I2C connection between arduino due and arduino mega 2560

Question

I am trying to interface a mega with a due per I2C. I have done:

• grounds are shared.
• SCL/SDA on mega 2560 (not 20/21) are connected to SCL1/SDA1 on due. SCL/SDA of due (20/21) are connected to an imu bno055 which is working fine.
• no pull up resistors are connected.

on mega I do:

#include <Time.h>
#include <DFRobot_sim808.h>
#include <SoftwareSerial.h>
#include <Wire.h>
#include <EasyTransferI2C.h>

EasyTransferI2C ET_GPS_data; 
struct SEND_DATA_STRUCTURE{
  double GPS_la;
  double GPS_lo;
  double GPS_alt;
};

SEND_DATA_STRUCTURE GPS_data;
#define I2C_SLAVE_ADDRESS 9

SoftwareSerial mySerial(PIN_TX,PIN_RX);
DFRobot_SIM808 sim808(&mySerial);

double GPS_la = 0.0;
double GPS_lo = 0.0;
double GPS_alt = 0.0;
#define GPS_Sampling_Time_ms 20
unsigned long currentMillis_GPS = 0;
unsigned long previousMillis_GPS = 0;

void getGPS();

void setup() {
  mySerial.begin(9600);
  Wire.begin();

  // ******** Initialize sim808 module *************
  while(!sim808.init())
  {
     delay(1000);
  }
  delay(3000);

  if(sim808.attachGPS()){
     //Serial.println("Open the GPS power success");
  }else{
     //Serial.println("Open the GPS power failure");
  }
  ET_GPS_data.begin(details(GPS_data), &Wire);

}

void loop() {
  currentMillis_GPS = millis();
  if (currentMillis_GPS - previousMillis_GPS > GPS_Sampling_Time_ms) {
    getGPS();
    sim808.detachGPS();
    previousMillis_GPS = currentMillis_GPS;
    ET_GPS_data.sendData(I2C_SLAVE_ADDRESS);
  }
  GPS_data.GPS_la = GPS_la;
  GPS_data.GPS_lo = GPS_lo;
  GPS_data.GPS_alt = GPS_alt;
}

void getGPS(){ 
  while(!sim808.attachGPS()){}
  delay(80);
    
  while(!sim808.getGPS()){}
  GPS_la = sim808.GPSdata.lat;
  GPS_lo = sim808.GPSdata.lon;
  GPS_alt = sim808.GPSdata.altitude;
}

on due:

#include <Arduino.h>
#include <Wire.h>
#include <EasyTransferI2C.h>
EasyTransferI2C ET_GPS_data; 
struct RECEIVE_DATA_STRUCTURE{
  double GPS_la;
  double GPS_lo;
  double GPS_alt;
};
RECEIVE_DATA_STRUCTURE GPS_data;
double GPS_la = 0.1;
double GPS_lo = 0.1;
double GPS_alt = 0.1;
#define GPS_Sampling_Time_ms 100
unsigned long currentMillis_GPS = 0;
unsigned long previousMillis_GPS = 0;

void setup() {
 Serial.begin(57600);
 Wire1.begin(I2C_SLAVE_ADDRESS);
 ET_GPS_data.begin(details(GPS_data), &Wire1);
 Wire1.onReceive(receive);
}

void loop() {
if(ET_GPS_data.receiveData()){
    GPS_la = GPS_data.GPS_la;
    GPS_lo = GPS_data.GPS_lo;
    GPS_alt = GPS_data.GPS_alt;
    Serial.println(GPS_alt);
  }
}

without the i2c, both of the boards work fine. What am I doing wrong? is this just because I have no pull up resistors? and if yes, how should I add it/them?

EDIT: I have followed another path and used serial connection instead.

Answer 1

Firstly, try to put pull up resistor in any I2C connection, Secondly, you should note that Due runs at 3.3V and Mega runs at 5V. therefore, there should be a voltage level shifter (3.3V to 5V) between these two I2C devices.