UART Serial Port Module in VHDL Programming Language

Introduction to UART Serial Port Module in VHDL Programming Language

Hello, fellow coders! Welcome to this blog post where I will introduce you to the fascinating world of UART Serial Port Module in

get="_blank" rel="noreferrer noopener">VHDL Programming Language, a widely used standard for serial communication in embedded systems. UART is essential for enabling devices to communicate with one another, allowing for the seamless exchange of data. In this post, I will explain the fundamental concepts and principles of the UART serial port module, including its architecture, operation, and the role it plays in data transmission. By the end of this article, you’ll have a clear understanding of how to implement a UART module in VHDL, empowering you to create efficient communication interfaces for your projects. Let’s dive in!

What is UART Serial Port Module in VHDL Programming Language?

The UART (Universal Asynchronous Receiver-Transmitter) serial port module plays a crucial role in digital communication systems by enabling asynchronous serial data transmission between devices. In VHDL (VHSIC Hardware Description Language), designers implement a UART module to facilitate this communication protocol, which is widely used in embedded systems, microcontrollers, and various electronic devices.

Key Components of a UART Module

1. Asynchronous Communication:

UART operates in an asynchronous manner, meaning that it does not require a shared clock signal between the transmitting and receiving devices. Instead, it uses start and stop bits to define the beginning and end of each data packet.

2. Data Framing:

A typical UART data packet consists of:

• Start Bit: A single bit indicating the start of a transmission, usually a logic low (0).
• Data Bits: Typically 5 to 9 bits representing the actual data being transmitted. The most common configurations are 8 data bits.
• Parity Bit (optional): An error-checking bit that can be used to ensure data integrity.
• Stop Bit(s): One or two bits indicating the end of the transmission, usually a logic high (1).

3. Transmitter and Receiver:

The UART module consists of two main components:

• Transmitter: Converts parallel data from a data bus into a serial format for transmission.
• Receiver: Converts incoming serial data back into parallel format for use by the receiving device.

4. Baud Rate:

The baud rate defines the speed of data transmission, typically measured in bits per second (bps). Both the transmitter and receiver must be configured to operate at the same baud rate to ensure successful communication.

5. Control Signals:

The UART module generates and utilizes control signals for data transmission and reception, including signals for data availability, transmit enable, and receive enable.

Implementation in VHDL:

1. Architecture:

Designers typically structure the UART module in VHDL with distinct processes or components for the transmitter and receiver. This modular design simplifies both testing and maintenance.

2. State Machines:

The design often employs finite state machines (FSMs) to manage the different states of data transmission and reception. For example, states can include idle, transmitting, receiving, and error-checking states.

3. Signal Handling:

VHDL’s ability to define and manipulate signals allows for effective handling of data, control signals, and status flags. This is critical for ensuring the proper flow of data and error handling.

4. Simulation and Testing:

VHDL provides robust simulation capabilities, enabling designers to test the UART module thoroughly before hardware implementation. Simulation tools can be used to verify the timing and functionality of the UART design under various scenarios.

Why do we need UART Serial Port Module in VHDL Programming Language?

The UART (Universal Asynchronous Receiver-Transmitter) serial port module is essential in digital systems for several key reasons. In VHDL (VHSIC Hardware Description Language), implementing a UART module enables efficient and flexible communication between devices, making it a valuable tool for hardware design. Here’s why the UART module is needed:

1. Serial Communication Between Devices

UART is one of the simplest and most widely used methods for enabling serial communication between two devices, such as microcontrollers, computers, sensors, or other peripherals. It allows data to be transmitted bit by bit over a single wire, reducing the number of connections compared to parallel communication.

2. Asynchronous Data Transfer

Unlike synchronous communication methods, UART does not require a shared clock signal between the transmitter and receiver. This asynchronous data transfer mechanism makes it highly flexible and simple to implement, even over longer distances where clock synchronization may be impractical.

3. Ease of Implementation

In VHDL, designing a UART module is relatively straightforward, as the architecture is well-defined and can be implemented using basic state machines and counters. The simplicity of UART makes it ideal for quick and efficient hardware communication interfaces, even in complex systems.

4. Low Overhead

UART uses minimal control signals (such as Start, Stop, and optionally Parity bits) to manage data transmission. This leads to lower complexity in the hardware design and reduces overhead, allowing for the seamless integration of serial communication in resource-constrained environments like embedded systems.

5. Compatibility with Various Devices

UART is a universal protocol, meaning it can communicate with a wide range of devices across different platforms. Many microcontrollers, computers, and embedded devices come with built-in UART interfaces, making it the preferred choice for communication in embedded designs.

6. Efficient Use of Hardware Resources

Since UART transmits data serially, it reduces the number of physical connections needed between devices compared to parallel communication, which requires multiple data lines. This efficient use of hardware resources is particularly beneficial in systems with limited I/O pins.

7. Error Detection with Parity

UART supports the use of a parity bit for simple error detection, which ensures that the transmitted data is correct. In VHDL implementations, this feature can be added to the UART design to enhance data reliability, particularly in systems where data integrity is critical.

8. Wide Range of Baud Rates

UART supports a wide range of baud rates (the rate at which data is transmitted), making it adaptable for different communication speeds. This flexibility allows designers to optimize communication for both low-speed and high-speed applications.

9. Debugging and Monitoring

UART is commonly used for debugging embedded systems. By connecting a UART interface to a PC or another system, designers can send and receive diagnostic information, making it easier to monitor and troubleshoot hardware in real-time.

10. Low-Power Applications

UART is suitable for low-power applications because it only requires data transmission when necessary, allowing devices to remain idle and conserve energy. This makes it an ideal choice for battery-powered or low-power embedded systems where energy efficiency is a priority.

Example of UART Serial Port Module in VHDL Programming Language

A UART (Universal Asynchronous Receiver-Transmitter) serial port module in VHDL typically includes both the transmitter and receiver components to handle asynchronous communication. In this example, we’ll go through the basic structure of a UART module that handles transmission (TX) and reception (RX) of data, focusing on the key elements like baud rate generation, data transmission, and reception.

This VHDL code explains the implementation of a UART transmitter, receiver, and baud rate generator. The design will be capable of handling data transmission with start, data, and stop bits as per standard UART protocol.

UART Transmitter (TX) in VHDL

The transmitter takes parallel data and sends it serially, including start, data, and stop bits.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity UART_TX is
    Port (
        clk : in std_logic;             -- Clock signal
        reset : in std_logic;           -- Reset signal
        tx_start : in std_logic;        -- Start transmission signal
        tx_data : in std_logic_vector(7 downto 0); -- 8-bit data to transmit
        tx_serial : out std_logic       -- Serial output (TX line)
    );
end UART_TX;

architecture Behavioral of UART_TX is
    signal tx_state : std_logic_vector(3 downto 0); -- State of TX
    signal tx_counter : integer := 0;              -- Baud rate counter
    signal tx_shift_reg : std_logic_vector(9 downto 0); -- Shift register for start, data, stop bits
    signal tx_busy : std_logic := '0';             -- Busy flag
begin

    process(clk, reset)
    begin
        if reset = '1' then
            tx_serial <= '1';          -- Idle state of the line is high
            tx_state <= (others => '0');
            tx_counter <= 0;
            tx_busy <= '0';
        elsif rising_edge(clk) then
            if tx_start = '1' and tx_busy = '0' then
                -- Load start bit (0), data, and stop bit (1) into shift register
                tx_shift_reg <= '0' & tx_data & '1'; 
                tx_busy <= '1';
                tx_counter <= 0;
            elsif tx_busy = '1' then
                if tx_counter = 434 then -- Baud rate count (for example, at 9600 baud with a specific clock frequency)
                    tx_serial <= tx_shift_reg(0); -- Transmit least significant bit
                    tx_shift_reg <= tx_shift_reg(9 downto 1) & '1'; -- Shift right
                    tx_counter <= 0;
                    if tx_shift_reg = "1111111111" then -- If all bits are transmitted
                        tx_busy <= '0';               -- Transmission complete
                    end if;
                else
                    tx_counter <= tx_counter + 1;
                end if;
            end if;
        end if;
    end process;

end Behavioral;

Explanation of UART Transmitter:

1. Input Ports:

• clk: The clock signal used to drive the UART.
• reset: Resets the module to its initial state.
• tx_start: Signals the beginning of transmission.
• tx_data: The 8-bit data to be transmitted.
• tx_serial: The serial output that sends data bit-by-bit.

2. Process:

• The UART transmitter takes 8 bits of data, appends a start bit (0) and a stop bit (1), and then serially shifts the bits out through tx_serial.
• The tx_busy flag ensures that the module does not accept new data until the current transmission is complete.
• The baud rate is controlled by a counter (tx_counter), which delays each bit transmission based on the desired communication speed (e.g., 9600 baud).

UART Receiver (RX) in VHDL

The receiver takes incoming serial data, strips the start and stop bits, and stores the data.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity UART_RX is
    Port (
        clk : in std_logic;
        reset : in std_logic;
        rx_serial : in std_logic;      -- Serial input (RX line)
        rx_data : out std_logic_vector(7 downto 0); -- Received data
        rx_ready : out std_logic       -- Indicates that data is ready to be read
    );
end UART_RX;

architecture Behavioral of UART_RX is
    signal rx_state : std_logic_vector(3 downto 0);
    signal rx_shift_reg : std_logic_vector(7 downto 0);
    signal rx_counter : integer := 0;
    signal bit_counter : integer := 0;
begin

    process(clk, reset)
    begin
        if reset = '1' then
            rx_ready <= '0';
            rx_state <= "0000";
            rx_counter <= 0;
            bit_counter <= 0;
        elsif rising_edge(clk) then
            if rx_state = "0000" then -- Idle state
                if rx_serial = '0' then -- Start bit detected
                    rx_state <= "0001";
                    rx_counter <= 0;
                    bit_counter <= 0;
                end if;
            elsif rx_state = "0001" then -- Start receiving data bits
                if rx_counter = 434 then -- Sample at middle of bit
                    rx_shift_reg <= rx_serial & rx_shift_reg(7 downto 1); -- Shift in received bit
                    bit_counter <= bit_counter + 1;
                    if bit_counter = 8 then -- All data bits received
                        rx_state <= "0010";
                    end if;
                    rx_counter <= 0;
                else
                    rx_counter <= rx_counter + 1;
                end if;
            elsif rx_state = "0010" then -- Stop bit received
                if rx_serial = '1' then -- Valid stop bit
                    rx_data <= rx_shift_reg;
                    rx_ready <= '1'; -- Data ready
                    rx_state <= "0000";
                end if;
            end if;
        end if;
    end process;

end Behavioral;

Explanation of UART Receiver:

1. Input Ports:

• clk: Clock signal.
• reset: Resets the receiver.
• rx_serial: The serial input where the data arrives.
• rx_data: 8-bit parallel data output after receiving a byte.
• rx_ready: Signal indicating that data has been received and is ready to be processed.

2. Process:

• The receiver detects the start bit (0), samples the incoming serial data at the center of each bit, and then stores the data bits.
• After receiving 8 data bits, it checks for the stop bit (1). If all conditions are met, the data is available on rx_data, and rx_ready is set to 1.

Baud Rate Generator for UART

A UART module requires a baud rate generator to sample the bits at the correct timing.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity baud_gen is
    Port (
        clk : in std_logic;            -- System clock
        reset : in std_logic;          -- Reset signal
        baud_clk : out std_logic       -- Output baud rate clock
    );
end baud_gen;

architecture Behavioral of baud_gen is
    signal baud_counter : integer := 0;
begin

    process(clk, reset)
    begin
        if reset = '1' then
            baud_counter <= 0;
            baud_clk <= '0';
        elsif rising_edge(clk) then
            if baud_counter = 434 then -- Adjust this for desired baud rate
                baud_clk <= not baud_clk;
                baud_counter <= 0;
            else
                baud_counter <= baud_counter + 1;
            end if;
        end if;
    end process;

end Behavioral;

Explanation of Baud Rate Generator:

Baud Clock: This module generates a clock signal that controls the transmission and reception speed of the UART module. The baud_counter determines the interval between each bit based on the system clock frequency and the desired baud rate.

Advantages of UART Serial Port Module in VHDL Programming Language

These are the Advantages of UART Serial Port Module in VHDL Programming Language:

1. Simple Communication Protocol

UART (Universal Asynchronous Receiver-Transmitter) is known for its simplicity in design. It uses just two main signal lines: Tx (Transmit) and Rx (Receive). Unlike more complex protocols like SPI or I2C, it doesn’t require additional handshaking or control lines, making it easy to implement in VHDL for communication between devices like microcontrollers, FPGAs, and sensors.

2. Resource Efficiency

Since UART requires only a few hardware resources, it’s ideal for FPGA designs with limited logic gates. A UART module implemented in VHDL can be compact, consuming fewer slices or lookup tables (LUTs) compared to other communication modules. This makes it a go-to choice when minimizing hardware usage is essential.

3. Customizable Baud Rate

One of UART’s strengths is the ability to adjust the baud rate easily in VHDL designs. The baud rate determines the communication speed between devices. By changing the configuration of the baud rate generator in the VHDL code, you can tailor the module to transmit data at the desired speed, ensuring compatibility with different systems.

4. Error Detection through Parity

A UART module in VHDL can be designed to include a parity bit for error checking. Parity bits help detect any corruption in data transmission. You can choose between even or odd parity schemes, ensuring the data sent over the communication channel is correct, improving the reliability of the system.

5. Asynchronous Communication

Unlike synchronous communication protocols that require a shared clock signal, UART operates asynchronously. This means the devices communicating through UART don’t need to be clock-synchronized, reducing design complexity. Data is transferred using a start bit, data bits, and stop bits to signal the beginning and end of communication.

6. Compatibility with Multiple Devices

UART is a widely adopted communication protocol, and most embedded devices (such as microcontrollers, FPGAs, and peripherals) support UART. This makes it easy to integrate multiple devices that use different architectures into the same system without compatibility issues, ensuring smooth communication across the board.

7. Full Duplex Communication

In UART, full-duplex communication means data can be sent and received at the same time on different lines (Tx and Rx). This bi-directional communication improves data transfer efficiency, as the system doesn’t need to wait for one direction of communication to complete before starting the other.

8. Easily Extendable to Multiple Devices

Although UART is typically used for point-to-point communication, it can be extended to communicate with multiple devices. For instance, in an FPGA, multiple UART modules can be implemented in VHDL to enable communication with different peripherals or devices. Alternatively, a multiplexer can be used to switch between devices on the same Tx/Rx line.

9. Wide Range of Applications

UART is used in a variety of applications, from simple serial communication with sensors to complex FPGA and embedded systems projects. Because it’s simple yet versatile, UART is found in areas like serial debugging, interfacing with GPS modules, and communication between microcontrollers and peripheral devices.

10. Ease of Debugging

Due to its simplicity and straightforward signaling method (start bit, data bits, stop bit, optional parity), UART communication is relatively easy to debug. Tools like oscilloscopes, logic analyzers, and serial monitors can be used to visualize the transmitted and received data, making it easier to pinpoint issues in communication compared to more complex protocols like I2C or SPI.

Disadvantages of UART Serial Port Module in VHDL Programming Language

These are the Disadvantages of UART Serial Port Module in VHDL Programming Language:

1. Limited Speed

UART (Universal Asynchronous Receiver-Transmitter) operates at slower speeds compared to other communication protocols like SPI or I2C. The speed of UART communication is determined by the baud rate (bits per second), which may be sufficient for basic applications, but for systems requiring fast data transfer, UART’s speed limitations can be a disadvantage.

2. No Flow Control

Unlike protocols such as SPI or I2C, UART does not inherently support hardware flow control. Flow control ensures that data is not sent faster than the receiving device can process it. In UART, if one device transmits data too quickly, the receiving device may not be able to keep up, leading to data loss or corruption. Additional mechanisms or software must be implemented to handle this limitation.

3. Limited Distance

UART communication is typically used for short-distance data transmission. Over long distances, signal quality deteriorates due to electrical noise or attenuation, which can lead to data errors. Without additional components, such as line drivers or transceivers, UART is not suitable for long-distance communication.

4. Point-to-Point Communication

UART is designed for communication between two devices (point-to-point). It does not natively support communication with multiple devices on the same bus like I2C or SPI. To achieve multi-device communication with UART, additional hardware or software techniques, such as using multiple UART modules or multiplexing, are required.

5. Asynchronous Nature

UART operates asynchronously, meaning that it does not use a shared clock signal between the transmitter and receiver. This can lead to timing issues if both devices are not precisely synchronized. Even small mismatches in the baud rate can cause data errors, making timing synchronization critical in UART systems.

6. High Latency

UART communication introduces latency due to the use of start and stop bits, as well as optional parity bits for error detection. These additional bits add overhead to each data transmission, reducing the effective data rate and increasing communication delay. In time-sensitive applications, this added latency may be a disadvantage.

7. Lack of Error Correction

UART detects transmission errors using parity bits, but lacks built-in error correction mechanisms. Although errors are detectable, the protocol doesn’t automatically request retransmission of corrupted data. Implementing error correction requires a separate mechanism in the system’s design.

8. Limited Data Size per Frame

Each UART frame typically consists of 8 data bits, which limits the amount of information that can be transmitted in a single frame. For applications requiring larger data transfers, multiple frames are necessary, which increases transmission time and overhead due to the additional start, stop, and parity bits.

9. Higher Resource Usage for Additional Features

In VHDL implementations of UART, adding extra features like flow control, error correction, or custom baud rate handling consumes additional hardware resources in the FPGA or ASIC. This may increase the overall resource usage, leaving fewer available resources for other components in the design.

10. Susceptible to Noise

UART signals are susceptible to electrical noise, particularly in environments with high electromagnetic interference (EMI) or over longer distances. Without proper shielding or noise reduction techniques, noise can interfere with the signal, causing data corruption. Since UART has limited error correction, it can struggle to maintain reliable communication in noisy environments.