I2C 2 way communication between Arduino Uno and Arduino Mega

Question

If I remove the transmission code from slave this works properly but I need to transmit from slave.

Here is my master code

#include <Wire.h>
void setup() {
 Wire.begin(3); 
 Wire.onReceive(receiveEvent);
}
byte x = 0;
void loop() {
 Wire.beginTransmission(9); 
 Wire.write("s"); 
 Wire.write(x); 
 Wire.endTransmission(); 
 x++;
 delay(500);
}
void receiveEvent(int howMany) {
 if(Wire.available()){
 int x = Wire.read();
 Serial.println(x);
 }
}

Here is my slave code:

#include <Wire.h>
void setup() {
 Wire.begin(9); // join i2c bus with address #9
 Wire.onReceive(receiveEvent); // register event
 Serial.begin(9600); // start serial for output
}
void loop() {
 delay(100);
}
void receiveEvent(int howMany) {
 if(Wire.available()){
 char c = Wire.read(); 
 Serial.print(c);
 if(c == 's'){
 int x = Wire.read();
 Serial.println(x);
 //transmit to master 
 Wire.beginTransmission(3);
 Wire.write(1);
 Wire.endTransmission();
 } else {
 Wire.read();
 }
 }
}

Is it possible not to use Wire.onRequest(requestCallback); to send data to master? Kindly help.

• James Cameron: When a sketch calls Wire.begin() and passes a number, it has nominated itself as a slave on the bus. So you appear to have two slaves. Normally one would have one master and one slave.
• James Cameron: What you seem to want is a combination of the Arduino Wire Tutorial arduino.cc/en/Tutorial/MasterReader and arduino.cc/en/Tutorial/MasterWriter do you agree?
• srinivas: i had a look but not able to implement it properly any suggestion ? if i give 2 wire.write the master is not reading it properly
• James Cameron: I don't know what you mean, sorry.
• srinivas: the Wire.requestFrom has only one wire.write i need multiple wire.write which is not supported so is there any workarounds ?

Answer 1

To join the bus as a master you cannot supply a 7-Bit slave address so your code has two slaves.

Below you will find 2 sketches that successfully sends data to a slave. I would suggest you start looking at the I2C protocol and understand there are 4 Modes:

• Master->Transmitter
• Master->Receiver
• Slave->Transmitter
• Slave->Receiver

All communications regardless of how the data is transferred requires a master. It's the master that controls the clock pulses, thats why its always the master that initiates the communcation.

The MasterReader/MasterWriter tutorials on the arduino forum should now possibly start to make a little more sense when understanding those modes.

This code shows how a master sends data to a slave, and then how it requests data from the slave. If we look at this code from the master point-of-view, you can see it essentially says, send this byte to the client (m->t & s->r), then request data from the slave (m->r,s->t)

Master Code

#include <Wire.h>
#define SLAVE_ADDRESS 0x60
void setup()
{
 Wire.begin(); 
 randomSeed(analogRead(3));
 Serial.begin(9600); 
}
byte x = 0;
void loop()
{
 x = random(0,255);
 Serial.print("Sent: ");
 Serial.print(x, HEX);
 Serial.print("\n");
 Wire.beginTransmission(0x60); 
 Wire.write(x); 
 Wire.endTransmission(); 
 delay(500);
 Serial.println("Requesting Data"); 
 Wire.requestFrom(SLAVE_ADDRESS, 1);
 int bytes = Wire.available();
 Serial.print("Slave sent ");
 Serial.print(bytes);
 Serial.print(" of information\n");
 for(int i = 0; i< bytes; i++)
 {
 x = Wire.read();
 Serial.print("Slave Sent: ");
 Serial.print(x, HEX);
 Serial.print("\n");
 } 
 delay(500);
}

Slave Code

#include <Wire.h>
#define SLAVE_ADDRESS 0x60
byte x = 0x00;
void setup()
{
 Wire.begin(SLAVE_ADDRESS);
 Wire.onReceive(receiveEvent);
 Wire.onRequest(requestEvent);
 Serial.begin(9600);
}
void loop()
{
 delay(100);
}
void requestEvent() 
{
 Serial.print("Request from Master. Sending: ");
 Serial.print(x, HEX);
 Serial.print("\n");
 Wire.write(x);
}
void receiveEvent(int bytes)
{
 if(Wire.available() != 0)
 {
 for(int i = 0; i< bytes; i++)
 {
 x = Wire.read();
 Serial.print("Received: ");
 Serial.print(x, HEX);
 Serial.print("\n");
 }
 }
}

• wajatimur: On master you could scan the address range for connected device. This would be a proper approach.
• Nick Gammon: In both of your event handlers: Don't do serial prints inside an ISR. It may work until the serial buffer fills up, then it will hang.
• Nick Gammon: You don't need to test if(Wire.available() != 0) because the bytes variable has the number of bytes you received. If anything, test if bytes > 0 however I presume that a receive event would have at least one byte.

Answer 2

I went through this problem recently. Your slave needs two functions: one for sending and the other for receiving data.

// function: what to do when asked for data
void requestEvent() {
Wire.write(t); 
}
// what to do when receiving data from master
void receiveEvent(int howMany)
{Val = Wire.read();}

Here is a code that works both ways of i2c communication in a single code

Sending and receiving different types of data via I2C in Arduino

Answer 3

You had:

 void receiveEvent(int howMany)
 {
 { 
 if(Wire.available()){
 char c = Wire.read(); 
 Serial.print(c);
 if(c == 's'){
 int x = Wire.read();
 Serial.println(x);
 //transmit to master 
 Wire.beginTransmission(3);
 Wire.write(1);
 Wire.endTransmission();
 }else{
 Wire.read();
 }
 }
 }

There are a number of problems with your slave code.

1. Don't do Serial prints in an ISR (interrupt service routine)
2. Don't do beginTransmission / endTransmission in a receive event.
3. Don't do any writes
4. To send data back use a requestEvent not a receiveEvent.

Better would be:

// add in setup()
Wire.onRequest (requestEvent); // interrupt handler for when data is wanted
...
volatile byte what;
void receiveEvent(int howMany)
{
 if (howMany < 2)
 return;
 char c = Wire.read ();
 if (c != 's')
 return;
 what = Wire.read (); // remember what we got
} // end of receiveEvent
void requestEvent()
{
 Wire.write (1); // return a response
} // end of requestEvent

---

the Wire.requestFrom has only one Wire.write. I need multiple Wire.write which is not supported so are there any workarounds?

Batch up the response and do one write:

void requestEvent ()
 {
 byte buf [4] = { 1, 2, 3, 4 };
 Wire.write (buf, sizeof buf); // send response
 } // end of requestEvent

---

Reference

There are a lot of examples at I2C - Two-Wire Peripheral Interface - for Arduino