Is UART a Bus?

The UART (Universal Asynchronous Receiver/Transmitter) is a widely used serial communication interface found in many microcontrollers and integrated circuits. But is UART considered a bus? The short answer is no, UART is not technically a bus. Here’s a more detailed explanation:

A bus is a communication system that transfers data between components inside a computer or between computers. Buses have multiple devices connected to a shared set of wires that provides a pathway for data transmission between devices. Some common examples of buses include PCI, PCIe, USB, I2C, SPI, CAN, etc.

In contrast, UART is a simple point-to-point serial communication protocol that allows two devices to communicate over a serial link. It does not have multiple slave devices connected to a shared bus. UART uses a single transmit line and a single receive line to full duplex data transfer between just two devices.

Key Differences Between UART and Bus

Here are some of the key differences between UART and bus:

• UART is a serial communication protocol whereas a bus allows parallel communication.
• UART has just two devices – a transmitter and a receiver connected over a serial link. A bus typically has one master and multiple slave devices.
• UART uses a single Tx line and Rx line. A bus uses a shared set of wires called bus signals.
• UART is a point-to-point protocol. A bus is a multi-drop communication system.
• UART communication is asynchronous in nature. Bus communication can be synchronous or asynchronous.
• UART requires just 2 wires for full duplex communication. A bus requires at least 3 wires – Data, Clock/Strobe and Reset.
• UART allows only one device to transmit at a time. Multiple devices can transmit on a bus based on an arbitration scheme.

Due to these differences, UART does not have the essential characteristics of a bus and is better classified as a serial communication protocol rather than a bus.

UART Communication Basics

Now that we have established UART is not a bus, let’s understand the basics of UART communication:

• UART stands for Universal Asynchronous Receiver/Transmitter. It’s asynchronous because it does not require a clock signal to synchronize the data transfer.
• UART uses two lines for sending (Tx) and receiving (Rx) data serially one bit at a time.
• UART communication is full duplex – Tx and Rx lines allow data to flow in both directions at the same time.
• A typical UART data frame contains a start bit, 5-9 data bits, an optional parity bit and stop bits.
• Common UART baud rates range from 300 baud to 115200 baud or even higher.
• UART interfaces can be implemented on silicon chips like microcontrollers and integrated circuits.
• UART is used for short-distance, low speed, low cost data exchange between devices like microcontrollers, GPS, Bluetooth etc.

Implementing UART Communication

The hardware circuitry needed to implement UART communication includes the following components:

• Transmitter – Converts parallel data from the transmission register to serial form.
• Receiver – Converts the incoming serial data back into parallel data for the receive register.
• First-In First-Out (FIFO) Buffers – Helps regulate data flow.
• Logic Circuits – Generates and interprets the UART data frames.
• Clock Generator – Provides clock signals to synchronize internal operations.
• Baud Rate Generator – Sets the desired data transmission rate.

The UART transmitter and receiver must be configured with communication parameters like baud rate, data bits, stop bits and parity to establish synchronized communication. Common UART configurations are 8N1 (8 data bits, no parity, 1 stop bit) and 8E1 (8 data bits, even parity, 1 stop bit).

The UART serial port is connected to external transceiver ICs like MAX232 to convert UART’s CMOS/TTL logic levels to RS-232 voltage levels for transmission over longer distances.

UART vs SPI

Both UART and SPI (Serial Peripheral Interface) are popular serial communication protocols. However there are some key differences:

• UART is asynchronous while SPI is synchronous.
• UART uses 2 wires whereas SPI uses 4 wires.
• UART range is limited to a few feet while SPI can communicate over longer distances.
• UART speed maxes out at ~10Mbps. SPI can reach over 100 Mbps speed.
• SPI is more suitable for short distance communication between chips on a board. UART is better for communicating with external peripherals.

UART vs I2C

I2C is also a synchronous serial protocol like SPI, but has some differences from UART:

• I2C is multi-master & multi-slave but UART is point-to-point.
• I2C needs just 2 wires similar to UART but supports multiple devices.
• UART has longer max distance than I2C which is typically used for on-board communication.
• UART is simpler and does not require sending device addresses like I2C.
• I2C supports up to 1008 nodes on a bus while UART connects only 2 devices.

Advantages of UART

Here are some of the advantages of using UART serial communication:

• Simple 2-wire interconnection reduces PCB traces and wires.
• No need for a precision clock source due to its asynchronous operation.
• UART ICs and protocols are easy to implement in hardware and software.
• Low cost compared to complex parallel buses.
• UARTs are available in many ICs like microcontrollers, FPGAs, ASICs etc.
• Mathematically calculated baud rates allow high precision timing.
• Wide range of baud rates possible from 300 baud to higher Mbps.
• UART communication is reliable and well suited for serial data transfer.

Disadvantages of UART

Some of the disadvantages or limitations of UART communication include:

• Only two devices can be connected in a UART link.
• UART is not suitable for transferring large parallel data streams.
• Complex hardware and software needed for buffering due to asynchronous data.
• Error detection limited to parity bits. Higher level protocols needed for robust error checking.
• Lower speeds and longer latency compared to parallel communication buses.
• Point-to-point topology limits UART’s applications.
• Slow speed makes UART unsuitable for time critical systems.
• Distance limited to 15 meters unless level converters are used.

Applications of UART

Some common applications and devices that use UART communication include:

• Connecting microcontrollers, processors or computers to peripheral devices like GPS, card readers, sensors etc.
• Embedded systems, IoT devices and industrial automation systems.
• Establishing console access for equipment monitoring, debugging and configuration.
• Interfacing with serial device servers, serial terminals and man-machine interfaces.
• Legacy mice, keyboards, printers and modems connections.
• Bluetooth, IrDA, RS-232, RS-485 based serial communications.
• Amateur radios like walkie-talkies using RF serial links.
• Low speed short serial links between boards or ICs on a PCB.

Conclusion

In summary, UART is not technically a bus but a simple and effective point-to-point serial communication protocol. It has a wide range of applications for modest data rate short-range serial communications. Compared to buses, UART offers a lightweight serial data transfer mechanism using just two wires. But its speed and connectivity is limited compared to advanced parallel buses.