May 2006

Rev 5

1/21

AN1048

Application note

ST7 software LCD driver

Introduction

This note describes a technique for driving Liquid Crystal Displays (LCD) with any standard

ST7 Microcontroller (MCU) i.e without any specific on-chip LCD driver hardware. This

technique offers a solution for applications which require a display at low cost together with

the versatile capabilities of the standard ST7 MCUs. This note also provides a technique to

control the LCD contrast through software.

After an introduction on LCDs in Section 1, Section 2 & 3 of this note describes the typical

waveforms required to drive an LCD with a multiplexing rate of 1 or 2 (duplex) and 4

(quadruplex). Section 3 presents a solution based on a standard ST7 MCU directly driving a

quadruplex LCD. This solution can be implemented with any ST7 MCU as it only requires

the standard I/O ports and one timer, both of which are standard features on all ST7 MCUs.

Section 4 describes how to control the contrast through software. Finally, Section 5 gives a

brief overview of the LCD demo board including the board schematics. The demo board,

based on a ST72F321B microcontroller, allows the user to develop and test applications

using an LCD device.

The program size (~300 bytes), the CPU load required for controlling the LCD (0.2%), and

the number of external components is kept to the minimum (two external resistors per COM

line). The number of I/O’s is the same as a solution using an on-chip LCD hardware driver or

an external hardware LCD driver. With software contrast control, it is a very flexible solution

that can be adapted easily to a range of applications.

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