Touch LCD Shield

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Introduction

This is a multifunctional Arduino/Arduino Mega compatible resistive touch screen. It can be used as display device or sketch pad. With a SD card module integrated also on this shield, this shield reserves great room for other expansions to your project.

Features

  • Big screen for easy and comfortable experience
  • Backlight controllable via programming
  • 65535 rich colors display
  • SPI pin-saving communication method
  • Full screen touch active range

Specification

Item Min Typical Max Unit
Voltage 4.5 5 5.5 VDC
Current / / 250 mA
LCD Panel Size 2.8 inch
View angle 60~120 Deg
Resolution 320x240 /
LCD color 65k /
Backlight Type LED /
LCD driver IC ILI9341 /
Interface Type SPI /
Touch Screen 4-Wire resistive touch screen /
Active area 43.2*57.3 mm
ESD contact discharge ±4 KV
ESD air discharge ±8 KV
Dimension 72.5x54.7x18 mm
Weight 24±2 g

Cautions

  • Don’t press too hard on the screen which might cause display distortion. 
  • Be careful to deposit the fragile screen.

Pins usage on Arduino


Pins Used for TFT Screen Control:
D4: TF_CS, SD card select input pin

D5: TFT_CS, TFT chip select input pin

D6: TFT_D/C, TFT Data/Command control pin

D7: BACKLIGHT, TFT backlight control pin

Pins Used for SPI Interface:
D10: SPI chip select
D11: SPI MOSI pin

D12: SPI MISO pin

D13: SPI serial clock pin

Pins Used for Touch Function:
A0 - Touch Screen Y- input pin.

A1 - Touch Screen X- input pin.

A2 - Touch Screen Y+ input pin.

A3 - Touch Screen X+ input pin.

TFT Programming

The TFT library provides the following Application Programming Interfaces(API). The library makes use of direct access to PORT registers instead of Arduino APIs. This is to increase the speed of communication between MCU and TFT. At present, the library supports Arduino, Arduino Mega (1280 or 2560) and Seeeduino ADK Main Board compatible boards. In Mega the 8bit data port of TFT is distributed to different pins belonging to different ports. This decreases the speed of graphics drawing when compared to Arduino. The choice of port pins are purely based on Arduino / Mega port pin arrangement.

General Functions

setXY(unsigned int poX, unsigned int poY)

Sets the cursor position to (poX,poY). This function is internally used by other graphics APIs.

setPixel(unsigned int poX, unsigned int poY,unsigned int color)

Sets the (poX,poY) pixel to color color. This function is internally used by other graphics APIs.

Lines

drawLine(unsigned int x0,unsigned int y0,unsigned int x1,unsigned int y1,unsigned int color)

Draws a line from pixel (x0,y0) to pixel (x1,y1) with color color.

drawVerticalLine(unsigned int poX, unsigned int poY,unsigned int length,unsigned int color)

Draws a Horizontal Line of length length with color color starting from pixel (poX,poY).

drawHorizontalLine(unsigned int poX, unsigned int poY,unsigned int length,unsigned int color)

Draws a Vertical Line of length length with color color starting from pixel (poX,poY).

Rectangle

drawRectangle(unsigned int poX, unsigned int poY, unsigned int length,unsigned int width,unsigned int color)

Draws a rectangle starting from (poX,poY) of length length, width width and color color.

fillRectangle(unsigned int poX, unsigned int poY, unsigned int length, unsigned int width, unsigned int color)

Draws a filled rectangle starting from pixel (poX,poY) of length length, width width and color color.

Circle

drawCircle(int poX, int poY, int r,unsigned int color)

Draws a circle at (poX,poY) of radius radius and color color.

fillCircle(int poX, int poY, int r,unsigned int color)

Draws a filled circle at (poX,poY) of radius radius and color color.

Text

drawChar(unsigned char ascii,unsigned int poX, unsigned int poY,unsigned int size, unsigned int fgcolor)

Draws a character starting from (poX,poY) using inbuilt font of size size and with color fgcolor. This function is used by drawString() function.

drawString(char *string,unsigned int poX, unsigned int poY,unsigned int size,unsigned int fgcolor)

Draws a string of text starting from (poX,poY) using inbuilt font of size size and with color fgcolor.

TouchScreen Programming

TFT Touch Shield uses the Adafruit Touch Screen Library. In short, a 4-wire resistive touch screen provides two voltage divider each for X and Y axis. By applying proper voltages for each axis and scanning the ADC values the position of the touch can be detected. These values are always prone to noise. Hence a digital filter is used.

  • To use the TouchScreen Library first create a TouchScreen object by

<syntaxhighlight lang="c"> TouchScreen ts = TouchScreen(XP, YP, XM, YM, 300); </syntaxhighlight>

Where XP, YP, XM and YM are ADC port pins connected to XPlus, YPlus, XMinus and YMinus pins of Touch Screen. 300 is the resistance across X plates.

  • Read the Raw ADC value using

<syntaxhighlight lang="c"> Point p = ts.getPoint(); </syntaxhighlight>

  • The Raw ADC value has to be converted to Pixel Co-ordinates. This is done with map function. This mapping changes for v0.9 and v1.0. The demo applications already takes care of this mapping.

<syntaxhighlight lang="c"> p.x = map(p.x, TS_MINX, TS_MAXX, 240, 0); p.y = map(p.y, TS_MINY, TS_MAXY, 320, 0); </syntaxhighlight>

  • The following sketch demonstrates use of TouchScreen Library. This can also be used to calibrate the touch screen co-ordinates.
  • Compile and upload the sketch.
  • Open serial port monitor and touch the points displayed on the screen.
  • See if the displayed X and Y values are correct. If not, we have to re-calibrate the touch screen coordinates.

How to calibrate the touch screen ?

    • The parameters TS_MINX, TS_MAXX, TS_MINY and TS_MAXY actually decides the extreme ends of the touch screen and actually forms the calibration parameters.
    • The values assigned to these variables are measured ADC values (i.e Raw X, Raw Y) when we touch the extreme diagonal ends of touch screen.
    • Touch points (0,0) and (239,319) and note down Raw X and Raw Y values. For better accuracy, try out many times and find the right value.
      • TS_MINX corresponds to ADC value when X = 0 ;
      • TS_MINY corresponds to ADC value when Y = 0 ;
      • TS_MAXX corresponds to ADC value when X = 240 -1 i.e 239 ;
      • TS_MAXY corresponds to ADC value when Y = 320 -1 i.e 319 ;
    • Change these parameters in the sketch, recompile and upload to Arduino.
    • Repeat the above steps if you still do not get accurate values.

Touch Screen Demo Sketch

<syntaxhighlight lang=arduino>

  1. include <stdint.h>
  2. include <TouchScreen.h>
  3. include <TFT.h>

//Measured ADC values for (0,0) and (240-1,320-1) //TS_MINX corresponds to ADC value when X = 0 //TS_MINY corresponds to ADC value when Y = 0 //TS_MAXX corresponds to ADC value when X = 240 -1 //TS_MAXY corresponds to ADC value when Y = 320 -1

static unsigned int TS_MINX, TS_MAXX, TS_MINY, TS_MAXY;

//Touch Screen Co-ordinate mapping register static unsigned int MapX1, MapX2, MapY1, MapY2;

// For better pressure precision, we need to know the resistance // between X+ and X- Use any multimeter to read it // The 2.8" TFT Touch shield has 300 ohms across the X plate

/* Usage: TouchScreen ts = TouchScreen(XP, YP, XM, YM, 300); 

Where, XP = X plus, YP = Y plus, XM = X minus and YM = Y minus */

//init TouchScreen port pins. This would be reinitialized in setup() based on the hardware detected. TouchScreen ts = TouchScreen(17, A2, A1, 14, 300);

void setup(void) { 

 Tft.init();//init TFT
 initTouchScreenParameters(); //initializes Touch Screen parameters based on the detected TFT Touch Schield hardware

 //Lines
 Tft.drawLine(0,0,50,50,RED);//draw a 45degree red line point(0,0) to point(50,50) 
 Tft.drawVerticalLine(25,0,50,GREEN);//draw a vertical green line point(25,0) to point(25,50)
 Tft.drawHorizontalLine(0,25,50,BLUE);//draw a horizontal blue line point(0,25) to point(50,25)

 //Rectangle
 Tft.drawRectangle(50,0,80,50,WHITE);//draw a white rectangle, length=80(X-AXIS), width=50(Y-AXIS)
 Tft.fillRectangle(0,50,50,80,CYAN);//fill a cyan rectangle, length=50(X-AXIS), width=80(Y-AXIS)

 //Circle
 Tft.drawCircle(75,75,25,RED);//draw a red circle, circle centre(75,75) radius=25
 Tft.fillCircle(150,100,50,GREEN);//fill a green circle, circle centre(150,100) radius=50

 //Text
 Tft.drawChar('S',0,150,2,RED);//draw a char, start from point(0,150) font size 2(16*16)
 Tft.drawString("Seeed Studio",8,166,2,GREEN);//draw a char, start from point(8,166) font size 2(16*16)

}

void loop(void) { 

 // a point object holds x y and z coordinates
 Point p = ts.getPoint();

 p.x = map(p.x, TS_MINX, TS_MAXX, MapX1, MapX2);
 p.y = map(p.y, TS_MINY, TS_MAXY, MapY1, MapY2);

 // we have some minimum pressure we consider 'valid'
 // pressure of 0 means no pressing!
 if (p.z > ts.pressureThreshhold) 
 {
   //p.x;
   //p.y;
   //p.z;
   Tft.fillCircle(p.x,p.y,2,GREEN);
 }

}

void initTouchScreenParameters() { 

 //This function initializes Touch Screen parameters based on the detected TFT Touch Schield hardware

 if(Tft.IC_CODE == 0x5408) //SPFD5408A TFT driver based Touchscreen hardware detected
 {
  1. if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
   ts = TouchScreen(54, A1, A2, 57, 300); //init TouchScreen port pins
  1. else
   ts = TouchScreen(14, A1, A2, 17, 300); //init TouchScreen port pins
  1. endif
   //Touchscreen parameters for this hardware
   TS_MINX = 120;
   TS_MAXX = 910;
   TS_MINY = 120;
   TS_MAXY = 950;

   MapX1 = 239;
   MapX2 = 0;
   MapY1 = 0;
   MapY2 = 319;
 }
 else //ST7781R TFT driver based Touchscreen hardware detected
 {
  1. if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
   ts = TouchScreen(57, A2, A1, 54, 300); //init TouchScreen port pins
  1. else
   ts = TouchScreen(17, A2, A1, 14, 300); //init TouchScreen port pins
  1. endif
   //Touchscreen parameters for this hardware
   TS_MINX = 140;
   TS_MAXX = 900;
   TS_MINY = 120;
   TS_MAXY = 940;

   MapX1 = 239;
   MapX2 = 0;
   MapY1 = 319;
   MapY2 = 0;
 }

} </syntaxhighlight>


Demo Code shown

  • Insert the TFT Touch Shield to Seeeduino as shown below.
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