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Fpga implements uart serial communication
**UART Uses Asynchronous Serial Communication**
Serial communication refers to the transmission of data bit by bit over a single line. The main advantage of this method is its simplicity, as it can be implemented with just a few wires, making it cost-effective and suitable for long-distance communication, although it is slower compared to parallel communication.
Asynchronous communication transmits data in character units. While the time interval between two characters can vary, the time between adjacent bits within the same character remains consistent. This allows the receiver to synchronize with the sender at the start of each character.
The speed of data transfer is measured in baud rate, which represents the number of bits transmitted per second. For example, if the system sends 120 characters per second, and each character consists of 10 bits (including 1 start bit, 7 data bits, 1 parity bit, and 1 stop bit), the baud rate would be 10 × 120 = 1200 bps.
The standard data format for UART communication includes:
- **Start Bit**: A logic "0" signal that marks the beginning of a character.
- **Data Bits**: Usually 5–8 bits representing the actual information being sent, such as ASCII or BCD codes.
- **Parity Bit**: Used for error checking, either even or odd parity.
- **Stop Bit**: A logic "1" that signals the end of a character, typically one or two bits long.
- **Idle Bit**: Represents the absence of data on the line when no transmission is happening.
In asynchronous communication, the receiver synchronizes with the transmitter upon detecting the start bit. It then samples the incoming bits at regular intervals until the stop bit is detected, after which it resets for the next character.
**Hardware Interface Design for Serial Communication**
For serial communication, the RS-232-C standard is commonly used. This interface typically uses a 9-pin D-type connector (DB9) instead of the original 25-pin version. The design in this project uses a DB9 connection.
The hardware circuit includes an FPGA serial port module, a MAX232 IC for level conversion, and the DB9 connector. The MAX232 is a low-power, cost-effective solution that converts logic-level signals to RS-232 levels, making it ideal for interfacing with PCs or other devices.
Data flows into the FPGA through the RxD pin of the DB9 connector, passes through the MAX232 for voltage conversion, and is then converted from serial to parallel by the FPGA's serial module. After processing, the data is reconverted to serial and sent back through the TxD pin.
**FPGA Serial Module Design**
The FPGA serial module is the core of the design. It consists of three main components: a baud rate generator, a transmit module, and a receive module. These modules are implemented using Verilog and operate based on the internal clock and control signals.
**Baud Rate Generation Module**
The baud rate generator produces a clock signal that is 16 times the desired baud rate. For example, if the target baud rate is 9600 bps, the generator outputs a clock of 153,600 Hz. This clock is used to control both the transmit and receive operations.
The frequency division factor is calculated using the formula:
`Division Factor = (External Clock Frequency / (16 × Desired Baud Rate)) - 1`
This allows the system to generate different baud rates dynamically.
**Transmit Module**
The transmit module takes parallel data from the CPU, loads it into a shift register, and sends it out bit by bit. It starts with a start bit, followed by the data bits, a parity bit, and ends with a stop bit. The entire process is controlled by a counter that ensures proper timing.
**Receive Module**
The receive module detects the falling edge of the input signal to identify the start bit. It then samples the incoming bits using a clock that is 16 times the baud rate. After receiving all bits, it checks the parity and stop bit to determine if the data is valid.
**Interface Control Module**
This module manages the interaction between the CPU and the UART. It handles control signals like reset, write, and read, and controls the flow of data between the external bus and the internal modules. It also manages the baud rate settings and ensures smooth communication.
**Performance Analysis**
Using Xilinx’s Spartan II xc2s100 FPGA, the design consumes only 56 LUTs, which is very efficient. The maximum operating frequency reaches 121.8 MHz, ensuring compatibility with all standard baud rates.
**Implementation and Testing**
The design was implemented using PADS for schematic and PCB layout. It was successfully tested on a physical board and integrated with a PC via a serial communication program using ActiveX controls.
The FPGA-based UART design successfully handles data transmission and reception, including parity and stop bit checks. It can be embedded in larger systems for remote communication, offering a reliable and resource-efficient solution for asynchronous serial communication.