💾Embedded Systems Design Unit 8 – Serial Communication Protocols

What's Serial Communication?

• Involves sending data sequentially, one bit at a time, over a single communication channel
• Contrasts with parallel communication, which sends multiple bits simultaneously over multiple channels
• Commonly used in embedded systems due to simplicity, low cost, and wide support
• Requires fewer wires and pins compared to parallel communication, making it suitable for resource-constrained devices
• Supports various data formats, such as ASCII, binary, and custom protocols
• Enables communication between microcontrollers, sensors, peripherals, and other devices
• Provides a standardized way to exchange data between devices from different manufacturers

Key Serial Protocols

• UART (Universal Asynchronous Receiver/Transmitter)
  • Asynchronous protocol that does not require a shared clock signal between devices
  • Commonly used for low-speed, short-distance communication (RS-232)
  • Utilizes start and stop bits to frame data packets
• I2C (Inter-Integrated Circuit)
  • Synchronous protocol that uses a shared clock signal (SCL) and data line (SDA)
  • Supports multiple master and slave devices on the same bus
  • Employs a unique addressing scheme to identify devices
  • Widely used for communication with sensors, EEPROMs, and other peripherals
• SPI (Serial Peripheral Interface)
  • Synchronous protocol that uses separate lines for clock (SCLK), data input (MOSI), and data output (MISO)
  • Typically faster than I2C but requires more wires and pins
  • Commonly used for high-speed communication with ADCs, DACs, and other devices
• CAN (Controller Area Network)
  • Robust protocol designed for automotive and industrial applications
  • Utilizes a multi-master bus with message prioritization and error detection
  • Supports data rates up to 1 Mbps and distances up to 40 meters
• USB (Universal Serial Bus)
  • High-speed serial protocol for connecting peripherals to computers and embedded devices
  • Supports plug-and-play functionality and power delivery
  • Offers various data transfer modes (control, bulk, interrupt, and isochronous)

Hardware Components

• UART modules integrated into microcontrollers for serial communication
• USB-to-UART bridges (FTDI, CP2102) for connecting UART devices to computers
• I2C and SPI peripherals built into microcontrollers for interfacing with external devices
• CAN controllers and transceivers for implementing CAN networks
• RS-232 and RS-485 transceivers for long-distance serial communication
• Logic level converters for interfacing devices with different voltage levels (3.3V and 5V)
• Connectors and cables (DB9, USB, JST) for physical connection between devices

Data Transmission Basics

• Baud rate represents the number of bits transmitted per second
  • Common baud rates include 9600, 19200, 38400, 57600, and 115200
  • Devices must agree on the same baud rate for successful communication
• Data bits determine the number of bits in each transmitted character (typically 7 or 8 bits)
• Parity bits used for basic error detection (odd, even, or none)
• Stop bits indicate the end of a transmitted character (usually 1 or 2 bits)
• Synchronization achieved through clock signals (I2C, SPI) or start/stop bits (UART)
• Flow control manages data transmission pace to prevent buffer overflows (hardware or software)
  • Hardware flow control uses additional signals (RTS/CTS) to control data flow
  • Software flow control employs special characters (XON/XOFF) to control data flow

Error Detection and Correction

• Parity checking detects single-bit errors by adding an extra bit to ensure even or odd parity
• Checksum calculation performs basic error detection by summing data bytes and comparing the result
• CRC (Cyclic Redundancy Check) detects burst errors using polynomial division
  • Sender calculates CRC value based on data and appends it to the message
  • Receiver recalculates CRC and compares it with the received value to detect errors
• Hamming codes enable single-bit error correction and double-bit error detection
  • Additional parity bits are added to the data based on a Hamming matrix
  • Receiver can identify and correct single-bit errors using the parity bits
• Retry mechanisms allow devices to retransmit data in case of detected errors
• Forward Error Correction (FEC) techniques, such as Reed-Solomon codes, enable error correction without retransmission

Implementing Serial Protocols

• Configure UART, I2C, or SPI peripherals in the microcontroller
  • Set baud rate, data bits, parity, stop bits, and other parameters
  • Enable interrupts for efficient handling of received data and transmission completion
• Implement protocol-specific functions for sending and receiving data
  • UART: Write data to transmit buffer and read from receive buffer
  • I2C: Initiate start condition, send device address, read/write data, and generate stop condition
  • SPI: Assert chip select, shift data in and out, and deassert chip select
• Handle data encoding and decoding according to the chosen protocol
  • Convert between binary data and ASCII or other formats
  • Pack and unpack structured data (e.g., sensor readings) into byte arrays
• Manage communication flow and synchronization
  • Implement handshaking mechanisms or acknowledgments
  • Use timers or delays to control timing-sensitive operations
• Incorporate error handling and recovery mechanisms
  • Detect and handle communication errors (parity, framing, overrun)
  • Implement retry or resynchronization procedures
• Test and debug the serial communication implementation
  • Use serial terminal programs (PuTTY, TeraTerm) to send and receive data
  • Analyze data traffic with logic analyzers or oscilloscopes

Real-World Applications

• Interfacing with GPS modules to obtain location data in embedded systems
• Connecting environmental sensors (temperature, humidity) to microcontrollers via I2C or SPI
• Implementing Modbus protocol over RS-485 for industrial automation and control systems
• Communicating with Bluetooth modules (HC-05, HC-06) using UART for wireless connectivity
• Controlling LCD displays and OLED screens using I2C or SPI interfaces
• Retrieving data from SD cards using SPI protocol in data logging applications
• Establishing communication between ECUs (Electronic Control Units) in automotive systems using CAN bus
• Connecting peripherals (keyboards, mice, printers) to embedded devices via USB

Challenges and Troubleshooting

• Signal integrity issues due to long wires, noise, or improper termination
  • Use proper termination resistors and shielded cables to minimize signal degradation
  • Reduce cable lengths and keep wires away from noise sources
• Mismatch in communication parameters (baud rate, parity, stop bits) between devices
  • Double-check and synchronize the configuration of all devices
  • Use datasheets and protocol specifications to ensure compatibility
• Timing violations and synchronization problems
  • Adhere to the specified timing requirements of the protocol
  • Implement proper synchronization mechanisms and timeouts
• Resource conflicts and bus contention in multi-device systems
  • Assign unique addresses to each device on the bus
  • Implement arbitration mechanisms to handle bus access conflicts
• Debugging and isolating communication issues
  • Use serial protocol analyzers or logic analyzers to capture and examine data traffic
  • Employ software debugging techniques, such as logging and breakpoints
  • Verify proper wiring and connections using multimeters or continuity testers
• Compatibility issues between devices from different manufacturers
  • Consult datasheets and application notes for device-specific requirements
  • Use protocol bridges or converters to interface between incompatible devices