In the office I hear these terms thrown around as if they are the same. My understanding is that USARTs can deliver the clock signal along with the data.

Are there any other differences? What are the advantages and disadvantages of each?


3 Answers 3


UART = Universal Asynchronous Receiver Transmitter

USART = Universal Synchronous Asynchronous Receiver Transmitter

A USART can act in Asynchronous mode just like a UART. But is has the added capability of acting Synchronously. This means that the data is clocked. The clock is either recovered from the data itself or sent as an external signal. The data is regular and bits synchronize with the clock signal. No start and stop bits are used. This allows for a higher baud rate when operating synchronously because bit timings have a certain guarantee and more bits can be used for data instead of as headers.

Whereas a UART has an internal clock signal and data on the bus can have somewhat sloppier and aregular timing. UARTs require start and stop bits and Asynchronous data is only synchronized with the start and stop bits.

  • 3
    \$\begingroup\$ It's worth noting that there are many different styles of synchronous communication, and the term "USART" is often used to mean, as far as I can tell, to mean "a UART which also supports at least some style of synchronous communication". One must read a USART data sheet to determine whether it will be able to handle any particular style of synchronous communication. \$\endgroup\$
    – supercat
    Commented Jun 14, 2012 at 20:17

That's it, synchronous communication is clocked, while asynchronous is self-timed. The asynchronous UART's main disadvantages:

  • transmitter and receiver have to be set to or agree on a common bit-rate.
  • timing must be accurate to at least a few %. Microcontrollers require a crystal based or calibrated RC clock.

Synchronous communication doesn't have these disadvantages, and doesn't need a fixed clock frequency. I2C for instance, allows a slave to slow down the clock if it's too fast, by stretching the master's clock pulse. Main disadvantages:

  • uses a separate line for the clock
  • clock pulses are shorter than a bit time, so the required bandwidth is wider than with NRZ UART.


UART requires only data signal.

In UART, the data does not have to be transmitted at a fixed rate.

In UART, data is normally transmitted one byte at a time.

In UART, data transfer speed is set around specific values like 4800, 9600, 38400 bps ,etc.

UART speed is limited around 115200 bps.

Full duplex.


In USART, Synchronous mode requires both data and a clock.

In USART’s synchronous mode, the data is transmitted at a fixed rate.

In USART, Synchronous data is normally transmitted in the form of blocks

Synchronous mode allows for a higher DTR (data transfer rate) than asynchronous mode does, if all other factors are held constant..

USART is faster than 115kb.

Half duplex.

For more details please refer to the following link :-



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