Specifications

The LCD has three control lines, Read/Write, enable and register select. It has a

processor on board with two registers that are accessed via its bus. In this case a 4-bit

bus was used due to a lack of I/O pins available. There are two registers to write too on

the LCD; the first is the control register the second is the data register. These use the

same bus but are selected by the register select control line. The Read/Write control line

is always kept low to signify that the registers are continuously being written to. The

control register allows the user to configure the type of cursor and style of display. It is

also important for being able to move the cursor around the display. To write to the 8-

bits of data to control register the actions taken are shown in figure 24.

Enable = 1

Write the high 4 bits of

the data to the data

lines

Toggle the enable low

then high

Delay to let data settle

Write the low 4 bits of

the data to the data

lines

Delay to let data settle

Enable = 0

Figure 24 Writing to the LCD command register

The register select is set to 0 to select the command register and the data line is then

enabled. The high 4 bits of the command are written to the data lines and a short delay

is allows the data lines to settle. Toggling the enable line initiates a new write cycle and

the low 4 bits of the command are written to the data lines. A small delay allows the

data to settle and enable is set back to zero and the select line is set to one to select the

data register.

To display characters, the character needs to be written to the data register. This is done

in the same way as writing to control register except that at the start Register select is

set to 1 instead of 0. Before data can be displayed on the screen. the LCD needs to go

through a strict initialisation procedure.

The initialisation of the LCD module is done as shown in figure 25.