Design of Communication Interface between FPGA and DSP

Designing a communication interface between an FPGA (Field-Programmable Gate Array) and a DSP (Digital Signal Processor) involves creating a high-speed, reliable communication path that allows the two devices to exchange data efficiently. FPGAs and DSPs often complement each other in embedded systems: the FPGA handles parallel processing tasks or I/O operations, while the DSP performs complex mathematical computations and signal processing.

There are multiple ways to design the communication interface between an FPGA and DSP, and the choice of interface depends on the system’s requirements in terms of speed, complexity, and resource utilization. Common methods for interfacing include parallel interfaces, serial interfaces (like SPI or UART), and standard protocols (like PCIe or Ethernet).

Here’s a step-by-step guide for designing a simple communication interface:

1. Communication Requirements Analysis
Before diving into the interface design, it’s important to define:

• Data Bandwidth Requirements: What is the data rate needed for communication between the FPGA and DSP?
• Control Signals: What control signals are needed for synchronization, handshaking, or error detection?
• Latency Tolerance: How much delay is acceptable for the data transfer between the two devices?
• Type of Data: Will the data be continuous or in bursts? Is the data size fixed or variable?
• Protocol Requirements: Do you need a well-defined, off-the-shelf protocol, or can a custom protocol be designed for higher efficiency?

2. Choose the Communication Protocol
There are several standard and custom protocols you can use. The following are common options:

A. Parallel Interface
A parallel interface is often used when high-speed data transfer is required with relatively simple handshaking.

Key Components:

• Data Bus: A parallel data bus to transfer multiple bits simultaneously. For example, an 8-bit or 16-bit data bus.
• Control Signals: Signals like Read/Write Enable, Chip Select, and Ready/Busy to synchronize the FPGA and DSP.

Advantages:

• High-speed transfer with low latency.
• Suitable for bulk data transfer.

Disadvantages:

• Requires more physical pins for a wider bus.
• Complexity increases with the width of the data bus.

Design Steps:

• Use a multiplexed parallel interface to minimize the number of signal lines.
• Define the handshaking protocol for synchronization. Typically, the FPGA and DSP will share control signals to indicate when data is valid and ready to be read or written.

B. Serial Interface (e.g., SPI, UART, or LVDS)
If you want to minimize the number of wires but still need reliable communication, a serial interface like SPI (Serial Peripheral Interface) or UART (Universal Asynchronous Receiver/Transmitter) might be a good choice.

SPI Interface:

• Master-Slave Communication: The FPGA could act as the master, controlling the clock (SCK), chip select (CS), and data transfer (MOSI/MISO lines).
• Full-Duplex Communication: Data can be transferred in both directions (from FPGA to DSP and vice versa).

Advantages:

• Fewer pins required compared to parallel interfaces.
• SPI and UART are widely supported on DSPs and FPGAs.

Disadvantages:

Slower compared to parallel interfaces, especially when data transfer rates are high.

Design Steps:

• Implement an SPI master/slave protocol on the FPGA and DSP.
• Use a clock divider on the FPGA side to match the clock frequency of the DSP.
• Add a handshaking mechanism to signal when data is ready for transmission.

C. High-Speed Serial Interfaces (e.g., PCIe, Ethernet)
For high-performance applications requiring large data throughput, you might consider using a standard high-speed serial interface like PCIe or Ethernet.

Advantages:

• Very high data transfer rates.
• Well-defined standards with built-in error checking and flow control.

Disadvantages:

• More complex to implement, requiring hardware blocks for protocol handling.
• Requires more resources.

Design Steps:

• For PCIe, you would need to implement a PCIe endpoint on the FPGA to interface with the DSP.
• For Ethernet, implement an Ethernet MAC (Media Access Controller) on the FPGA, enabling the two devices to communicate over a network.

3. Interface Design for Example (SPI-based)
For simplicity, let’s design an SPI interface between the FPGA and DSP. The FPGA will act as the SPI master, and the DSP will be the SPI slave.

A. FPGA Side Design (SPI Master)
1.Clock Generation:

• Use a clock generator or clock divider to generate the SPI clock (SCK) at the required speed.
• The FPGA will act as the master and drive the clock for synchronization.

2.SPI Handshaking:

• Chip Select (CS): Controls whether the FPGA is communicating with the DSP.
• MOSI (Master Out Slave In): The data line where the FPGA sends data to the DSP.
• MISO (Master In Slave Out): The data line where the DSP sends data to the FPGA.
• SCK (Serial Clock): Clock signal to synchronize data transfer.

3.Data Transfer Logic:

Implement a state machine to manage data transfer from the FPGA to the DSP. The state machine will:

• Send data from the FPGA to the DSP (write operation).
• Receive data from the DSP (read operation).

Handle data framing and timing to ensure that the transfer occurs at the correct time with respect to the SPI clock.

4.Simple Verilog Code for SPI Master:

verilog

module spi_master (
    input clk,                  // System clock
    input reset,                // Reset signal
    input [7:0] data_in,        // Data to send to DSP
    output reg spi_clk,         // SPI Clock (SCK)
    output reg mosi,            // SPI Data Out (MOSI)
    output reg cs,              // Chip Select
    input miso,                 // SPI Data In (MISO)
    output reg [7:0] data_out   // Received data from DSP
);

reg [7:0] shift_reg;
reg [2:0] bit_counter;

always @(posedge clk or posedge reset) begin
    if (reset) begin
        cs <= 1;               // Deactivate chip select
        spi_clk <= 0;          // Reset clock
        bit_counter <= 3'b000; // Reset bit counter
        mosi <= 0;             // Reset MOSI
    end else begin
        // Chip Select Control (activate during transmission)
        cs <= 0;

        // Shift Register for Data Transmission
        if (bit_counter < 8) begin
            mosi <= data_in[7];    // Send MSB first
            data_in <= data_in << 1; // Shift left after sending a bit
            spi_clk <= ~spi_clk;     // Toggle SPI clock
            bit_counter <= bit_counter + 1;
        end else begin
            // Finished transmitting
            cs <= 1;
            spi_clk <= 0;
        end
    end
end
endmodule

B. DSP Side Design (SPI Slave)
1.SPI Slave Configuration:

• The DSP will be configured as the SPI slave. The DSP’s SPI peripheral will be set to listen for the incoming clock signal (SCK) and data (MISO/MOSI).
• The DSP needs to process the data received from the FPGA or send data back to the FPGA when requested.

2.Data Reception and Transmission:

• The DSP will sample the MISO line and load the incoming data into an internal register.
• Similarly, the DSP can transmit data to the FPGA using the MOSI line when the FPGA requests data.

4. Error Checking and Flow Control
To ensure robust communication, consider adding:

• Parity Checking: To detect errors in transmitted data.
• Acknowledgment Protocol: To ensure data is successfully transferred (e.g., using a READY or ACK signal).
• Timeout Mechanism: To prevent the system from hanging if data is not received in a certain time frame.

5. Conclusion
Designing an interface between an FPGA and DSP depends on the specific requirements of the application, including the data rate, complexity, and physical constraints. An SPI-based communication interface is one of the simplest and most effective methods for linking an FPGA with a DSP. By creating a custom protocol and synchronizing data transfers, you can efficiently exchange data between the two devices.

For more complex systems, you might explore parallel interfaces, high-speed serial interfaces like PCIe or Ethernet, or custom protocols depending on the speed, resource availability, and processing requirements.