.. currentmodule:: machine .. _machine.UART:

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::

uart.read(10)       # read 10 characters, returns a bytes object
uart.read()         # 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

Constructors

.. class:: UART(id, ...)

Construct a UART object of the given id.

Methods

.. method:: 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.

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 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.

.. note:: 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.

.. method:: UART.deinit()

Turn off the UART bus.

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

.. method:: UART.any()

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)
poll.poll(timeout)

.. method:: UART.read([nbytes])

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.

.. method:: 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.

.. method:: UART.readline()

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.

.. method:: UART.write(buf)

Write the buffer of bytes to the bus.

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

.. method:: UART.sendbreak()

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.

.. method:: UART.flush()

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.

.. note::

   For the esp8266 and nrf ports the call returns while the last byte is sent.
   If required, a one character wait time has to be added in the calling script.

Availability: rp2, esp32, esp8266, mimxrt, cc3200, stm32, nrf ports, renesas-ra

.. method:: UART.txdone()

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.

.. note::

   For the esp8266 and nrf ports the call may return ``True`` even if the last byte
   of a transfer is still being sent. If required, a one character wait time has to be
   added in the calling script.

Availability: rp2, esp32, esp8266, mimxrt, cc3200, stm32, nrf ports, renesas-ra

.. method:: UART.irq(handler=None, trigger=0, hard=False)

Configure an interrupt handler to be called when a UART event occurs.

The arguments are:

 - *handler* is an optional function to be called when the interrupt event
   triggers.  The handler must take exactly one argument which is the
   ``UART`` instance.

 - *trigger* configures the event(s) which can generate an interrupt.
   Possible values are a mask of one or more of the following:

   - ``UART.IRQ_RXIDLE`` interrupt after receiving at least one character
     and then the RX line goes idle.
   - ``UART.IRQ_RX`` interrupt after each received character.
   - ``UART.IRQ_TXIDLE`` interrupt after or while the last character(s) of
     a message are or have been sent.
   - ``UART.IRQ_BREAK`` interrupt when a break state is detected at RX

 - *hard* if true a hardware interrupt is used.  This reduces the delay
   between the pin change and the handler being called. Hard interrupt
   handlers may not allocate memory; see :ref:`isr_rules`.

Returns an irq object.

Due to limitations of the hardware not all trigger events are available on all ports.

.. table:: Availability of triggers :align: center

  ============== ========== ====== ========== =========
  Port / Trigger IRQ_RXIDLE IRQ_RX IRQ_TXIDLE IRQ_BREAK
  ============== ========== ====== ========== =========
  CC3200                      yes
  ESP32            yes        yes                yes
  MIMXRT           yes                yes
  NRF                         yes     yes
  RENESAS-RA       yes        yes
  RP2              yes                yes        yes
  SAMD             yes        yes     yes
  STM32            yes        yes
  ============== ========== ====== ========== =========

.. note:: - The ESP32 port does not support the option hard=True. It uses Timer(0) for UART.IRQ_RXIDLE, so this timer cannot be used for other means.

 - The rp2 port's UART.IRQ_TXIDLE is only triggered when the message
   is longer than 5 characters and the trigger happens when still 5 characters
   are to be sent.

 - The rp2 port's UART.IRQ_BREAK needs receiving valid characters for triggering
   again.

 - The SAMD port's UART.IRQ_TXIDLE is triggered while the last character is sent.

 - On STM32F4xx MCU's, using the trigger UART.IRQ_RXIDLE the handler will be called once
   after the first character and then after the end of the message, when the line is
   idle.

Availability: cc3200, esp32, mimxrt, nrf, renesas-ra, rp2, samd, stm32.

Constants

.. data:: UART.RTS UART.CTS

Flow control options.

Availability: esp32, mimxrt, renesas-ra, rp2, stm32.

.. data:: UART.IRQ_RXIDLE UART.IRQ_RX UART.IRQ_TXIDLE UART.IRQ_BREAK

IRQ trigger sources.

Availability: renesas-ra, stm32, esp32, rp2040, mimxrt, samd, cc3200.