SaltForTheThirsty

    

/*
 * PingBridge.c
 *
 * Created: 12/8/2012 9:23:20 PM
 * Author: Daniel Mack
 * 
 * Function:
 * to serve as an inbetween for a microcontroller and the Parallax Ping)) sonar.
 * Triggers the sonar and stores the distance and sends it out the SPI
 * in slave mode. Can return about 55 measurements a second at
 * distance = 10 feet, which about the max of the Parallax Ping))) 
 * 
 * PORT B PIN 0 is connected to the Sig pin of the Ping)))
 * 
 * code good at 3.69 MHz
 */ 

// includes
#include 
#include 
#include 

// defines
// TIMEOUT should be set at (.0185*fclk)/8
#define TIMEOUT 8533	// timeout at about 10.3 feet at 3.69 MHz
#define low(x) ((x) & 0x00FF)
#define high(x) (((x)>>8)& 0x00FF)

// function prototypes
uint16_t time_stamp(uint8_t);

// global variables
volatile uint8_t edgeDetect = 0;
volatile uint8_t conversionComplete = 0;
volatile uint16_t distance = 0;

int main(){
  // initialize hardware
  // Ports
  DDRB = (1<<PB0) | (1<<PB4); // MISO and PB0
  // spi
  SPCR = (1<<SPIE) | (1<<SPE);	// interrupt, f/4
  SPDR = 0;
  // timer counter 1
  OCR1A = TIMEOUT;
  uint16_t risingEdge = 0;
  uint16_t fallingEdge = 0;

  while(1){
    // trigger the sonar with a 5 micro second pulse
    DDRB |= (1<<PB0);
    cli();
    // atmega88pa pins can be flipped by writing to them
    // hence the two writes and not one write and one
    // clear
    PINB |= (_BV(PB0));
    _delay_us(5);
    PINB |= (_BV(PB0));
    sei();
    // PB0 now input to receive pulse from Ping)))
    DDRB &=~(1<<PB0);
    // Start Timer
    // set timer up to capture incoming pulse
    // rising edge, ctc, OCR1A as top, f/8
    TCCR1B = (1<<ICES1) | (1<<WGM12) | (1<<CS11);
    // clear pending interrupts
    TIFR1 |= (1<<ICF1) | (1<<OCF1B) | (1<<OCF1A);
    // input capture compA interrupt enabled
    TIMSK1 = (1<<ICIE1) | (1<<OCIE1A);

    // look for rising edge, timestamp it then look for falling edge, timestamp it.
    // Do math to find distance. Distance is just a number of counts not normal units
    // ie inches or cm.
    while(conversionComplete == 0){
      if(edgeDetect == 1)
        risingEdge = time_stamp(0);
      if(edgeDetect == 3){
        cli();
        fallingEdge = time_stamp(1);
        distance = fallingEdge - risingEdge;
        SPDR = low(distance);
        conversionComplete = 1;
        sei();
      }
    }
    
    // delay 200 microseconds before next reading 
    // as instructed by Ping))) datasheet
    // stop and reset timer
    conversionComplete = 0;
    TCCR1B = 0x00;
    TCNT1 = 0x0000;
    TIMSK1 = 0x00;
    _delay_us(200);
  }
}

// functions

// returns timer value at rising edge, sets timer to 
// look for falling edge and returns timer value
// at falling edge
uint16_t time_stamp(uint8_t c){
  switch(c){
    case 0:
      edgeDetect++;
      TCCR1B &= ~_BV(ICES1);
      break;
    case 1:
      edgeDetect = 0;
      break;
}
  return(ICR1);
}

// ISR

ISR(SPI_STC_vect){
  SPDR = high(distance);
}

// sets resets after timeout
ISR(TIMER1_COMPA_vect){
  conversionComplete = 1;
  edgeDetect = 0;
  SPDR = 0;
  distance = 0;
}

ISR(TIMER1_CAPT_vect){
  edgeDetect++;
}