UART vs. USART: Key Differences Explained

This article clarifies the distinctions between UART (Universal Asynchronous Receiver/Transmitter) and USART (Universal Synchronous/Asynchronous Receiver/Transmitter). We’ll delve into their characteristics and explore the scenarios where each is most suitable. You can also find links to comparisons of UART with SPI and I2C.

UART (Universal Asynchronous Receiver/Transmitter)

• Full Name: Universal Asynchronous Receiver/Transmitter.

• Data Rate: Generally supports lower data rates compared to USART.

• Baud Rate Requirement: The receiver must know the transmitter’s baud rate before communication can begin. This means you need to agree on the speed beforehand.

• Protocol Simplicity: UART is a relatively simple protocol. It relies on:

  • A start bit (typically a low logic level) before the data word.
  • One or two stop bits (typically a high logic level) after the data word.
  • An optional parity bit (even or odd) for basic error detection.

  The following image shows the structure:

  

  • UART Packet Structure: 1 start bit (low), 8 data bits (including parity), 1 or 2 stop bits (high).
  • Transmission Style: Data is transmitted byte by byte.
  • Clocking: UART generates its own internal clock and synchronizes it with the incoming data stream based on the transition of the start bit.
  • RS232 Connection: UART is often referred to as RS232, a common serial communication standard.
  • Voltage Levels: For longer distances, the standard 5V UART signal is often converted to higher voltages, such as +12V for logic 0 and -12V for logic 1 to improve signal integrity.

  The following image shows the data mapping.

  

USART (Universal Synchronous/Asynchronous Receiver/Transmitter)

• Full Name: Universal Synchronous/Asynchronous Receiver/Transmitter.

• Data Rate: Supports higher data rates because it can generate a clock signal along with the data (synchronous mode).

• Baud Rate Requirement: In synchronous mode, the receiver does not need to know the transmitter’s baud rate beforehand. This information is derived from the clock signal accompanying the data.

• Transmission Style: Synchronous data is generally transmitted in blocks.

• UART Compatibility: USART can be configured to operate like a UART, making it a more versatile option. However, a standard UART cannot emulate a USART’s synchronous capabilities.

• Protocol Complexity: USART is more complex and can support various protocols for data transmission, including LIN, SPI, I2C, Modbus, and IrDA.

• Clocked Data: The USART utilizes a clock signal to reliably detect and decode the data, typically sampling in the middle of each bit period.