Connecting Mega and Uno with I2C communication.

Projects Networking, Protocols, and Devices
1

Hello, I'm making single axis robots which have Wi-fi shield with Mega and Uno.

I designed my robot as follows:

  1. Using Mega board with wifi shield, and I2C master mode. The mega board gets magnetometer sensing data(from UART sensor) and posts data to server.

  2. The mega board gets moving information data from server, and transmits to uno board with I2C communication.

  3. The uno board gets moving information data, and controls the step motor driver.

I think that the mega board sends a signal to the uno board and works independently with posting data to server, when the uno board receives a signal and operates the For loop.

Although the motor control(using the for loop) performed in the uno board, the robot doesn't posting sensing data to server when he is moving with motor.

Is there any other issue that I don't know about I2C communication?

Here is my code. Sorry for some deleted code.

Mega board ::

void loop(){
  if(millis() - check_time > 500){
    check_time = millis();
    TX_loop(); // get sensor data and post data to server
  }
  else
    RX_loop(); // send moving data to uno
}

void TX_loop(){//   Posting protocol to server
  digitalWrite(49,HIGH);
  getSensorData();    // sensing data function
  
  //          Creating Magnetometer data
  s_val = '{' + jsonform("x", String(s_data[3])) + ','; 
  s_val += jsonform("y", String(s_data[4])) + ',';
  s_val += jsonform("z", String(s_data[5])) + '}';

  s_tmp = s_val.length();
  s_len = (int)s_tmp.toInt();
  
  s_tmp.toCharArray(s_msg,s_len+1); //s_msg :: char array , s_val :: String class

  s_msg[s_len] = '\0';
  
  if(client.connected()){
    client.write(s_msg);
    Serial.write(s_msg);
    tag++;
  }
  else{
    Serial.println("client error");
  }
}
void RX_loop(){//   Getting protocol from server

  if(client.available()) {
    c_tmp = client.readStringUntil('\n');
    c_tmp.trim();
    c_tmp = c_tmp.substring(c_tmp.indexOf('{'), c_tmp.lastIndexOf('}') + 1);
    c_tmp2 = c_tmp.length();
    c_len = (int)c_tmp2.toInt();

    c_tmp.toCharArray(c_msg, c_len+1);
    c_msg[c_len]='\0';    
  }    
  Serial.write(c_msg);
  Wire.beginTransmission(4); //transmit to device #4
  Wire.write(c_msg,c_len+1);
  Wire.write('\n');
  Wire.endTransmission();      
}

Uno board ::

void setup(){
  Wire.begin(4); // Address : #4
  Wire.onReceive(requestEvent);
}

void loop(){}

void requestEvent(int num){
  digitalWrite(13,LOW);

  String val = "";
  String _len = "";
  int len = 0;
  char json[200];
  if(Wire.available()){   
    val = Wire.readStringUntil('\n');
    val.trim();
    Serial.println(val);
    _len = val.length();
    len = (int)_len.toInt();
    val.toCharArray(json,len+1);
    json[len]='\0';
  }
  parser(json);
}

void parser(char* json) { //    Receiving data


  StaticJsonBuffer<200> RXBuffer;

  JsonObject& RXdata = RXBuffer.parseObject(json);

  if (!RXdata.success()) {
    digitalWrite(13,HIGH);
  } else {
    digitalWrite(13,LOW);
    robot._angle = RXdata["angle"];
    robot._meter = RXdata["meter"];
    robot._speed = RXdata["speed"];
    robot._startsign = RXdata["startsign"];

    motor_control(robot);
  }
}

int motor_control(motor_signal motor) {
      pulse = (int) (motor._meter * (int)pow(2,stepmode) * 1630);
      pulsefunc(pulse, pwm)
}

void pulsefunc(int pulse, int pwm) {
  for (int i = 0; i < pulse; i++) {
    digitalWrite(L_clk, HIGH);
    digitalWrite(R_clk, HIGH);
    delay(pwm);
    digitalWrite(L_clk, LOW);
    digitalWrite(R_clk, LOW);
    delay(pwm);
  }
}
2

Sending information via I2C as text is almost never done. I know that is in the examples, but everyone uses binary data.

The requestEvent() is run inside an interrupt handler, keep it as short as possible. Don't use the String object, don't use the Serial functions, certainly don't use any delay() !
Can the Uno handle all those memory eating operations with String objects ? Many would say that the String object should not be used for the Arduino Uno at all.

Could you rewrite it ? Send binary data to the Slave. Keep the requestEvent() very short, copy it to a 'volatile' global variable. Do all the actions in the loop().

3

Thank you for your reply.

Unlike your worries, Uno can handle the signal when the robot just receives moving data.
I was wonder, Since I2c communication is synchronous system, so when operating a for loop in uno board, will share with the mega board to stop.

I solved my problems with using Serial communications.