The AXI UART (Universal Asynchronous Receiver/Transmitter) is a peripheral that enables communication between an ARM processor and external devices using the UART protocol. The AXI interface provides a high-performance, efficient connection for serial communication, particularly for low-speed, low-latency data transfer. AXI UART is commonly used in embedded systems for communication with peripherals like sensors, modems, and debugging tools.

UART (Universal Asynchronous Receiver/Transmitter) is a widely used protocol for serial communication. It enables data transmission over a single wire pair, typically used for communication between microcontrollers and peripherals or between computers and peripheral devices. UART communication involves asynchronous transmission, meaning that data is sent without a clock signal, with the sender and receiver synchronizing based on pre-defined baud rates.

The AXI interface provides a standard communication protocol between the processor and the UART peripheral. Using AXI ensures efficient data transfer, low overhead, and seamless integration with the rest of the SoC, such as memory and other peripherals. AXI UART typically uses AXI4-Lite for control register access, allowing the processor to configure parameters like baud rate, data bits, stop bits, and enable interrupts for data transmission.

The AXI UART peripheral consists of several key components that work together to facilitate serial communication. These components handle data transmission, reception, and various UART configurations. Key components include: • Transmit (TX) FIFO: A FIFO (First-In-First-Out) buffer used to store outgoing data. The CPU writes data to the TX FIFO, and the UART sends it serially over the communication line. • Receive (RX) FIFO: A FIFO buffer that stores incoming data from the serial line. The UART receives data, places it in the RX FIFO, and the CPU reads it from there. • Control Registers: Registers used to configure the UART parameters, such as baud rate, data length, parity, stop bits, and enable interrupts. • Interrupts: Interrupt mechanisms that notify the processor when certain conditions occur, such as data being available in the RX FIFO or the TX FIFO being empty.

AXI UART Operation involves three main actions: • Write Operation: The CPU writes data to the TX FIFO, which is then transmitted asynchronously over the UART interface. • Read Operation: The CPU reads received data from the RX FIFO, which holds incoming data from the external UART device. • Interrupts: The UART peripheral can generate interrupts to signal the CPU when data is available in the RX FIFO or when the TX FIFO is ready for more data.

AXI UART peripherals are equipped with several features that enhance their performance and functionality: • Baud Rate Control: The UART peripheral supports configurable baud rates to adjust the speed of data transmission. The baud rate is determined by dividing the system clock by a divisor stored in a control register. • FIFO Buffers: Both the TX and RX FIFOs improve the efficiency of data transfers by allowing the UART peripheral to handle multiple data bytes at once, reducing the need for constant intervention from the CPU. • Interrupt Handling: AXI UART supports interrupts to notify the CPU when the RX FIFO contains data or when the TX FIFO is empty. This helps in managing data flow without polling. • Parity and Framing Error Detection: The UART peripheral can detect errors in incoming data, such as parity errors and framing errors, and signal these errors to the CPU. • Flow Control: UART can implement hardware flow control using RTS (Request to Send) and CTS (Clear to Send) lines to manage the flow of data between the transmitter and receiver.

Both AXI UART and AXI4-Stream are crucial peripherals in modern ARM-based SoC designs, each serving unique purposes. AXI UART is designed for low-throughput, asynchronous communication, typically used in control applications, while AXI4-Stream is optimized for high-bandwidth, continuous data transfer, making it ideal for multimedia and networking applications. Together, these peripherals enable flexible and efficient communication within ARM-based systems, supporting a wide range of embedded system designs.