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231 killagreg 1
#include <inttypes.h>
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#include <avr/io.h>
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#include <avr/interrupt.h>
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#include "main.h"
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volatile uint16_t CountMilliseconds = 0;
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/*****************************************************/
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/*              Initialize Timer 0                   */
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/*****************************************************/
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// timer 0 is used for the PWM generation to control the offset voltage at the air pressure sensor
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// Its overflow interrupt routine is used to generate the beep signal and the flight control motor update rate
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void TIMER0_Init(void)
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{
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        uint8_t sreg = SREG;
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        // disable all interrupts before reconfiguration
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        cli();
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        // Timer/Counter 0 Control Register A
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        // Waveform Generation Mode is Fast PWM (Bits WGM02 = 0, WGM01 = 1, WGM00 = 1)
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    // Clear OC0A on Compare Match, set OC0A at BOTTOM, noninverting PWM (Bits COM0A1 = 1, COM0A0 = 0)
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    // Clear OC0B on Compare Match, set OC0B at BOTTOM, (Bits COM0B1 = 1, COM0B0 = 0)
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    TCCR0A &= ~((1<<COM0A0)|(1<<COM0B0));
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    TCCR0A |= (1<<COM0A1)|(1<<COM0B1)|(1<<WGM01)|(1<<WGM00);
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        // Timer/Counter 0 Control Register B
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        // set clock devider for timer 0 to SYSKLOCK/8 = 20MHz / 8 = 2.5MHz
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        // i.e. the timer increments from 0x00 to 0xFF with an update rate of 2.5 MHz
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        // hence the timer overflow interrupt frequency is 2.5 MHz / 256 = 9.765 kHz
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        // divider 8 (Bits CS02 = 0, CS01 = 1, CS00 = 0)
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        TCCR0B &= ~((1<<FOC0A)|(1<<FOC0B)|(1<<WGM02));
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    TCCR0B = (TCCR0B & 0xF8)|(0<<CS02)|(1<<CS01)|(0<<CS00);
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        // initialize the Output Compare Register A & B used for PWM generation on port PB3 & PB4
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    OCR0A =  0;  // for PB3
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    OCR0B = 120; // for PB4
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        // init Timer/Counter 0 Register
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    TCNT0 = 0;
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        // Timer/Counter 0 Interrupt Mask Register
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        // enable timer overflow interrupt only
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        TIMSK0 &= ~((1<<OCIE0B)|(1<<OCIE0A));
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        TIMSK0 |= (1<<TOIE0);
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        SREG = sreg;
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}
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/*****************************************************/
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/*          Interrupt Routine of Timer 0             */
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/*****************************************************/
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ISR(TIMER0_OVF_vect)    // 9.765 kHz
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{
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    static uint8_t cnt = 0;
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        if(!cnt--) // every 10th run (9.765kHz/10 = 976Hz)
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        {
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                cnt = 9;
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                CountMilliseconds++; // increment millisecond counter
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        }
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}
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// -----------------------------------------------------------------------
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uint16_t SetDelay (uint16_t t)
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{
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  return(CountMilliseconds + t - 1);
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}
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// -----------------------------------------------------------------------
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int8_t CheckDelay(uint16_t t)
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{
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  return(((t - CountMilliseconds) & 0x8000) >> 8); // check sign bit
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}
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// -----------------------------------------------------------------------
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void Delay_ms(uint16_t w)
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{
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 unsigned int t_stop;
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 t_stop = SetDelay(w);
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 while (!CheckDelay(t_stop));
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}
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