i2c: minimum code

[last updated: 2020-05-16]
i2c home page
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      On This Page:
• Alternate modes of operation:
  • simplest: master sending single byte to single slave:
  • Master sending a multiple-byte packet to Slave:
  • master requests data from slave
  • example from deepblue?
• ...
• ...

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• simplest: master sending single byte to single slave:
  • Master code:
    • #include <Wire.h>
      int x = 65; // some nominal number to send to slave
      // -or- char x = 65;
      // -or- char x = 'A';
    • in setup:
      Wire.begin(); // start the i2c bus as master
    • in loop:
      Wire.beginTransmission(9); // open bus to transmit to slave device at address 9
      Wire.write(x); // add var x to queue
      Wire.endTransmission(); // send queue to slave, then stop transmitting and release bus

      
      for troubleshooting:
      Serial.print("data sent: "); Serial.println(x);
      if (x < 75) {
          Serial.println("incrementing x");
          x = x + 1;
      } else {
          x = 65;
      }
      -----

      With x initially defined as: int x = 65;
      data sent will print out "65", "66", etc with each loop.
      -----
      With x initially defined as: char x = 65;
          or as: char x = 'A';
      it will print: "A", "B", etc.

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  • Single-byte Slave code:
    • #include <Wire.h>
      int x; // to contain value of data received from master
      // -or- char x;
      // -or- byte x;
    • setup:
      Wire.begin(9); // start the i2c bus as slave at address 9
      Wire.onReceive(receiveEvent); // attach a function (named "receiveEvent") that will execute when something is received.
    • loop:
      nothing needed
    • function defs:
      void receiveEvent(int numBytes) {
          // takes a single int parameter (the number of bytes to be read from the master) and returns nothing.
          x = Wire.read(); // read one character from the bus
      }

      
      for troubleshooting:
      Serial.print("rcvd: "); Serial.println(x);
      -----

      if x is initially defined as: int x = 65;
          print rcvd will print "65", "66" etc as master loop increments the value
          it will print the same regardless whether the master thinks its sending a char or an int
      -----
      if x is initially defined as: char x = 'A';
          then it will print "A", "B" etc as master loop increments the value
          regardless whether the master thinks its sending a char or an int
      -----

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• Master sending a multiple-byte packet to Slave:

        go here for code that did NOT work

  • Master code:
    • #include <Wire.h>
      char msgToSend[] = {"print this"};
      int numChars = sizeof(msgToSend)-1; // gives # of chars in the msg - well ... no, it doesn't ...
    • in setup:
      Wire.begin(); // start the i2c bus as master
    • in loop:
      Wire.beginTransmission(9); // open bus to transmit to slave device at address 9
      for (int n = 0; n < numChars; n++) {
          Wire.write(msgToSend[n]); // put next element of msgToSend into local buffer/queue
      }
      Wire.endTransmission(); // send queue to slave, then stop transmitting and release bus

  • Slave code:
    • #include <Wire.h>
      char x = '3'; // dummy initial value for received char
    • setup:
      Wire.begin(9); // start the i2c bus as slave at address 9
      Wire.onReceive(receiveEvent); // attach a function (named "receiveEvent") that will execute when something is received.
    • loop:
      nothing needed
    • function defs:
      void receiveEvent(int numBytes) {
          while (1 < Wire.available()) { // loop through all but the last
              char c = Wire.read(); // receive byte as a character
              Serial.print(c); // print the character
          }
      }

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• Master requests data from Slave:
  • #include <Wire.h>

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