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ST9 USER GUIDE

tween one electrode and the backplane, the area delimited on the front panel by the shape of

the electrode takes the reverse state, i.e. opaque or transparent, respectively. Using a seven-

segment pattern, it is possible to display numbers, by activating the appropriate segments to

show the desired figures.

A liquid-crystal display presents a high impedance between its electrodes, ranging typically in

the megaohm region. It is thus a voltage-controlled device, with a threshold of about 3 V to

change from one state to the other. Driving such a display is then a simple matter, but one fun-

damental precaution must be taken to avoid premature destruction of the display: it must be

driven with alternating voltage, to avoid electrolysis within the inter-electrode solution. The fre-

quency is of little importance, and the best values range between 30 and 100 Hz.

This basic explanation does not cover all the details and precautions relative to LCDs but is

sufficient to understand the use of the SPI as a LCD driver.

In this application, the display is installed on a separate board. Since it has 40 pins, it is not

convenient to connect this board with a 40-wire cable. Using a serial synchronous transmis-

sion, only four wires are required. The block diagram of the circuit of the display board is:

Figure 45

. LCD Interface Example

As shown by this diagram, to make it easy to wire our display board, the correspondence be-

tween the segment positions and the bit positions in the bytes transferred is actually:

And the conversion between the figures (0 to 9) and the display pattern is done by a constant

array defined in this case as:

; Table of characters. Gives the patterns for the last 3 digits, from 0 to 9.

Bit position

Segment

Liquid Crystal Display

One digit

32 bit serial input, parallel output shift register

Ground

Serial Input

Shift Clock

+5V

+1 back panel

dp g f e d c b a

ST9 Datasheet(PDF) 89 Page - STMicroelectronics