Tooltron

#include "current.h"

#include <avr/io.h>

#include <avr/interrupt.h>

#include <math.h>

#define CYCLES_PER_SECOND 60 // wall power

#define SAMPLES_PER_CYCLE 16

#define N_SAMPLES 48

unsigned int samples[N_SAMPLES];

int sample_idx;

unsigned int sum;

unsigned long sum_sq;

void current_init() {

  /* TODO reduce power consumption with DIDR* */

  /*

   * COM1A = COM1B = 0, disconnect pins

   * WGM1 = 4, clear timer on compare A

   * CS1 = 1, no prescaler

   */

  TCCR1B = _BV(WGM12) | _BV(CS10);

  /* Timer is cleared on A, ADC is triggered on B */

  OCR1A = F_CPU / SAMPLES_PER_CYCLE / CYCLES_PER_SECOND;

  OCR1B = OCR1A;

  /*

   * REFS = 0, Vcc reference (set to 2 for internal 1.1V reference)

   * MUX = 8, PB3(ADC8)

   */

  ADMUX = _BV(MUX3);

  /*

   * ADLAR = 0, right adjust result

   * ADTS = 5, start on timer 1 compare match B

   */

  ADCSRB = _BV(ADTS2) | _BV(ADTS0);

  /*

   * ADEN = 1, enable

   * ADSC = 0, don't start yet

   * ADATE = 1, auto trigger

   * ADIE = 1, enable interrupt

   * ADPS = 6, prescale clock by 64

   */

  ADCSRA = _BV(ADEN) | _BV(ADATE) | _BV(ADIE) | _BV(ADPS2) | _BV(ADPS1);

}

ISR(ADC_vect) {

  unsigned int old, new;

  /* clear the timer interrupt flag so that the ADC will be triggered again

   * next time the timer resets */

  TIFR |= _BV(OCF1B);

  new = ADC;

  /* put sample into ring buffer */

  old = samples[sample_idx];

  samples[sample_idx++] = new;

  if (sample_idx == N_SAMPLES)

    sample_idx = 0;

  /* keep a running total of samples and samples squared */

  sum += new;

  sum -= old;

  sum_sq += (unsigned long)new*new;

  sum_sq -= (unsigned long)old*old;

}

unsigned int current_read() {

  unsigned int _sum;

  unsigned long _sum_sq;

  cli();

  _sum = sum;

  _sum_sq = sum_sq;

  sei();

  /* calculate the variance using sum and sum_sq */

  return (N_SAMPLES*_sum_sq - (unsigned long)_sum*_sum) /

    (N_SAMPLES*(N_SAMPLES-1));

}