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/*#######################################################################################

Decodieren eines RC Summen Signals

#######################################################################################*/

// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

// + Copyright (c) 04.2007 Holger Buss

// + only for non-profit use

// + www.MikroKopter.com

// + see the File "License.txt" for further Informations

// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#include <stdlib.h>

#include <avr/io.h>

#include <avr/interrupt.h>

#include "rc.h"

volatile int16_t PPM_in[11];

volatile int16_t PPM_diff[11];

volatile uint8_t NewPpmData = 1;

volatile uint8_t SenderOkay = 0;

/***************************************************************/

/*  16bit timer 1 is used to decode the PPM-Signal            */

/***************************************************************/

void RC_Init (void)

{

        uint8_t sreg = SREG;

        // disable all interrupts before reconfiguration

        cli();

        // PPM-signal is connected to the Input Capture Pin (PD6) of timer 1

        DDRD &= ~(1<<DDD6);

        PORTD |= (1<<PORTD6);

        // Channel 5,6,7 is decoded to servo signals at pin PD5 (J3), PD4(J4), PD3(J5)

        // set as output

        DDRD |= (1<<DDD5)|(1<<DDD4)|(1<<DDD3);

        // low level

        PORTD &= ~((1<<PORTD5)|(1<<PORTD4)|(1<<PORTD3));

        // Timer/Counter1 Control Register A, B, C

        // Normal Mode (bits: WGM13=0, WGM12=0, WGM11=0, WGM10=0)

        // Compare output pin A & B is disabled (bits: COM1A1=0, COM1A0=0, COM1B1=0, COM1B0=0)

        // Set clock source to SYSCLK/64 (bit: CS12=0, CS11=1, CS10=1)

        // Enable input capture noise cancler (bit: ICNC1=1)

        // Trigger on positive edge of the input capture pin (bit: ICES1=1),

        // Therefore the counter incremets at a clock of 20 MHz/64 = 312.5 kHz or 3.2µs

    // The longest period is 0xFFFF / 312.5 kHz = 0.209712 s.

        TCCR1A &= ~((1<<COM1A1)|(1<<COM1A0)|(1<<COM1B1)|(1<<COM1B0)|(1<<WGM11)|(1<<WGM10));

        TCCR1B &= ~((1<<WGM13)|(1<<WGM12)|(1<<CS12));

        TCCR1B |= (1<<CS11)|(1<<CS10)|(1<<ICES1)|(1<<ICNC1);

        TCCR1C &= ~((1<<FOC1A)|(1<<FOC1B));

        // Timer/Counter1 Interrupt Mask Register

        // Enable Input Capture Interrupt (bit: ICIE1=1)

        // Disable Output Compare A & B Match Interrupts (bit: OCIE1B=0, OICIE1A=0)

        // Disable Overflow Interrupt (bit: TOIE1=0)

        TIMSK1 &= ~((1<<OCIE1B)|(1<<OCIE1A)|(1<<TOIE1));

    TIMSK1 |= (1<<ICIE1);

    SREG = sreg;

}

/********************************************************************/

/*         Every time a positive edge is detected at PD6            */

/********************************************************************/

/*

The PPM-Frame length is 22.5 ms.

Channel high pulse width range is 0.7 ms to 1.7 ms completed by an 0.3 ms low pulse.

The mininimum time delay of two events coding a channel is ( 0.7 + 0.3) ms = 1 ms.

The maximum time delay of two events coding a chanel is ( 1.7 + 0.3) ms = 2 ms.

The minimum duration of all channels at minimum value is  8 * 1 ms = 8 ms.

The maximum duration of all channels at maximum value is  8 * 2 ms = 16 ms.

The remaining time of (22.5 - 8 ms) ms = 14.5 ms  to (22.5 - 16 ms) ms = 6.5 ms is

the syncronization gap.

*/

ISR(TIMER1_CAPT_vect) // typical rate of 1 ms to 2 ms

{

        static uint16_t oldICR1 = 0;

    int16_t signal = 0, tmp;

        static int16_t index;

        // 16bit Input Capture Register ICR1 contains the timer value TCNT1

        // at the time the edge was detected

        // calculate the time delay to the previous event time which is stored in oldICR1

        signal = (uint16_t) ICR1 - oldICR1;

        oldICR1 = ICR1;

    //sync gap? (3.52 ms < signal < 25.6 ms)

        if((signal > 1100) && (signal < 8000))

        {

                // if a sync gap happens and there where at least 4 channels decoded before

                // then the NewPpmData flag is reset indicating valid data in the PPM_in[] array.

                if(index >= 4)  NewPpmData = 0;  // Null means NewData

                // synchronize channel index

                index = 1;

        }

        else // within the PPM frame

        {

        if(index < 10) // channel limit is 9 because of the frame length of 22.5 ms

        {

                        // check for valid signal length (0.8 ms < signal < 2.1984 ms)

                        // signal range is from 1.0ms/3.2us = 312 to 2.0ms/3.2us = 625

            if((signal > 250) && (signal < 687))

            {

                                // shift signal to zero symmetric range  -154 to 159

                signal -= 466; // offset of 1.4912 ms ??? (469 * 3.2µs = 1.5008 ms)

                // check for stable signal

                // the deviation of the current signal level from the average must be less than 6 (aprox. 1%)

                if(abs(signal - PPM_in[index]) < 6)

                {

                                        // a good signal condition increases SenderOkay by 10

                                        // SignalOkay is decremented every 2 ms in main.c

                                        // this variable is a level for the average rate of a noiseless rc signal

                                        if(SenderOkay < 200) SenderOkay += 10;

                                }

                                // calculate exponential history for signal

                tmp = (3 * (PPM_in[index]) + signal) / 4;

                if(tmp > signal+1) tmp--; else

                if(tmp < signal-1) tmp++;

                // calculate signal difference on good signal level

                if(SenderOkay >= 195)  PPM_diff[index] = ((tmp - PPM_in[index]) / 3) * 3; // cut off lower 3 bit for nois reduction

                else PPM_diff[index] = 0;

                PPM_in[index] = tmp; // update channel value

            }

            index++; // next channel

            // demux sum signal for channels 5 to 7 to J3, J4, J5

                if(index == 5) PORTD |= (1<<PORTD5); else PORTD &= ~(1<<PORTD5);

                if(index == 6) PORTD |= (1<<PORTD4); else PORTD &= ~(1<<PORTD4);

                if(index == 7) PORTD |= (1<<PORTD3); else PORTD &= ~(1<<PORTD3);

        }

        }

}