class UART – duplex serial communication bus

UART implements the standard UART/USART duplex serial communications protocol. At the physical level it consists of 2 lines: RX and TX. The unit of communication is a character (not to be confused with a string character) which can be 8 or 9 bits wide.

UART objects can be created and initialised using:

from machine import UART

uart = UART(3, 9600)                         # init with given baudrate
uart.init(9600, bits=8, parity=None, stop=1) # init with given parameters

Bits can be 7, 8 or 9. Stop can be 1 or 2. With parity=None, only 8 and 9 bits are supported. With parity enabled, only 7 and 8 bits are supported.

A UART object acts like a stream object and reading and writing is done using the standard stream methods:       # read 10 characters, returns a bytes object         # read all available characters
uart.readline()     # read a line
uart.readinto(buf)  # read and store into the given buffer
uart.write('abc')   # write the 3 characters


class machine.UART(id, ...)

Construct a UART object of the given id.


UART.init(baudrate=9600, bits=8, parity=None, stop=1, \*, ...)

Initialise the UART bus with the given parameters:

  • baudrate is the clock rate.

  • bits is the number of bits per character, 7, 8 or 9.

  • parity is the parity, None, 0 (even) or 1 (odd).

  • stop is the number of stop bits, 1 or 2.

Additional keyword-only parameters that may be supported by a port are:

  • tx specifies the TX pin to use.

  • rx specifies the RX pin to use.

  • rts specifies the RTS (output) pin to use for hardware receive flow control.

  • cts specifies the CTS (input) pin to use for hardware transmit flow control.

  • txbuf specifies the length in characters of the TX buffer.

  • rxbuf specifies the length in characters of the RX buffer.

  • timeout specifies the time to wait for the first character (in ms).

  • timeout_char specifies the time to wait between characters (in ms).

  • invert specifies which lines to invert.

    • 0 will not invert lines (idle state of both lines is logic high).

    • UART.INV_TX will invert TX line (idle state of TX line now logic low).

    • UART.INV_RX will invert RX line (idle state of RX line now logic low).

    • UART.INV_TX | UART.INV_RX will invert both lines (idle state at logic low).

  • flow specifies which hardware flow control signals to use. The value is a bitmask.

    • 0 will ignore hardware flow control signals.

    • UART.RTS will enable receive flow control by using the RTS output pin to signal if the receive FIFO has sufficient space to accept more data.

    • UART.CTS will enable transmit flow control by pausing transmission when the CTS input pin signals that the receiver is running low on buffer space.

    • UART.RTS | UART.CTS will enable both, for full hardware flow control.


It is possible to call init() multiple times on the same object in order to reconfigure UART on the fly. That allows using single UART peripheral to serve different devices attached to different GPIO pins. Only one device can be served at a time in that case. Also do not call deinit() as it will prevent calling init() again.


Turn off the UART bus.


You will not be able to call init() on the object after deinit(). A new instance needs to be created in that case.


Returns an integer counting the number of characters that can be read without blocking. It will return 0 if there are no characters available and a positive number if there are characters. The method may return 1 even if there is more than one character available for reading.

For more sophisticated querying of available characters use select.poll:

poll = select.poll()
poll.register(uart, select.POLLIN)

Read characters. If nbytes is specified then read at most that many bytes, otherwise read as much data as possible.

Return value: a bytes object containing the bytes read in. Returns None on timeout.

UART.readinto(buf[, nbytes])

Read bytes into the buf. If nbytes is specified then read at most that many bytes. Otherwise, read at most len(buf) bytes.

Return value: number of bytes read and stored into buf or None on timeout.


Read a line, ending in a newline character.

Return value: the line read or None on timeout.


Write the buffer of bytes to the bus.

Return value: number of bytes written or None on timeout.


Send a break condition on the bus. This drives the bus low for a duration longer than required for a normal transmission of a character.


Waits until all data has been sent. In case of a timeout, an exception is raised. The timeout duration depends on the tx buffer size and the baud rate. Unless flow control is enabled, a timeout should not occur.


Tells whether all data has been sent or no data transfer is happening. In this case, it returns True. If a data transmission is ongoing it returns False.