void startup(void) __attribute__((naked)) __attribute__((section (".start_section")) );

void startup ( void )
{
__asm volatile(
	" LDR R0,=0x2001C000\n"		/* set stack */
	" MOV SP,R0\n"
	" BL main\n"				/* call main */
	"_exit: B .\n"				/* never return */
	) ;
}

#define GPIOD_BASE 0x40020C00
#define GPIOE_BASE 0x40021000

#define SYSTICK_BASE 0xE000E010
#define STK_CTRL ((volatile unsigned long *) (SYSTICK_BASE))
#define STK_LOAD ((volatile unsigned long *) (SYSTICK_BASE + 0x04))
#define STK_VAL ((volatile unsigned long *) (SYSTICK_BASE + 0x08))

#define GPIOD_MODER  	* ((unsigned long *)    (GPIOD_BASE))
#define GPIOD_OTYPER 	* ((unsigned short *)   (GPIOD_BASE + 0x04))
#define GPIOD_PUPDR  	* ((unsigned long *)    (GPIOD_BASE + 0x0C))
#define GPIOD_IDR 		* ((unsigned short *)   (GPIOD_BASE + 0x10))
#define GPIOD_IDR_HIGH 	* ((unsigned char *)   	(GPIOD_BASE + 0x11))
#define GPIOD_ODR 		* ((unsigned short *)   (GPIOD_BASE + 0x14))
#define GPIOD_ODR_LOW 	* ((unsigned char *)   	(GPIOD_BASE + 0x14))
#define GPIOD_ODR_HIGH 	* ((unsigned char *)   	(GPIOD_BASE + 0x15))

#define GPIOE_MODER 	* ((unsigned long *)    (GPIOE_BASE))
#define GPIOE_OTYPER 	* ((unsigned short *)   (GPIOE_BASE + 0x04))
#define GPIOE_PUPDR  	* ((unsigned long *)    (GPIOE_BASE + 0x0C))
#define GPIOE_IDR 		* ((unsigned short *)   (GPIOE_BASE + 0x10))
#define GPIOE_IDR_LOW 	* ((unsigned char *)   	(GPIOE_BASE + 0x10))
#define GPIOE_IDR_HIGH 	* ((unsigned char *)   	(GPIOE_BASE + 0x11))
#define GPIOE_ODR 		* ((unsigned short *)   (GPIOE_BASE + 0x14))
#define GPIOE_ODR_LOW 	* ((unsigned char *)   	(GPIOE_BASE + 0x14))
#define GPIOE_ODR_HIGH 	* ((unsigned char *)   	(GPIOE_BASE + 0x15))

void app_init (void);
unsigned char keyb(void);
void ActivateRow(int);
unsigned int ReadColumn(void);
void out7seg(unsigned char c);
void irq_handler(void);
int measure_distance(void);

static volatile int unit 			= 58*10;		// 58*centimeters
static volatile int measure_delay 	= 500;     		// denotes how long delay in ms between distance measurement



void main(void){
	app_init();
	unsigned char distance;
	while(1){
		distance = measure_distance();
		out7seg(distance);
		delay_x_ms(measure_delay);
	}


}

int measure_distance(void){
	int counter = 0;
	int distance = 0;
	GPIOE_ODR_LOW = 0;
	delay_10mikro();
	GPIOE_ODR_LOW = 1;
	delay_10mikro();
	GPIOE_ODR_LOW = 0;

	while(~GPIOE_IDR_LOW & 2){
	}
	while(GPIOE_IDR_LOW & 2){
		delay_1mikro();
		counter++;
	}
	distance = counter/unit;

	return distance;
}

void delay_1mikro(void){
	*STK_CTRL = 0;
	*STK_LOAD = (168-1);
	*STK_VAL = 0;
	*STK_CTRL = 5;
	while((*STK_CTRL & 0x10000) == 0);
	*STK_CTRL = 0;
}

void delay_10mikro(void){
	for(int i = 0; i < 10; i++){
	delay_1mikro();
	}
}

void delay_x_ms(int x){
	for(int i = 0; i < (x*1000); i++){
		delay_1mikro();
	}
}

void app_init(void) {
    GPIOD_MODER = 0x5555;
	GPIOE_MODER = 0x00000001; //set PE0 to output, PE1 to input
    GPIOD_OTYPER &= 0x00FF;
    GPIOD_PUPDR &= 0x0000FFFF;
    GPIOD_PUPDR |= 0x00AA0000;
}

void out7seg(unsigned char c) {
	GPIOD_ODR_LOW = c;
}