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#include <stdint.h>
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#include <avr/io.h>
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#include <avr/interrupt.h>
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#include "time.h"
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/* F_CPU / PRESCALE = OCR * 1000 + ERROR */
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#define OCR (F_CPU / PRESCALE / 1000UL)
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#define ERROR (F_CPU / PRESCALE - OCR * 1000UL)
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static char ready, ms;
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static uint16_t error;
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ISR(TIMER0_COMPA_vect) {
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  error += ERROR;
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  if (error >= 1000) {
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    error -= 1000;
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    OCR0A = OCR+1;
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  } else {
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    OCR0A = OCR;
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  }
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  if (++ms == TICK_MS) {
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    ready = 1;
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    ms = 0;
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  }
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}
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void time_init() {
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  OCR0A = OCR;
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  TIMSK = _BV(OCIE0A);
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  TCCR0A = _BV(WGM01);
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  TCCR0B = CLOCK_SEL;
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}
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void time_wait() {
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  while (!ready);
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  ready = 0;
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}