A Touchscreen Chinese Chess App

Software Design and Implementation

Overview

The most important part for the software design is the touch screen control and the rule implementations.

LCD drawing

The LCD we use is a 3.2" TFT screen. It has a resolution of 320 × 240, as the following figure shows. There are 240 pixels each line and 320 pixels each column. We can use (x, y) coordinate to represent each pixel on screen.

Key control signals for SSD1289 TFT Driver are listed: lcd_reset, lcd_cs(chip select), lcd_rs(0 for register address, 1 for value), lcd_wr(0 for write), lcd_rd(set 0 all the time in this project).

Timing characteristic of 16-bit interface

The set up for writing data to a specified register can be figured out from the image shown above.

We have three related functions to fulfill the communication between the Atmega1284P and the LCD.

void LCD_Write_Com( unsigned int reg )//specify the register you are going to write

void LCD_Write_Data( unsigned int value )//write the register you specified

void LCD_Write_Com_Data( unsigned int reg, unsigned int value )// write a specified register

 

Initialize of LCD screen

We use 16-bit data for the TFT LCD touch screen. The SSD1289 TFT Driver has more than 40 registers. Data are wrote to these registers to initialize the TFT LCD touch screen. Related code is in the "LCD_Init()" function. 

Useful registers for drawing on the touch screen:

R4Eh and R4Fh make initial settings for the GDDRAM X address counter and GDDRAM Y address counter(AC). After GDDRAM data are written, the address counter is automatically updated according to the settings with AM. I/D bits and settings for a new GDDRAM address is not required in the address counter. Therefore, data are written consecutively without setting an address.

R44h specify the start/end positions of the window address in the horizontal direction by and address unit. High 8 bits are for the end position of the window, while low 8 bits are for the start position of the window. Then R45h/R46h are for the start/end positions of the window address in the vertical direction. R22h transforms all the GDDRAM data into 18-bit, and writes the data. Format for transforming data into 18-bit depends on the interface used. After writing to GDDRAM, address is automatically updated according to AM bit and ID bit. Access to GDDRAM during stand-by mode is not available. So from the registers above we can draw a pixel and draw a set of area. Write the addresses for start horizontal/vertical position to R4E/R4F, then write color to R22, and then by setting the CS to 0, the function of drawing a pixel is realized. Then by set the initial position to R4E and R4F, and then set the area to be drawn with R44h, R45h and R46h. After doing this, write the related color to R22h. For example, set the horizontal start position to 0, end position to 239; set the vertical start position to 0, end position to 319; set R4Eh and R4Fh to 0; then write a for loop to write the color to R22 for 240*320 times, then the whole screen will the paint the color you wrote. With the method above we construct some basic functions necessary for drawing chessboard and the menu.

void Paint(color) // Paint the total screen to one color

void draw_line(unsigned int x1,unsigned int y1,unsigned int x2,unsigned int y2,unsigned int color)

void draw_rectangle(unsigned int x1, unsigned int y1,unsigned int x2,unsigned int y2, unsigned color)

void video_chessmate(int x, int y, char c,unsigned color)//draw a single chessman

LCD touch detecting

The Touch Panel is of the same size as the LCD Screen and is transparent. We have to link the position of the place we touch with the display's position.

The ADS7843 is a resistive touch screen controller that incorporates a 12-bit 125 kHz sampling SAR typeA/D converter. It can detect the pressed screen location by performing two A/D conversions.

The control signals for touch panel are DOUT, BUSY, DIN,  CS and DCLK. Timing characteristics are as below:

 

Timing Characteristic for touch panel

We have two functions for communication with the touch panel: WriteSPI() for writing commands to the touch panel, write 0xD0 for the X-position and 0x90 for the Y-position, ReadSPI() to receive the X-position/Y-position according to the command we gave. (0xD0 and 0x90 mentioned above are for 12 bits data)

This X/Y position is a 12 bits value, and can be mapped exactly to the coordinate (0,239)/(0,319). For the reason that our screen is not big enough, we would have a lot of deviation if we map the coordinate to a smaller scale. So we still use the 12bit value for verifying the position.

The PENIRQ of touch panel is connected to PIND2 of the microcontroller, which is the pin for INT0 external interrupt. Usually the touch panel is not pressed, and the output of PENIRQ stays high. Whenever the touch panel is pressed, the PENIRQ would change to low. And this falling edge is caught by INT0, X/Y positions are read using WriteSPI and ReadSPI in INT0. But a problem came out, that is more than one falling edge will be generated when the touch panel is pressed. We solved this problem by using timer0.

Debounce strategy

Whenever the touchscreen is pressed, there will be a voltage drop, and causes an external interrupt. In the external interrupt ISR we record the coordinate of the point being touched. Based on observation of the number of coordinates being recorded for each touch on Putty, we made two integer buffer of size 40 to store the X-coordinate and Y-coordinate. Whenever a touch is made, it enters the timer0 external interrupt. In this ISR, a XY coordinate is recorded, and we store the coordinates in the buffer. Before leaving this external interrupt ISR, we set the debounce timer to 500, the timer0 compare match interrupt ISR will be entered every 1ms, and in this ISR the debounce timer will be decreased by 1, when the debounce timer becomes 0, this means there is no touch for 0.5s, all the coordinates for the same touch should be all stored in the buffer. So we calculate the average value, and clear buffer for next touch.

Chinese Chess Game Implementations

We construct a structure “chessman” to help us implement the Chinese chess game.

typedef struct {
int x,y; // store the current position of the chessman
char c; // the character of the chessman, such as „k‟ for king, „m‟ for minister
int pre_x, pre_y;// the previous position of the chessman, used for play back
int blong; // „0‟ for the red part, „1‟ for black part
int id;// id for each chessman from 1 to 32
char property; // there are 7 kinds of chessman, they have different rules when make a move
} chessman;

And there are three main functions helping us implement the game.

In the function move_chessman(int id,int to_x,int to_y), there is a switch case, according to the property of selected chessman, different rules are implemented, if the move is not legal, then the move can not be made, and sign will be set to 0, means no move. 1 means a move is made successfully. The function eat_chessman(int id,int to_x,int to_y,int eaten_id) is similar to function move_chessman().
In the function task(), identify a point on the chessboard according to the touched coordinates, if there is chessman on this point, and is of the same party as the before selected chessman, or there is no selected chessman, then set this chessman as the selected chessman, if there is a chessman, but it is of the other side, then call function eat_chessman, if the move is made successfully, then change the flag, means it is other ones turn to make a move, and clear the selected chessman. If there is no chessman on this point, and there is a selected chessman, then call function move_chessman(),if the move is made successfully, then change the flag, means it is other one‟s turn to make a move, and clear the selected chessman, means there is no selected chessman for now.

 

Mainpage

The mainpage of our program will draw the user interface. We start drawing the mainpage by drawing two rectangles with different colors. We also shows the string in the rectangles to tell the user how to start. One figure of what our mainpage looks like is shown below. When the user touch on either of the color bar, the LCD will jump to the corresponding page.

Play Page

In the play page, two players can play the chess. One side is red, and the other is black. The red one must start first, and the player click on the chessman he/she wants to move, and then click the point or the chessman he/she wants to move to or eat. The game is over until one side been checkmate.

Chinese Chess Game rules' description: There are 7 kinds of chessman: King ('k'), Minister(M),Bishop('B'), Knight('K'),Rook('R'), Gun('G'), Pawn('P'). The red side uses red chessmen with character of lowcase letter on it. The black side uses black chessmen with uppercase letter on it.

Generals are labeled with the 'Q' on the black side and 'k' on the red side.

The general starts the game at the midpoint of the back edge, and can only move within the palace. The general may move and capture one point orthogonally.

Ministers are labeled 'M' for Black and 'm' for Red.
The minister start on either side of the general. They move and capture one point diagonally and may not leave the palace, which confines them to five points on the board. The advisor is probably derived from the mantri in chaturanga, like the queen in Western chess.

Bishops also known as elephants, are labeled 'B' for Black and 'b' for Red. They are located next to the ministers. These pieces move and capture exactly two points diagonally and may not jump over intervening pieces; the move is described as being like the character 田 Tián ("field"). If a bishop cannot move due to a diagonally adjacent piece, it is known as "blocking the elephant's eye". Bishop may not cross the river, and serve as defensive pieces.

Knight also known as horses are labeled 'K' for Black and 'k' Red. Horses begin the game next to the elephants, on their outside flanks. A horse moves and captures one point orthogonally and then one point diagonally away from its former position, a move which is traditionally described as being like the character 日 Rì. The horse can be blocked by a piece located one point horizontally or vertically adjacent to it. Blocking a horse is called "hobbling the horse's leg".

Rooks are labeled 'R' for Black and 'r' for Red. The rook moves and captures any distance orthogonally, but may not jump over intervening pieces. The rooks begin the game on the points at the corners of the board.

Guns (aka cannons) are labeled 'G'for Black and 'r' for Red. Each player has two cannons, which start on the row behind the soldiers, two points in front of the horses. Cannons move like chariots, any distance orthogonally without jumping, but can only capture by jumping a single piece, friend or foe, along the path of attack. The piece over which the cannon jumps is called the "cannon platform". Any number of unoccupied spaces, including none, may exist between the cannon, screen, and the piece to be captured.

Each side has five pawns also known as soldiers, labeled 'P' for Black and 'p' for Red. Soldiers begin the game located on every other point one row back from the edge of the river. They move and capture by advancing one point. Once they have crossed the river, they may also move and capture one point horizontally. Soldiers cannot move backward, and therefore cannot retreat.

There is one way to go back to main page, manually tap the left bottom of the screen, where there shows"Tap_to_main_menu"

Settings Page

In the Settings mode, users can set the background color. The picture below shows what our setting page looks like. Users can click on the color bar, and the background color of the play page will be set to that corresponding color.

The figure below shows the setting page

Settings Page