ppm.cpp

#include "ppm.h"
#include <avr/io.h>
#include <avr/interrupt.h>



#define CHANNELS (8)
volatile unsigned int ppm[CHANNELS];
volatile int chan=-1;

volatile char ppmNewData;
volatile unsigned int timeOld;

/**
* Interrupt: Compare 1A match = timeout
*/
ISR (TIMER1_COMPA_vect) {
  if (chan!=-1) {
    ppmNewData=1;
  }
  chan=-1;
}

/**
* Interrupt: ICP-Edge detect
*/
ISR (TIMER1_CAPT_vect) {

  unsigned int time = ICR1;

  OCR1A = time + T_OUT;

  if (time>timeOld) {
    time = time - timeOld;
  } else {
    time = time + (0xffff - timeOld) + 1;
  }
  timeOld = ICR1;
  if (chan >= 0 && chan < CHANNELS) {
    ppm[chan] = time;
  }
  chan++;
}

/**
* Init Timer1
*/
void ppmInit() {

#ifndef TIMSK
  // for Mega48/88/168
#define TICIE1 ICIE1
#define TIMSK TIMSK1
#endif

  TCCR1A = 0; // without this Timer1 on atmega328p is only 8 bit long
  TCCR1B = 0; // ..for good measure..
  // tested with ATMega8
  TCCR1B=(1<<CS10); // TClk=CPUClk (no prescaler)
  TCCR1B|=(1<<ICNC1); // enable input noise chancelor (wenn schon, denn schon)
  //TCCR1B|=(1<<ICES1); // positive edge (comment out for inverted signals)
  TIMSK=(1<<TICIE1)|(1<<OCIE1A); // ICP-Interrupt and Output Compare A Match enable

  ppmNewData=0;
  chan=-1;
}

/**
* Get PPM-Channel n (8-bit resolution)
*/
unsigned char ppmGet(int n) {
  unsigned int t=ppm[n];

  // conversion T_MIN...T_MAX -> 0...255
  if (t<T_MIN) t=T_MIN;
  t-=T_MIN;
  t=t/((T_MAX-T_MIN)/255);
  if (t>255) t=255;

  return ((unsigned char)t);
}