adc.c

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

#include "adc.h"

#define ADC_VOLTAGE_REFERENCE 5.22

#define NUM_ADC_READS 200

/*
 *Funtion that sets uo the ADC with the required congifurations
*/
void adc_setup(void){
   DDRC &= ~(1 << OAF_OUT); // Set ADC0 as input just in case
   DDRC &= ~(1 << OAR_OUT); // Set ADC1 as input just in case

   // Define the prescaler to 128 so that the ADC input frequency is set to 125Khz, a value \
      between 50Khz and 200Khz according to the datasshet, 24.4, page 240
   ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);

   // Set the ADC voltage reference as AVCC with an external cap at AVREF according to the  \
      24.9.1, page 248
   ADMUX |= (1 << REFS0);

   //Set ADC as Single Conversion mode
   PRR &= ~(1 << PRADC);

   // Set right adjust ADC result to allow 10 bit full resolution.
   ADMUX &= ~(1 << ADLAR);
}


/*
 * Function that starts the reading and conversion of the ADC readed values
*/
void adc_start(uint8_t adc_ch){
   //Set the channel to read values from
   ADMUX = (ADMUX & 0xf0) | (adc_ch & 0x0f);

   // Enable ADC to start reading values
   ADCSRA |= (1 << ADEN);

   // Start the A2D conversion
   ADCSRA |= (1 << ADSC);
}

/*
 * Function that read, convert and store the vswr value
*/
void adc_read(float *vswr_val){
   uint8_t forward[NUM_ADC_READS] = {0}, reverse[NUM_ADC_READS] = {0};
   float forward_avg = 0, reverse_avg = 0;

   // Read NUM_ADC_READS times both ADC0 and ADC1 channels and store them
   for(uint8_t i = 0; i < NUM_ADC_READS; i++){
      //Read forward value
      adc_start(OAF_OUT);
      while(ADCSRA & (1 << ADSC));
      forward[i] = ADC;

      //Read reverse value
      adc_start(OAR_OUT);
      while(ADCSRA & (1 << ADSC));
      reverse[i] = ADC;
   }

   // Evaluate average value for forward and reverse
   for(uint8_t i = 0; i < NUM_ADC_READS; i++){
      forward_avg += forward[i];
      reverse_avg += reverse[i];
   }
   forward_avg /= NUM_ADC_READS;
   reverse_avg /= NUM_ADC_READS;

   // Convert to voltage value
   forward_avg *= (ADC_VOLTAGE_REFERENCE / 1024);
   reverse_avg *= (ADC_VOLTAGE_REFERENCE / 1024);

   //Evaluate VSWR value
   if(reverse_avg >= forward_avg) //To prevent dividing by zero or get negative VSWR
      *vswr_val = 9999;
   else
      *vswr_val = (forward_avg + reverse_avg) / (forward_avg - reverse_avg);
}