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Chapter 5: Interfacing and Communication Protocols

Introduction

Digital systems rely on interfaces and protocols to exchange data with other components, devices, and the outside world. Communication may happen within a system or across systems using well-defined protocols. In FPGA designs, understanding and implementing these protocols ensures seamless connectivity, performance, and integration.

5.1 I/O Blocks and GPIO

General Purpose Input/Output (GPIO) pins provide basic digital signaling capabilities. These pins can be configured as input or output and are commonly used to interface with LEDs, switches, sensors, and buttons.

  • Input mode: Reads logic levels from external devices.
  • Output mode: Sends logic levels to drive other components.
  • Tri-state buffers: Allow GPIOs to be disabled (high-Z state) for shared buses.

Example GPIO Verilog stub:

module gpio_example(
      input btn,
      output led
    );
      assign led = btn; // direct connection
    endmodule

5.2 UART, SPI, and I2C Protocols

Serial communication protocols are essential for compact and reliable data transmission between devices. Each has its own advantages:

  • UART (Universal Asynchronous Receiver/Transmitter): Simple two-wire protocol (TX/RX). Commonly used for terminal interfaces.
  • SPI (Serial Peripheral Interface): Synchronous, full-duplex protocol with master-slave topology. Requires MOSI, MISO, SCLK, and SS signals.
  • I2C (Inter-Integrated Circuit): Two-wire protocol (SDA, SCL) that supports multiple devices via addressing. Ideal for low-speed peripherals.

Implementing these in Verilog typically involves FSMs to handle transmission timing and data framing.

5.3 Parallel Interfaces and High-Speed Buses

Parallel buses transmit multiple bits simultaneously, enabling faster data throughput but requiring more pins. Common types include:

  • AXI (Advanced eXtensible Interface): Used in ARM SoCs and Xilinx FPGAs for high-performance interconnects.
  • Wishbone: An open-source interconnect bus used in many embedded designs.
  • Custom buses: Designers may implement custom protocols for specific use cases, especially in FPGA-based systems.

Timing synchronization and handshake signals (e.g., valid/ready, strobe) are critical in parallel interfaces.

5.4 Interfacing with External Memories

FPGA systems often need to connect to external memory devices for expanded storage. Memory interface considerations include timing, data width, and control signals.

  • SRAM: Easy to interface due to asynchronous or synchronous operation and predictable timing.
  • DRAM: Requires complex controllers (e.g., DDR memory interfaces) and calibration for high-speed access.
  • Flash: Non-volatile storage for configuration or firmware; often accessed through SPI or parallel memory bus.

Memory controllers handle protocol-specific timing and transaction management.

Summary

  • GPIO enables basic input/output operations in FPGA systems.
  • UART, SPI, and I2C are widely used serial communication protocols.
  • Parallel buses like AXI and Wishbone enable high-speed, multi-bit data exchange.
  • Interfacing with external memory requires understanding protocol timing and control schemes.

🧪 MicroSim

Link to Simulation Demo

✅ Quiz: Check Your Understanding

1. What is the key difference between UART and SPI?

  • A) UART is synchronous, SPI is asynchronous
  • B) UART uses addressing, SPI does not
  • C) UART is asynchronous, SPI is synchronous
  • D) SPI uses fewer wires than UART
Show Answer

Correct answer: C) UART is asynchronous, SPI is synchronous

2. Which of the following is NOT a signal used in SPI?

  • A) MISO
  • B) SCLK
  • C) SDA
  • D) MOSI
Show Answer

Correct answer: C) SDA

3. What is the role of the Chip Enable (CE) signal in memory interfacing?

  • A) Enables output data
  • B) Writes data to memory
  • C) Activates the memory chip
  • D) Controls read/write direction
Show Answer

Correct answer: C) Activates the memory chip