Implementing a serial port on an FPGA using Verilog is a fundamental task in embedded system design. It allows for communication with external devices, enabling data exchange and control. This article delves into the process of implementing a serial port on an FPGA, covering the fundamental concepts, design considerations, and Verilog code implementation.
Understanding Serial Communication
Serial communication transmits data bit-by-bit over a single channel, as opposed to parallel communication which transmits multiple bits simultaneously. This makes serial communication more efficient for long-distance transmission, as it requires fewer wires. The most common serial communication protocol is the Universal Asynchronous Receiver/Transmitter (UART).
UART Protocol
UART is a synchronous protocol that uses a start bit, data bits, a parity bit (optional), and a stop bit to frame the data transmission.
- Start Bit: Indicates the beginning of a data frame.
- Data Bits: The actual data being transmitted.
- Parity Bit: An optional bit used for error checking.
- Stop Bit: Indicates the end of a data frame.
The number of data bits, the parity bit, and the number of stop bits can be configured according to the application's requirements.
Design Considerations for FPGA Serial Port
Implementing a serial port on an FPGA requires careful consideration of various aspects:
1. Clock Speed and Baud Rate:
The FPGA clock speed determines the maximum baud rate achievable. The baud rate is the number of data bits transmitted per second. To ensure reliable communication, the baud rate should be set lower than the FPGA clock frequency.
2. Data Width:
The data width defines the number of bits transmitted per frame. Common data widths are 8, 9, and 10 bits.
3. Parity Bit:
A parity bit can be used to detect single-bit errors. It is typically set to either even or odd parity, depending on the application.
4. Stop Bits:
The number of stop bits determines the duration of the stop condition. One or two stop bits are commonly used.
5. Flow Control:
Flow control mechanisms can be implemented to prevent data loss when the receiving device is unable to process data at the same rate as the transmitting device. Common flow control methods include XON/XOFF and hardware flow control.
Verilog Implementation of a UART
The following Verilog code demonstrates the implementation of a basic UART transmitter and receiver:
module uart_tx(
input clk,
input rst,
input [7:0] data_in,
input tx_enable,
output tx_data
);
// Internal signals
reg [9:0] tx_shift_reg;
reg tx_busy;
// Assign the data to the shift register
assign tx_data = tx_shift_reg[0];
// Transmit logic
always @(posedge clk or posedge rst) begin
if (rst) begin
tx_busy <= 1'b0;
tx_shift_reg <= 0;
end else begin
if (tx_enable && !tx_busy) begin
tx_busy <= 1'b1;
tx_shift_reg <= {1'b0, data_in, 1'b1}; // Start bit, data, stop bit
end else if (tx_busy) begin
tx_shift_reg <= tx_shift_reg >> 1;
if (tx_shift_reg[9] == 1'b1) begin
tx_busy <= 1'b0;
end
end
end
end
endmodule
module uart_rx(
input clk,
input rst,
input rx_data,
output [7:0] data_out,
output rx_ready
);
// Internal signals
reg [9:0] rx_shift_reg;
reg rx_busy;
reg [7:0] rx_data_out;
reg rx_ready_flag;
// Receive logic
always @(posedge clk or posedge rst) begin
if (rst) begin
rx_busy <= 1'b0;
rx_shift_reg <= 0;
rx_ready_flag <= 1'b0;
end else begin
if (rx_data == 1'b0 && !rx_busy) begin
rx_busy <= 1'b1;
rx_shift_reg <= 0;
end else if (rx_busy) begin
rx_shift_reg <= {rx_shift_reg[8:0], rx_data};
if (rx_shift_reg[9] == 1'b1) begin
rx_busy <= 1'b0;
rx_ready_flag <= 1'b1;
rx_data_out <= rx_shift_reg[7:0];
end
end
end
end
// Assign the output
assign rx_ready = rx_ready_flag;
assign data_out = rx_data_out;
endmodule
Explanation:
- uart_tx: The transmitter module takes the data to be transmitted, enables the transmission, and outputs the serial data.
- uart_rx: The receiver module takes the serial data input, and outputs the received data and a ready flag.
- Shift Register: The shift registers are used to handle the serial transmission and reception of data bits.
- Start Bit: When a new data frame is to be transmitted, a '0' is shifted into the shift register, indicating the start of the frame.
- Data Bits: The data to be transmitted is then shifted into the register.
- Stop Bit: A '1' is shifted in at the end of the data frame to indicate the end of transmission.
- Receiver Logic: The receiver module looks for a start bit to initiate data reception and then shifts the data into the register. Once the stop bit is received, the received data is available.
Conclusion
Implementing a serial port on an FPGA using Verilog provides a versatile communication interface for embedded systems. By understanding the fundamental concepts of serial communication, the UART protocol, and the design considerations, you can create efficient and reliable serial port implementations on FPGAs. This article provides a basic framework for implementing a UART. Depending on the specific application, you can further customize the design to include features like parity, flow control, and different data widths.