ECEN 449 Lecture 8: Serial Communications and I2C Protocol

Serial Communication Overview

• Serial communication: information is transmitted serially (bit-by-bit) over the communication medium
• Parallel communication: information is transferred in chunks of bits (bytes, 32-bit, 64-bit words) at a time
• Synchronous communication: transmission requires a reference clock
• Asynchronous communication: no reference clock, hand-shaking is required

I2C Serial Communications Protocol

• I2C is a two-wire serial communication protocol invented by Philips Semiconductors (now NXP) in 1982
• I2C connects microcontroller master devices with lower-speed peripheral slave devices
• Only two lines are used: Serial Data Line (SDA) and Serial Clock Line (SCL)
• Communication occurs in a synchronous, half-duplex manner

I2C Bus Overview

• I2C bus is a multi-master serial single-ended computer bus to communicate with low-voltage devices
• The I2C bus offers four data transfer speeds: standard, fast, high-speed, ultra high-speed modes
• I2C supports 7-bit and 10-bit address space devices and its operation varies with low voltages
• The I2C protocol connects a maximum of 128 devices

I2C Communication Modes

• Standard Mode: Up to 100 kbps
• Fast Mode: Up to 400 kbps
• Fast Mode Plus: Up to 1 Mbps
• High-Speed Mode: Up to 3.4 Mbps
• Ultra-Fast Mode: Up to 5 Mbps

I2C Protocol - Communication Steps

• Start Condition: Master initiates communication
• Address + Read/Write Bit: Master sends the slave’s address
• Data Transfer: Master writes or reads data
• Stop Condition: Master ends communication

I2C Message Format

• Message-oriented data transfer with four parts:
  • Start condition
  • Slave Address transmission
  • Data fields
  • Stop condition

Read/Write Control

• Start Condition: SDA goes from High to Low, while SCL is High
• Direction of Transfer:
  • 𝑅𝑅|𝑤𝑤 = 0: Controller (Master) sending data to peripheral
  • 𝑅𝑅|𝑤𝑤 = 1: Controller is requesting to receive data from peripheral

mbed Code Example: Reading Sensor Data

• Initialization: Create an instance of the I2C object and specify the pins for SCL and SDA lines
• Addressing: Specify the address of the device to communicate with
• Reading and Writing: Use the "read" and "write" methods to communicate with the device
• Acknowledgment: Receiving device sends an acknowledgment bit after each byte of data is transferred
• Repeated Start: Allows for repeated start conditions for reading or writing multiple registers
• Error Handling: Mbed provides methods for error handling, including timeouts and retries

I2C Terminology

• Transmitter: The device that sends data to the I2C bus
• Receiver: The device that receives data from the I2C bus
• Controller (Master): The device that initiates/terminates transfers and generates clock signals
• Peripheral (Slave): The device addressed by a master
• Multi-Master: More than one master can control the bus at the same time without corrupting the message
• Arbitration: Procedure to ensure that only one master controls the bus at a time
• Synchronization: Procedure to synchronize the clock signals of two or more devices