.. currentmodule:: pyb
.. _pyb.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 pyb import UART

    # init with the given baudrate
    uart = UART(3, 9600, timeout_char=1000)

    # init with explicit parameters
    uart.init(9600, bits=8, parity=None, stop=1, timeout_char=1000)

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 :std:term:`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

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 the number of characters waiting

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

Constructors
------------

.. class:: UART(bus: Union[int, str], *args, **kwargs)

   Construct a UART object on the given ``bus`` (an integer peripheral
   index, e.g. ``3`` for ``UART3``). With no additional parameters the
   object is created but not initialised (it retains the previous bus
   settings, if any); if extra arguments are given the bus is initialised.
   See :meth:`init` for the available parameters.

   ``UART(3)`` is wired to the same header pins on every STM32 OpenMV Cam:

   .. list-table::
      :header-rows: 1
      :widths: 30 30

      * - Signal
        - Header pin
      * - ``TX``
        - ``P4``
      * - ``RX``
        - ``P5``

   Additional UART buses are available on some boards:

   .. list-table::
      :header-rows: 1
      :widths: 14 18 18 50

      * - Bus
        - TX pin
        - RX pin
        - Available on
      * - ``UART(1)``
        - ``P1``
        - ``P0``
        - OpenMV Cam M7 / H7 / H7 Plus / Pure Thermal
      * - ``UART(4)``
        - ``P2``
        - ``P3``
        - OpenMV Cam N6
      * - ``UART(7)``
        - ``P14``
        - ``P13``
        - OpenMV Cam N6

   Methods
   -------

   .. method:: init(baudrate: int, bits: int = 8, parity: Optional[int] = None, stop: int = 1, *, timeout: int = 1000, flow: int = 0, timeout_char: int = 0, read_buf_len: int = 64) -> None

      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 0, ``UART.RTS``, ``UART.CTS``
          or ``UART.RTS | UART.CTS``.
        - ``timeout`` is the timeout in milliseconds to wait for writing/reading the first character.
        - ``timeout_char`` is the timeout in milliseconds to wait between characters while writing or reading.
        - ``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.

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

   .. method:: deinit() -> None

      Turn off the UART bus.

   .. method:: any() -> int

      Returns the number of bytes waiting (may be 0).

   .. method:: read(nbytes: Optional[int] = None) -> Optional[bytes]

      Read characters.  If ``nbytes`` is specified then read at most that many bytes.
      If ``nbytes`` are available in the buffer, returns immediately, otherwise returns
      when sufficient characters arrive or the timeout elapses.

      If ``nbytes`` is not given then the method reads as much data as possible.  It
      returns after the timeout has elapsed.

      *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.

   .. method:: readchar() -> int

      Receive a single character on the bus.

      Return value: The character read, as an integer.  Returns -1 on timeout.

   .. method:: readinto(buf: bytearray, nbytes: Optional[int] = None) -> Optional[int]

      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.

   .. method:: readline() -> Optional[bytes]

      Read a line, ending in a newline character. If such a line exists, return is
      immediate. If the timeout elapses, all available data is returned regardless
      of whether a newline exists.

      Return value: the line read or ``None`` on timeout if no data is available.

   .. method:: write(buf: Union[bytes, bytearray, str]) -> Optional[int]

      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. If a timeout occurs and no bytes
      were written returns ``None``.

   .. method:: writechar(char: int) -> None

      Write a single character on the bus. ``char`` is an integer to
      write. See the *CTS flow control* section below for blocking
      semantics when CTS flow control is enabled.

   .. method:: sendbreak() -> None

      Send a break condition on the bus. This drives the bus low for a
      duration of 13 bits.

   Constants
   ---------

   .. data:: RTS
      :type: int

      Bit flag for the ``flow`` argument of :meth:`init`; enables RTS
      (request-to-send) hardware flow control on the receive path.

   .. data:: CTS
      :type: int

      Bit flag for the ``flow`` argument of :meth:`init`; enables CTS
      (clear-to-send) hardware flow control on the transmit path. May
      be OR-ed with :data:`RTS` to enable both directions.

Flow Control
------------

``UART(3)`` supports RTS/CTS hardware flow control. On the OpenMV Cam M7,
H7, H7 Plus and Pure Thermal the flow-control pins are:

    ``(TX, RX, nRTS, nCTS) = (P4, P5, P1, P2)``

On the OpenMV Cam N6 only ``nRTS`` is exposed (on header pin ``P7``);
``nCTS`` is not routed to the I/O header.

In the following paragraphs the term "target" refers to the device
connected to the UART.

When the UART's :meth:`init` method is called with ``flow`` set to one or
both of ``UART.RTS`` and ``UART.CTS``, the relevant flow-control pins are
configured. ``nRTS`` is an active-low output and ``nCTS`` is an active-low
input with pull-up enabled. To wire up flow control, connect the OpenMV
Cam's ``nCTS`` to the target's ``nRTS`` and the OpenMV Cam's ``nRTS`` to
the target's ``nCTS``.

CTS: target controls OpenMV Cam transmitter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If CTS flow control is enabled the write behaviour is as follows:

If the OpenMV Cam's ``UART.write(buf)`` method is called, transmission will stall for
any periods when ``nCTS`` is ``False``. This will result in a timeout if the entire
buffer was not transmitted in the timeout period. The method returns the number of
bytes written, enabling the user to write the remainder of the data if required. In
the event of a timeout, a character will remain in the UART pending ``nCTS``. The
number of bytes composing this character will be included in the return value.

If ``UART.writechar()`` is called when ``nCTS`` is ``False`` the method will time
out unless the target asserts ``nCTS`` in time. If it times out ``OSError 116``
will be raised. The character will be transmitted as soon as the target asserts ``nCTS``.

RTS: OpenMV Cam controls target's transmitter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If RTS flow control is enabled, behaviour is as follows:

If buffered input is used (``read_buf_len`` > 0), incoming characters are buffered.
If the buffer becomes full, the next character to arrive will cause ``nRTS`` to go
``False``: the target should cease transmission. ``nRTS`` will go ``True`` when
characters are read from the buffer.

Note that the ``any()`` method returns the number of bytes in the buffer. Assume a
buffer length of ``N`` bytes. If the buffer becomes full, and another character arrives,
``nRTS`` will be set False, and ``any()`` will return the count ``N``. When
characters are read the additional character will be placed in the buffer and will
be included in the result of a subsequent ``any()`` call.

If buffered input is not used (``read_buf_len`` == 0) the arrival of a character will
cause ``nRTS`` to go ``False`` until the character is read.