class UART – Universal Asynchronous Receiver/Transmitter

UART implements the standard UART/USART duplex serial communications protocol. At the physical level it consists of 2 lines: RXD and TXD. The unit of communication is a character (not to be confused with a string character) which can be 5, 6, 7 or 8 bits wide.

UART objects can be created and initialised using:

from machine import UART

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

Bits can be 5, 6, 7, 8. Parity can be None, UART.EVEN or UART.ODD. Stop can be 1, 1.5 or 2.

A UART object acts like a stream object therefore reading and writing is done using the standard stream methods:       # read 10 characters, returns a bytes object
uart.readall()      # 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

To check if there is anything to be read, use:

uart.any()               # returns the number of characters available for reading

Quick Usage Example

from machine import UART
# this uses the UART_1 default pins for TXD and RXD (``P3`` and ``P4``)
uart = UART(1, baudrate=9600)
uart.write('hello') # read up to 5 bytes


class machine.UART(bus, ...)

Construct a UART object on the given bus. bus can be 0 or 1. If the bus is not given, the default one will be selected (0) or the selection will be made based on the given pins.


uart.init(baudrate=9600, bits=8, parity=None, stop=1, * , timeout_chars=2, pins=(TXD, RXD, RTS, CTS))

Initialise the UART bus with the given parameters:

  • baudrate is the clock rate.
  • bits is the number of bits per character. Can be 5, 6, 7 or 8.
  • parity is the parity, None, UART.EVEN or UART.ODD.
  • stop is the number of stop bits, 1 or 2.
  • timeout_chars Rx timeout defined in number of characters. The value given here will be multiplied by the time a characters takes to be transmitted at the configured baudrate.
  • pins is a 4 or 2 item list indicating the TXD, RXD, RTS and CTS pins (in that order). Any of the pins can be None if one wants the UART to operate with limited functionality. If the RTS pin is given the the RX pin must be given as well. The same applies to CTS. When no pins are given, then the default set of TXD (P1) and RXD (P0) pins is taken, and hardware flow control will be disabled. If pins=None, no pin assignment will be made.

Turn off the UART bus.


Return the number of characters available for reading.[nbytes])

Read characters. If nbytes is specified then read at most that many bytes.

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


Read as much data as possible.

Return value: a bytes object or 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. If such a line exists, return is immediate. If the timeout elapses, all available data is returned regardless of whether a newline exists.

Return value: the line read or None on timeout if no data is available.


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 of 13 bits. Return value: None.



parity types (along with None)


IRQ trigger sources

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