Dynamic lcd driver using gpio pins

Freescale Semiconductor Application Note

Document Number: AN3412 Rev. 0, 4/2010

Dynamic LCD Driver Using GPIO Pins
by: Juan Cazares Applications Engineer Mexico Guadalajara

1 Introduction
This application note explains how the twisted nematic (TN) liquid crystal display (LCD) works and how this kind of LCD can be controlled using the microcontroller's GPIO pins. There are two ways tocontrol the TN LCD, statically and dynamically. The driver discussed in this document is intended to drive dynamic LCDs. The dynamically driven LCDs require a pure AC voltage waveform applied to the LCD electrodes. Each electrode needs one backplane electrode and one frontplane terminal to be displayed. A dynamic LCD makes the situation more complicated because the segments are organized into amatrix, but then again this kind of LCD reduces the number of terminals used. The Freescale application note titled XGATE Library: TN/STN LCD Driver Driving Bare TN and STN LCDs Using GPIO Pins (document AN3219) explains in depth the dynamically driven LCDs performance.

Contents
1 Introduction...........................................................1 2 LCDOverview......................................................2 3 Hardware ..............................................................3 4 Functional Description..........................................6 4.1 Configuring the Driver..................................6 4.2 Driver Initialization.......................................7 4.3 Driver Control...............................................7 4.4 LCD Refresh CycleFrequency.....................7 4.5 Contrast Control............................................7 4.6 Maximum Number of Digits.........................7 4.7 Configuring Backplanes................................8 4.8 LCD Buffer...................................................8 4.9 Displaying Characters...................................8 4.10 Creating New Characters..............................9 4.11Displaying Message......................................9 4.12 Code Size and Execution Time...................10 5 Conclusion...........................................................10

© 2010 Freescale Semiconductor, Inc.

LCD Overview

2 LCD Overview
To reduce the number of terminals used to drive the LCD, the segments are multiplexed. This means each terminal drives two or more segments. Theterminals are also called frontplanes. The amount of segments driven depends on the number of LCD backplanes. The LCD manufacturer must specify the number of backplanes used to drive the LCD. This driver is intended to manage a dynamically driven LCD with 4 backplanes, although the number of backplanes can be changed. The number of backplanes is also called a duty ratio, in this case ¼ dutyratio. The LCD model used in this application note is the VIM-878-DP. Each digit in the display is made up of 16 segments in total. The 16 segments of each digit can be driven with four terminals, this is because the segments are organized into a matrix. Figure 1 and Figure 2 show how the display segments are organized.

Figure 1. Display segments
To polarize one segment a differential voltage mustexist between the desired frontplane terminal and the appropriate backplane. Table 1 shows the backplane and frontplane connections required to polarize the 16 segments shown in Figure 1.

Table 1. Backplane and frontplane connections
Backplane0 Frontplane0 Frontplane1 Frontplane2 Frontplane3 DP D L M Backplane1 C E K N Backplane2 B F J G Backplane3 A CA I H

The polarity of the voltagewaveform applied to each segment must be inverted periodically to maintain a constant electric field between electrodes and to avoid damage on segments because of the DC component. All the visible segments must be refreshed with a minimal refreshing frequency of 50 Hz or 60 Hz. The software driver provided in this application note (AN3412SW) refreshes each segment with a frequency equal to 400 Hz....