circuitpython/docs/library/machine.UART.rst

.. 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 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.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``.

   .. note::

      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.

Constants
---------

.. data:: UART.RX_ANY

    IRQ trigger sources

    Availability: WiPy.