PPM output is noisy when reading from sensors over i2c

Question

Please excuse me for the lengthiness of the problem but its beyond my knowhow, I dont know where else to go.

I am trying to send a PPM signal to a FlySkyi10 radio over the trainer cable. When using the PPM code below I am able to send the correct PPM signals to my radio.

//this programm will put out a PPM signal

//////////////////////CONFIGURATION///////////////////////////////
#define chanel_number 8  //set the number of chanels
#define default_servo_value 1500  //set the default servo value
#define PPM_FrLen 22500  //set the PPM frame length in microseconds (1ms = 1000µs)
#define PPM_PulseLen 300  //set the pulse length
#define onState 1  //set polarity of the pulses: 1 is positive, 0 is negative
#define sigPin 10  //set PPM signal output pin on the arduino
//////////////////////////////////////////////////////////////////


/*this array holds the servo values for the ppm signal
 change theese values in your code (usually servo values move between 1000 and 2000)*/
int ppm[chanel_number];

void setup(){  
  //initiallize default ppm values
  for(int i=0; i<chanel_number; i++){
    ppm[i]= default_servo_value;
  }

  pinMode(sigPin, OUTPUT);
  digitalWrite(sigPin, !onState);  //set the PPM signal pin to the default state (off)

  cli();
  TCCR1A = 0; // set entire TCCR1 register to 0
  TCCR1B = 0;

  OCR1A = 100;  // compare match register, change this
  TCCR1B |= (1 << WGM12);  // turn on CTC mode
  TCCR1B |= (1 << CS11);  // 8 prescaler: 0,5 microseconds at 16mhz
  TIMSK1 |= (1 << OCIE1A); // enable timer compare interrupt
  sei();
}

void loop(){
  //put main code here
  static int val = 1;

  ppm[0] = ppm[0] + val;
  if(ppm[0] >= 2000){ val = -1; }
  if(ppm[0] <= 1000){ val = 1; }
  delay(10);
}

ISR(TIMER1_COMPA_vect){  //leave this alone
  static boolean state = true;

  TCNT1 = 0;

  if(state) {  //start pulse
    digitalWrite(sigPin, onState);
    OCR1A = PPM_PulseLen * 2;
    state = false;
  }
  else{  //end pulse and calculate when to start the next pulse
    static byte cur_chan_numb;
    static unsigned int calc_rest;

    digitalWrite(sigPin, !onState);
    state = true;

    if(cur_chan_numb >= chanel_number){
      cur_chan_numb = 0;
      calc_rest = calc_rest + PPM_PulseLen;// 
      OCR1A = (PPM_FrLen - calc_rest) * 2;
      calc_rest = 0;
    }
    else{
      OCR1A = (ppm[cur_chan_numb] - PPM_PulseLen) * 2;
      calc_rest = calc_rest + ppm[cur_chan_numb];
      cur_chan_numb++;
    }     
  }
}

But, the problem comes in when I read data from a sensor over i2c and some other functions. As soon as I use the i2c code, the PPM signal becomes noisy, and what should be a 1500ms signal, has noise of +/-20ms.

I dont want to make this post too bulky, I have used the i2c library for the LSM9DS0 to read sensor data. I have not used interrupts anywhere and I will have to have a closer look at the library. unfortunately i dont understand much of it. The simplified code for reading from the seensors is below. the website for the libraries is here: https://learn.adafruit.com/adafruit-lsm9ds0-accelerometer-gyro-magnetometer-9-dof-breakouts/arduino-code

#include <Wire.h>
#include <SPI.h>
#include <Adafruit_LSM9DS0.h>
#include <Adafruit_Sensor.h>  // not used in this demo but required!

// i2c
Adafruit_LSM9DS0 lsm = Adafruit_LSM9DS0();

// You can also use software SPI
//Adafruit_LSM9DS0 lsm = Adafruit_LSM9DS0(13, 12, 11, 10, 9);
// Or hardware SPI! In this case, only CS pins are passed in
//Adafruit_LSM9DS0 lsm = Adafruit_LSM9DS0(10, 9);

void setupSensor()
{
  // 1.) Set the accelerometer range
  lsm.setupAccel(lsm.LSM9DS0_ACCELRANGE_2G);
  //lsm.setupAccel(lsm.LSM9DS0_ACCELRANGE_4G);
  //lsm.setupAccel(lsm.LSM9DS0_ACCELRANGE_6G);
  //lsm.setupAccel(lsm.LSM9DS0_ACCELRANGE_8G);
  //lsm.setupAccel(lsm.LSM9DS0_ACCELRANGE_16G);

  // 2.) Set the magnetometer sensitivity
  lsm.setupMag(lsm.LSM9DS0_MAGGAIN_2GAUSS);
  //lsm.setupMag(lsm.LSM9DS0_MAGGAIN_4GAUSS);
  //lsm.setupMag(lsm.LSM9DS0_MAGGAIN_8GAUSS);
  //lsm.setupMag(lsm.LSM9DS0_MAGGAIN_12GAUSS);

  // 3.) Setup the gyroscope
  lsm.setupGyro(lsm.LSM9DS0_GYROSCALE_245DPS);
  //lsm.setupGyro(lsm.LSM9DS0_GYROSCALE_500DPS);
  //lsm.setupGyro(lsm.LSM9DS0_GYROSCALE_2000DPS);
}

void setup() 
{
#ifndef ESP8266
  while (!Serial);     // will pause Zero, Leonardo, etc until serial console opens
#endif
  Serial.begin(9600);
  Serial.println("LSM raw read demo");

  // Try to initialise and warn if we couldn't detect the chip
  if (!lsm.begin())
  {
    Serial.println("Oops ... unable to initialize the LSM9DS0. Check your wiring!");
    while (1);
  }
  Serial.println("Found LSM9DS0 9DOF");
  Serial.println("");
  Serial.println("");
}

void loop(void) 
{  
  /* Get a new sensor event */ 
  sensors_event_t accel, mag, gyro, temp;
  lsm.getEvent(&accel, &mag, &gyro, &temp); 

  // print out accelleration data
  Serial.print("Accel X: "); Serial.print(accel.acceleration.x); Serial.print(" ");
  Serial.print("  \tY: "); Serial.print(accel.acceleration.y);       Serial.print(" ");
  Serial.print("  \tZ: "); Serial.print(accel.acceleration.z);     Serial.println("  \tm/s^2");

  // print out magnetometer data
  Serial.print("Magn. X: "); Serial.print(mag.magnetic.x); Serial.print(" ");
  Serial.print("  \tY: "); Serial.print(mag.magnetic.y);       Serial.print(" ");
  Serial.print("  \tZ: "); Serial.print(mag.magnetic.z);     Serial.println("  \tgauss");

  // print out gyroscopic data
  Serial.print("Gyro  X: "); Serial.print(gyro.gyro.x); Serial.print(" ");
  Serial.print("  \tY: "); Serial.print(gyro.gyro.y);       Serial.print(" ");
  Serial.print("  \tZ: "); Serial.print(gyro.gyro.z);     Serial.println("  \tdps");

  // print out temperature data
  Serial.print("Temp: "); Serial.print(temp.temperature); Serial.println(" *C");

  Serial.println("**********************\n");

  delay(250);
}

Does anyone have an suggestions?

Answer 1

I changed the method of reading data from the sensors to SPI, which solved the problem. All noise in the ppm signal has been eliminated.