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 5, 6, 7, 8. Parity can be None, 0 (even) or 1 (odd). Stop can be 1 or 2.

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 or 2. 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.


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

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.

Turn off the UART bus.


Return True if any characters waiting, else False.[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 b'' on timeout.


Read as much data as possible.

Return value: a bytes object.


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.


Read a line, ending in a newline character.

Return value: the line read.


Write the buffer of bytes to the bus.

Return value: number of bytes written.


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


Send a break condition on the bus. This drives the bus low for a duration of 13 bits. Return value: None.

uart.callback(value, priority=1, handler=None)

Create a callback to be triggered when data is received on the UART.

  • value sets the size in bytes of the Rx buffer. Every character received is put into this buffer as long as there’s space free.
  • priority level of the interrupt. Can take values in the range 1-7. Higher values represent higher priorities.
  • handler an optional function to be called when new characters arrive.


The handler will be called whenever any of the following two conditions are met:

  • 4 new characters have been received.
  • At least 1 new character is waiting in the Rx buffer and the Rx line has been silent for the duration of 1 complete frame.

This means that when the handler function is called there might be 1, 2, 3 or 4 characters waiting.

Return a callback object.



to select the flow control type