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

Supported parameters differ on a board:

Pyboard: 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.

WiPy/CC3200: Bits can be 5, 6, 7, 8. 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         # 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.

  • 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.

On the WiPy only the following keyword-only parameter is supported:

  • pins is a 4 or 2 item list indicating the TX, RX, 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 TX and RX pins is taken, and hardware flow control will be disabled. If pins is None, no pin assignment will be made.


Turn off the UART bus.


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. It may return sooner if a timeout is reached. The timeout is configurable in the constructor.

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. It may return sooner if a timeout is reached. The timeout is configurable in the constructor.

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


Read a line, ending in a newline character. It may return sooner if a timeout is reached. The timeout is configurable in the constructor.

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.

UART.irq(trigger, priority=1, handler=None, wake=machine.IDLE)

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

  • trigger can only be UART.RX_ANY

  • 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.

  • wake can only be machine.IDLE.


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

  • 8 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 will be between 1 to 8 characters waiting.

Returns an irq object.

Availability: WiPy.



IRQ trigger sources

Availability: WiPy.