Playing Snake Game using Raspberry Pi Game Pad- (Part 38/38)

This article discusses about how to develop simple game pad hardware and interface it with the Raspberrypi board and also about the technique of interfacing a game code written in HTML5 with the game pad. Here a Snake game written in HTML5 and JavaScript is modified in such a way that it can be played with the new game pad having four push buttons. In this project the techniques of signals, pipe, fork etc. are used get the game running.

The Snake game running on a browser window forms the GUI or front end of the entire system. In a Linux operating system each hardware device is represented as a file. The device can be controlled by simply reading and writing into that file. The hardware of an operating system is on the one side and the user trying to access the hardware is on the other side, and in between them there might be several layers of process running which communicates each other using inter process communication methods. In this project there is game pad which is the hardware and there is a process which reads from the game pad and there is also a Pipe file or FIFO in-between the game and the game pad reading process.

[[wysiwyg_imageupload:11011:]]

The game runs in a browser window and it communicates with the game pad through a Named Pipe or FIFO. There are multiple processes running which can read from the game pad and write the required commands to the FIFO for controlling the game. The JavaScript written in the game code simply reads from the FIFO for the input control data.

Fig. 2: Block Diagram Of Raspberry Pi Gamepad

The parent process here creates four child processes which are dedicated for each of the keys in the gamepad. Whenever a key is pressed or released, they will generate a signal and send it to the parent process with a value. Using this method the parent process can identify which key has been pressed or released.

As soon as the parent process detects a key press from the value it received along with the signal, it simply writes a particular command value to the NAMED PIPE or FIFO. The HTML game will be reading continuously from the FIFO every 100ms and change the direction of the snake according to the command read.

The following part of the JavaScript reads from the file every 100ms and changes the direction of movement of the snake in the game.

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//

function loadFile()

{

reader = new XMLHttpRequest() || new ActiveXObject(‘MSXML2.XMLHTTP’);

reader.open(‘get’, ‘xyz’, true);

reader.onreadystatechange = displayContents;

reader.send(null);

}

function displayContents()

{

ch = reader.responseText[0];

if(ch == ‘l’)

heading = (heading == EAST) ? EAST : WEST;

else if(ch == ‘r’)

heading = (heading == WEST) ? WEST : EAST;

else if(ch == ‘u’)

heading = (heading == SOUTH) ? SOUTH : NORTH;

else if(ch == ‘d’)

heading = (heading == NORTH) ? NORTH : SOUTH;

}

//++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++//

The functions used in the coding of the Gamepad code for writing operations on the FIFO are explained in the following section.

The FIFO needs to be created one using the following command in the same directory where the executable for the gamepad and the HTML game files exist.

mkfifo xyz

The FIFO can also be created using the code itself by adding the function mkfifo(), open() as explained in a previous documentation.

The user can write data into the temporary file from the terminal itself using the ‘echo’ command as shown in the following example.

echo abcdef >> /tmp/my_fifo

The FIFO writing parent process is written in such a way that it will write commands to the FIFO in respond to the signals received from the child process. The commands that will be written to the FIFO for controlling the direction of the snake are listed below;

u -> Snake Upwards

d -> Snake Downwards

l -> Snake Left

r -> Snake RIght

These commands can be written into the FIFO with the help of the system() function.

system ( “echo r >> xyz );

The above function call will turn the Snake towards right.

Project Source Code

###

#include <bcm2835.h>
#include <pthread.h>
#include <unistd.h>
#include <signal.h>
#include <sys/time.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define IN_PIN1 RPI_GPIO_P1_07
#define IN_PIN2 RPI_GPIO_P1_22
#define IN_PIN3 RPI_GPIO_P1_18
#define IN_PIN4 RPI_GPIO_P1_16
#define OUT_PIN1 RPI_GPIO_P1_15
#define OUT_PIN2 RPI_V2_GPIO_P1_13
#define OUT_PIN3 RPI_GPIO_P1_12
#define OUT_PIN4 RPI_GPIO_P1_11
#define SNAKE_FIFO_NAME "xyz"
void set_pins_input ( void );
void set_pins_output ( void );
void set_output_pins_low ( void );
void button_signal_handler ( int sig, siginfo_t *siginfo, void *context );
void signal_on_state_change_pin1 ( void );
void signal_on_state_change_pin2 ( void );
void signal_on_state_change_pin3 ( void );
void signal_on_state_change_pin4 ( void );
void sig_set_handler ( int signo, void *handler );
void sig_send_val ( pid_t id, int signo, int val );

pid_t child_id [ 5 ];
int st = 0;
int main ( void )
{
int i;
int snake_fifo = -1;
if (!bcm2835_init())
return 1;

set_pins_output ();
set_output_pins_low ();
set_pins_input ();
delay ( 100 );

sig_set_handler ( SIGUSR1, &button_signal_handler );
if ( ! ( child_id [ 0 ] = fork () ) )
{
signal_on_state_change_pin1 ();
_exit ( 0 );
}
else;
if ( ! ( child_id [ 1 ] = fork () ) )
{
signal_on_state_change_pin2 ();
_exit ( 0 );
}
else;
if ( ! ( child_id [ 2 ] = fork () ) )
{
signal_on_state_change_pin3 ();
_exit ( 0 );
}
else;
if ( ! ( child_id [ 3 ] = fork () ) )
{
signal_on_state_change_pin4 ();
_exit ( 0 );
}
else;
while ( 1 )
{
system ( "echo x >> xyz" );
delay ( 100 );
}
bcm2835_close();
return 0;
}
void signal_on_state_change_pin1 ( void )
{
while ( 1 )
{
if ( bcm2835_gpio_lev ( IN_PIN1 ) )
{
delay ( 50 );
if ( bcm2835_gpio_lev ( IN_PIN1 ) )
{
sig_send_val ( getppid (), SIGUSR1, 1 );

do
{
while ( bcm2835_gpio_lev ( IN_PIN1 ) )
delay ( 1 );
delay ( 50 );
}
while ( bcm2835_gpio_lev ( IN_PIN1 ) );
sig_send_val ( getppid (), SIGUSR1, 1 );
}
else;
}else;
delay ( 1 );
}
}
void signal_on_state_change_pin2 ( void )
{
while ( 1 )
{
if ( bcm2835_gpio_lev ( IN_PIN2 ) )
{
delay ( 50 );
if ( bcm2835_gpio_lev ( IN_PIN2 ) )
{
sig_send_val ( getppid (), SIGUSR1, 2 );

do
{
while ( bcm2835_gpio_lev ( IN_PIN2 ) )
delay ( 1 );
delay ( 50 );
}
while ( bcm2835_gpio_lev ( IN_PIN2 ) );

sig_send_val ( getppid (), SIGUSR1, 2 );
}
else;
}else;
delay ( 1 );
}
}
void signal_on_state_change_pin3 ( void )
{
while ( 1 )
{
if ( bcm2835_gpio_lev ( IN_PIN3 ) )
{
delay ( 50 );
if ( bcm2835_gpio_lev ( IN_PIN3 ) )
{
sig_send_val ( getppid (), SIGUSR1, 3 );

do
{
while ( bcm2835_gpio_lev ( IN_PIN3 ) )
delay ( 1 );
delay ( 50 );
}
while ( bcm2835_gpio_lev ( IN_PIN3 ) );
sig_send_val ( getppid (), SIGUSR1, 3 );
}
else;
}else;
delay ( 1 );
}
}
void signal_on_state_change_pin4 ( void )
{
while ( 1 )
{
if ( bcm2835_gpio_lev ( IN_PIN4 ) )
{
delay ( 50 );
if ( bcm2835_gpio_lev ( IN_PIN4 ) )
{
sig_send_val ( getppid (), SIGUSR1, 4 );

do
{
while ( bcm2835_gpio_lev ( IN_PIN4 ) )
delay ( 1 );
delay ( 50 );
}
while ( bcm2835_gpio_lev ( IN_PIN4 ) );
sig_send_val ( getppid (), SIGUSR1, 4 );
}
else;
}else;
delay ( 1 );
}
}
void button_signal_handler ( int sig, siginfo_t *siginfo, void *context )
{
if ( 1 == *( ( int * ) &siginfo -> si_value ) )
system ( "echo u >> xyz" );
else if ( 2 == *( ( int * ) &siginfo -> si_value ) )
system ( "echo r >> xyz" );
else if ( 3 == *( ( int * ) &siginfo -> si_value ) )
system ( "echo l >> xyz" );
else if ( 4 == *( ( int * ) &siginfo -> si_value ) )
system ( "echo d >> xyz" );
else;

bcm2835_gpio_write ( OUT_PIN1, st );
st = ~ st;
}
void set_output_pins_low ( void )
{
bcm2835_gpio_write ( OUT_PIN1, LOW);
bcm2835_gpio_write ( OUT_PIN2, LOW);
bcm2835_gpio_write ( OUT_PIN3, LOW);
bcm2835_gpio_write ( OUT_PIN4, LOW);
}
void set_pins_output ( void )
{
bcm2835_gpio_fsel ( OUT_PIN1, BCM2835_GPIO_FSEL_OUTP );
bcm2835_gpio_fsel ( OUT_PIN2, BCM2835_GPIO_FSEL_OUTP );
bcm2835_gpio_fsel ( OUT_PIN3, BCM2835_GPIO_FSEL_OUTP );
bcm2835_gpio_fsel ( OUT_PIN4, BCM2835_GPIO_FSEL_OUTP );
}
void set_pins_input ( void )
{
bcm2835_gpio_fsel ( IN_PIN1, BCM2835_GPIO_FSEL_INPT );
bcm2835_gpio_set_pud ( IN_PIN1, BCM2835_GPIO_PUD_OFF );
bcm2835_gpio_fsel ( IN_PIN2, BCM2835_GPIO_FSEL_INPT );
bcm2835_gpio_set_pud ( IN_PIN2, BCM2835_GPIO_PUD_OFF );
bcm2835_gpio_fsel ( IN_PIN3, BCM2835_GPIO_FSEL_INPT );
bcm2835_gpio_fsel ( IN_PIN3, BCM2835_GPIO_FSEL_INPT );
bcm2835_gpio_set_pud ( IN_PIN4, BCM2835_GPIO_PUD_OFF );
bcm2835_gpio_set_pud ( IN_PIN4, BCM2835_GPIO_PUD_OFF );
}
void sig_send_msg ( pid_t id, int signo, char *msg )
{
union sigval *sigdata;

sigdata = malloc ( sizeof ( union sigval ) );
sigdata -> sival_ptr = msg;
sigqueue ( id, signo, *sigdata );

free ( sigdata );
}
void sig_send_val ( pid_t id, int signo, int val )
{
union sigval *sigdata;

sigdata = malloc ( sizeof ( union sigval ) );
sigdata -> sival_int = val;
sigqueue ( id, signo, *sigdata );
free ( sigdata );
}
void sig_set_handler ( int signo, void *handler )
{
struct sigaction *act;
act = malloc ( sizeof ( struct sigaction ) );
act -> sa_sigaction = handler;
act -> sa_flags = SA_SIGINFO;
sigaction ( signo, act, NULL );
}
 
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