ping.c

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#include "main.h"

#define period 20
//we found these by trial and error basically
//( 20ms - 1.5 )* 16,000 = 296000 should be the neutral position at 90 degrees, but we had to calibrate
#define clockwise 0x4E000 //319488
#define cclockwise 0x47500 //292096
#define TIMER4B_PRESCALER 50


volatile int overflow;
volatile int rise_time; // start time of the return pulse
volatile int fall_time;// end time of the return pulse
volatile int j;

volatile float servo_angle;


void TIMER3B_Handler(void)
{
    TIMER3_ICR_R |= TIMER_ICR_CBECINT; //clear interrupt flag

    if(j%2)
    {
        //if j is even, it must be the fall time
        fall_time = TIMER3_TBR_R;
    }
    else
    {
        //must be a rise time
        rise_time = TIMER3_TBR_R;
    }

    //j is incremented every clock cycle
    j++;
}


void ping_init()
{
    //enable the clock to port B; turn on the clock
    SYSCTL_RCGCGPIO_R |= SYSCTL_RCGCGPIO_R1;
    GPIO_PORTB_PCTL_R|=0x7000;
    GPIO_PORTB_DEN_R|=0x08;
}



void send_pulse()
{
    GPIO_PORTB_DIR_R|=0x08;
    GPIO_PORTB_AFSEL_R&=~0x08;
    GPIO_PORTB_DATA_R|=0x08;
    //wait at least 5 microseconds
    timer_waitMicros(5);
    GPIO_PORTB_DATA_R&=~0x08;
    GPIO_PORTB_AFSEL_R|=0x08;
    GPIO_PORTB_DIR_R&=~0x08;
}


int ping_read()
{
    send_pulse();

    //wait for interrupts
    timer_waitMillis(3);

    //we don't want the width to be negative, so we have to account for overflow
    if(fall_time < rise_time)
    {
        overflow++;

        return ping_read();
    }

    timer_waitMillis(4);

    //fall time - rise time = pulse width in clock cycles
    return (fall_time - rise_time);
}


double time2dist(int time)
{
    //(speed of sound in ms / 16MHz clock) / 2
    //340000/ 16000000 / 2
    return time*0.010622;
}


void clock_timer_init3(void)
{
    SYSCTL_RCGCTIMER_R |= SYSCTL_RCGCTIMER_R3; // Enable clock

    //Page724 in datasheet
    //Configuration settings for timer
    TIMER3_CTL_R &= 0x04; //disable timerB
    TIMER3_CFG_R |= 0x04; 
    TIMER3_CTL_R |= TIMER_CTL_TBEVENT_BOTH;  //enable both edges
    TIMER3_TBMR_R |= 0x03; //enable capture
    TIMER3_TBMR_R |= TIMER_TBMR_TBCDIR; //count
    TIMER3_TBMR_R |= TIMER_TBMR_TBCMR; //edge-time mode
    TIMER3_ICR_R = 0x400;; //clear TIMER3B interrupt flags
    TIMER3_IMR_R |= TIMER_IMR_CBEIM; // Setting interrupt mask to event
    //initialize local interrupts
    NVIC_EN1_R = 0x18; ;// 0001 1000; //#warning "enable interrupts for TIMER3A and TIMER3B" n = 0, 1, 2, 3, or 4
    IntRegister(INT_TIMER3B, TIMER3B_Handler); //register TIMER3B interrupt handler
    IntMasterEnable(); //intialize global interrupts
    TIMER3_CTL_R |= (TIMER_CTL_TBEN); //Enable TIMER3A & TIMER3B
}