54. I2C Scanner

Two-wire communication protocol(Serial-synchronous)

• SDA (Serial Data Line) – Bidirectional data line.
• SCL (Serial Clock Line) – Clock signal generated by the master.

The master initiates communication always.
It is a Short-distance communication protocol with a distance of <10cm.
With  I²C, multi-master and multi-slave or both at the same time are possible.
I²C communication is commonly used to interface such as LCDs, OLED displays, RTC modules, and various sensors.

 I²C Frame

I²C Frame Description

Note: SDA never changes when SCL is HIGH. If it changes, it will be either a start or stop condition.
Example frames

1.I²C master writing data (0x65) to slave (address = 0x35)

     
2. I²C master reading data (0x66) from slave (address = 0x08)

Note: 1) Data direction →  MSB to LSB (MSB 1st)
           2) The ideal state of SCL and SDA is HIGH.

3. DSO output of I²C master sending 0x7B to slave at address 0x33.

     DSO Output

• SDA and SCL use open-drain connections.

• Open drain Output→ I²C devices can only pull the bus LOW. They cannot drive HIGH actively.
• To pull the line High actively → An Internal / External Pull-up needs to be used. Without it, the bus floats and gives garbage data.
• Pull-up resistor can be from 1kΩ  to 10kΩ. For long distances or high speeds, use a lower value resistor.

 I²C modes are defined based on communication speed. Most of the sensors come with only the standard mode.

Generally, addresses from 0x00 to 0x07 are reserved for special purposes. 

Address (0x00): It is used to broadcast a message sent to all slave devices, known as a General Call.

A slave device can pause communication by holding the SCL clock line LOW to force the master to wait.

Gives slaves time to:

• Process received data
• Prepare data to send

Mixed voltages (e.g., 3.3V MCU ↔ 5V sensor) can result in invalid logic levels and can damage devices (overvoltage on I/O pins).

To avoid this, a bidirectional level shifter or I²C buffer with voltage translation can be used.

In multi-master I²C, when both masters try to initiate communication at the same time instance, then the master that transmits a ZERO (LOW ) bit first gets control of the I²C bus.

Why: Since the Master transmitting ZERO (LOW) will force the SDA line to LOW, although the other master tries to send ONE (HIGH).

Example

Master 1: 0xB2 → 10110010
Master 2: 0xB4 → 10110100

Bus matches for 5 bits, at bit-6, master-1 wins and continues.

(Note : In I2C on data line, transmission starts with MSB first.)

As an important on-board communication protocol,  I²C is always present in most of the controllers, i.e., AVR, PIC, ARM, and RISC.

Generally following registers are present in controllers. (with reference to AVR ATmega 328P- also known as TWI (Two-Wire Interface)).

• Set Bitrate - Set the prescaler and desired  I²C bitrate.
• Send Start Condition - Set the bits in the control register to initiate the start condition.
• Wait for TWI0NT- wait for action to perform.
• Send Slave Address + Write Bit .
• Wait for TWINT - wait for action to perform
• Send a Data Byte to the slave.
• Wait for TWINT -wait for action to perform.
• Send Stop Condition - Stop  I²C & bus release.

I²C communication pins in Arduino UNO

In Arduino UNO, the Wire.h library is used for  I²C.
The default speed is 100 kHz, but it can be changed to 400 kHz.

Send character ‘A’  from master( Arduino UNO) to slave 0x08 (Arduino UNO) after every 1 second.

Master Code

#include <Wire.h>
void setup() {
  Wire.begin(); // Initialize  I²C as master
}
void loop() {
  Wire.beginTransmission(0x08);  // Slave address 0x08
  Wire.write('A'); // Send character
  Wire.endTransmission();
  delay(1000);
}  

Slave Code

#include <Wire.h>
void setup() {
  Serial.begin(9600); // initialize the serial communication
  Wire.begin(0x08);  // Initialize I2C in slave mode
  Wire.onReceive(receiveEvent);  // Register callback
}
void loop() { 
}
// Callback function definition
void receiveEvent(int bytesReceived) {
  if (Wire.available()) {
    char receiveData = Wire.read(); // read the received data from master
    // display data received from master
    Serial.print(receiveData);
  }
}