LCD Character Display Primer

Introduction

Liquid Crystal Displays are display devices used to convey information through arrangements of pixels. Graphic and Text LCDs are the most common types available for electronic products. LCDs range from something as simple as the relatively simple 16-120 character displays available for small electronics to large 60+" high definition television sets.

Principles of operation

Reflective twisted nematic liquid crystal display.

1. Polarizing filter film with a vertical axis to polarize light as it enters.
2. Glass substrate with Indium tin oxide (ITO) electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth.
3. Twisted nematic liquid crystal.
4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.
5. Polarizing filter film with a horizontal axis to block/pass light.
6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source.)

Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer. The surfaces of the electrodes in contact with the liquid crystal material are treated in order to align the crystal molecules in a certain direction. The electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).

In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. When light passes through the liquid crystal molecules the rotation of the polarization changes slightly and reduces the effect so the light can pass through mostly unaffected. If a large voltage is applied, the liquid crystal molecules become almost completely untwisted. This causes the light to be heavily polarized again effectively making the pixel appear black. By adjusting the voltage, the polarization can be controlled precisely allowing you to display a full range of grey-scale.

Text LCDs

Text LCDs display full text strings set in software. Since text characters are defined from the ASCII standard library, other ASCII standard set characters and glyphs can also be sent to the text LCD. This can be done easily by using unicode characters within your text string. In C#, this may look something like this:

In this example, the string \u indicates that a unicode character follows, and the unicode character 0041 (which references the hexadecimal character code 0x41) represents the capital letter A. After the LCD converts the unicode character, the above example would cause the LCD screen to read Apple starts with A. A chart of all ASCII standard set character codes is available in the Appendix at the end of this manual.

tLCD.rows[0].DisplayString = "Apple starts with \u0041";

Custom Characters

Custom characters can also be generated for the PhidgetTextLCD. A custom character can be any arrangement of pixels within the space allotted for a single character. Single characters are made up of pixels arranged in a grid 5 pixels wide by 8 pixels high. Once generated, custom characters can be stored in any one of eight volatile memory locations on the PhidgetTextLCD, and can be recalled with a simple API command from software. When custom characters are designed, a formula is used to change the pixel design into a pair of numerical values. The first value relates to the design of the top 4 rows of the character, and the second value relates to the design of the bottom 4 rows of the character. Unlike the unicode characters used in the Special Characters section above, the calculated number is not in hexadecimal format but is an integer value up to six characters in length. The calculation for custom characters can be done by hand, or can be completed for you by using the form available at www.phidgets.com/documentation/customchar.html. Done by hand, each integer value represents the sum of two to the power of each individual on-pixel’s location within that integer-value’s half of the character. Pixels not turned on are valued at zero. For example, a custom character happy-face with pixels 6, 8, 11 and 13 in the upper half turned on, pixels 1, 3, 6, 8, 11, 12 and 13 in the lower half turned on, and all other pixels turned off, would result in the following integer values.

These two values are then stored in one of eight memory locations (CG-RAM 0 to 7) on the PhidgetTextLCD by using the Set Custom Character method in software. In C#, this may look something like this:

tLCD.customCharacters[0].setCustomCharacter(10560, 14666)

Once stored, characters can be recalled into a text string by either using the unicode value for the location as referenced in the ASCII chart (Appendix A) or by using the String Code method from the API. Examples in C# of both methods are shown below:

tLCD.rows[0].DisplayString = "I am happy \u0008";

tLCD.rows[0].DisplayString = "I am happy " + tLCD.customCharacters[0].StringCode;

Connector Pin Out and Cable Coloring Scheme

A (Anode) Red K (Cathode) Black VCC Red GND Black VO Blue RS Blue R/W Blue EN Blue DB0 Green DB1 White DB2 Green DB3 White DB4 Green DB5 White DB6 Green DB7 White

File:Lcdadapterview.png

Supported Screen Sizes

The API supports the following screen sizes:

1x8	2x8
1x16	2x16	4x16
2x20	4x20
2x24
1x40	2x40	4x40

Appendix - ASCII Chart