=(CC)+R7

650 GOSUB 800

660 D=(CC)+R8

670 GOSUB 800