#include <16F648A.h>
#fuses INTRC_IO,NOWDT,PUT,NOPROTECT,NOBROWNOUT,NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600,parity=N,xmit=PIN_B2,rcv=PIN_B1,bits=9)

#define  trigger PIN_B4
#define  led1    PIN_B5

long rise,fall,pulse_width;

#int_ccp1
void isr() {
   fall = CCP_1;

   pulse_width = fall - rise;     // CCP_1 is the time the pulse went high
}                                 // CCP_2 is the time the pulse went low
                                  // pulse_width/(clock/4) is the time
                                  //with 4,000,000 clk each tmr1 increment = 1uS

                                  // In order for this to work the ISR
                                  // overhead must be less than the
                                  // low time.  For this program the
                                  // overhead is 45 instructions.  The
                                  // low time must then be at least
                                  // 9 us.
long convert(long echo){
   long distance;
   distance = echo / 74;
   return distance;
}

void main() {
   long dist;
   output_low(trigger);
   output_high(led1);
   printf("\r\nHigh time (sampled every second):\r\n");
   setup_ccp1(CCP_CAPTURE_FE);    // Configure CCP1 to capture fall

   setup_timer_1(T1_INTERNAL);    // Start timer 1

   enable_interrupts(INT_CCP1);   // Setup interrupt on falling edge
   enable_interrupts(GLOBAL);

   while(TRUE) {
      output_high(trigger);
      delay_us(20);
      output_low(trigger);
      rise = get_timer1();
      delay_ms(20);
      dist = convert(pulse_width);
      if (dist< 75){
         output_low(led1);
         printf("\r dist:%lu inches",dist);
         }
      else {
         //timeout occurred, no obastacle
         output_high(led1);
      }
   }

}