#include <avr/io.h>
#include <avr/interrupt.h>

#define PE4_Clear (PORTE &= 0xEF)
#define PE4_Set  (PORTE |= 0x10)
#define PE4_IN  (DDRE &= 0xEF)
#define PE4_OUT  (DDRE |= 0x10)

#define LED_ON   (PORTA |= 0x01)
#define LED_OFF      (PORTA &= 0xFE)

#define TEMPERATURE 25

unsigned short tick=0, pulse_check=0, pulse_end=0;

void us_delay(unsigned int us_time)
{
 unsigned int i;

 for(i=0; i<us_time; i++) // 4 cycle +
 {
  asm("PUSH R0"); // 2 cycle +
  asm("POP R0");   // 2 cycle +
  asm("PUSH R0"); // 2 cycle +
  asm("POP R0");   // 2 cycle + =12 cycle for 11.0592MHZ
  asm("PUSH R0"); // 2 cycle +
  asm("POP R0");   // 2 cycle = 16 cycle = 1us for 16MHz
 }
}

void ms_delay(unsigned int ms_time)
{
    unsigned int i;
   
    for(i=0; i<ms_time;i++)
        us_delay(1000);
}

//========================================
// ultra sound
void init_EXT_INT(void)
{
 EIMSK |= (1<<INT4);  // INT4 Interrupt Set
 EICRB |= (1<<ISC41)|(1<<ISC40); //INT4 Rising Edge / Interrupt Set
 EIFR   = 1<<INT4;  // Interrupt Flag
}

void init_TIMER0_COMPA(void)
{
 TCCR0A |= (1<<WGM01); // CTC Mode
 TCCR0A |= (1<<CS01); // clk/8 Prescaler
 TCNT0 = 0;
 OCR0A = 19;    //1cycle->20us = 1/(16Mhz/(2*8*(19+1)))
 TIMSK0 = (1<<OCIE0A); // T/C0 Output Compare Match Interrup Enable
 TIFR0 = 0x00;
}

SIGNAL(TIMER0_COMP_vect) 
{
 tick++;   // 20usec 단위 업 카운트
}
  
SIGNAL(INT4_vect)
{
 unsigned short pulse_tick;

 pulse_tick = tick;

 if(EICRB & 0x03) // 
 {
    EICRB &= 0x00; // INT4 low state set
    tick = 0;  // tick clear
 }
 else
 {
  EICRB |= (1<<ISC41)|(1<<ISC40); //INT4 Rising Edge/Interrupt Set
  pulse_end = pulse_tick; // echo pulse count
 }
}

//======================================
// port init
void initPort(void)
{
 DDRA  = 0xff;
 DDRE  = 0xef;
 PORTE = 0x10;
}

void main(void)
{
 float distance;
 
 initPort();

 init_EXT_INT();
 init_TIMER0_COMPA();
 
 while(1)
 {
  cli();   // interrupt clear
  PE4_Clear;
  PE4_OUT;  // PE4 pin is output
  us_delay(500);

  PE4_Set;  // output set during 5us
  us_delay(5);

  PE4_Clear;  // output clear
  us_delay(100);

  PE4_IN;   // PE4 pin is input
  us_delay(100);

  sei();   // interrupt enable

  /* distance = velocity * time */

      distance=(331.5+(0.6*TEMPERATURE))*(pulse_end*0.00001/2)*1000;
  
  if(distance>500){   // 50~100cm
   if(distance<1500) LED_ON;
  }
  else LED_OFF;
 }
}

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