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 pyb 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 7, 8 or 9. Parity can be None, 0 (even) or 1 (odd). Stop can be 1 or 2.

Note: 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
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

Individual characters can be read/written using:

uart.readchar()     # read 1 character and returns it as an integer
uart.writechar(42)  # write 1 character

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

uart.any()               # returns True if any characters waiting

Note: The stream functions read, write, etc. are new in MicroPython v1.3.4. Earlier versions use uart.send and uart.recv.


class pyb.UART(bus, ...)

Construct a UART object on the given bus. bus can be 1-6, or ‘XA’, ‘XB’, ‘YA’, or ‘YB’. With no additional parameters, the UART object is created but not initialised (it has the settings from the last initialisation of the bus, if any). If extra arguments are given, the bus is initialised. See init for parameters of initialisation.

The physical pins of the UART busses are:

  • UART(4) is on XA: (TX, RX) = (X1, X2) = (PA0, PA1)
  • UART(1) is on XB: (TX, RX) = (X9, X10) = (PB6, PB7)
  • UART(6) is on YA: (TX, RX) = (Y1, Y2) = (PC6, PC7)
  • UART(3) is on YB: (TX, RX) = (Y9, Y10) = (PB10, PB11)
  • UART(2) is on: (TX, RX) = (X3, X4) = (PA2, PA3)


uart.init(baudrate, bits=8, parity=None, stop=1, *, timeout=1000, flow=None, timeout_char=0, read_buf_len=64)

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.
  • flow sets the flow control type. Can be None, UART.RTS, UART.CTS or UART.RTS | UART.CTS.
  • timeout is the timeout in milliseconds to wait for the first character.
  • timeout_char is the timeout in milliseconds to wait between characters.
  • read_buf_len is the character length of the read buffer (0 to disable).

This method will raise an exception if the baudrate could not be set within 5% of the desired value. The minimum baudrate is dictated by the frequency of the bus that the UART is on; UART(1) and UART(6) are APB2, the rest are on APB1. The default bus frequencies give a minimum baudrate of 1300 for UART(1) and UART(6) and 650 for the others. Use pyb.freq to reduce the bus frequencies to get lower baudrates.

Note: with parity=None, only 8 and 9 bits are supported. With parity enabled, only 7 and 8 bits are supported.


Turn off the UART bus.


Return True if any characters waiting, else False.


Write a single character on the bus. char is an integer to write. Return value: None.[nbytes])

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

Note: for 9 bit characters each character takes two bytes, nbytes must be even, and the number of characters is nbytes/2.

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.


Receive a single character on the bus.

Return value: The character read, as an integer. Returns -1 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. If characters are 7 or 8 bits wide then each byte is one character. If characters are 9 bits wide then two bytes are used for each character (little endian), and buf must contain an even number of bytes.

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.



to select the flow control type