.. currentmodule:: pyb

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 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 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

Methods

.. method:: UART.init(baudrate, bits=8, parity=None, stop=1, *, timeout=0, flow=0, timeout_char=0, read_buf_len=64)

 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. The minimum baudrate is dictated by the frequency
 of the bus that the UART is on; UART(1) and UART(6) are APB2, the rest are on
 APB1. The default bus frequencies give a minimum baudrate of 1300 for
 UART(1) and UART(6) and 650 for the others. Use :func:`pyb.freq <pyb.freq>`
 to reduce the bus frequencies to get lower baudrates.

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

.. method:: UART.deinit()

 Turn off the UART bus.

.. method:: UART.any()

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

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

 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:: UART.readchar()

 Receive a single character on the bus.

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

 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. 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:: UART.write(buf)

 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:: UART.writechar(char)

 Write a single character on the bus. ``char`` is an integer to write.
 Return value: ``None``. See note below if CTS flow control is used.

.. method:: UART.sendbreak()

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

Constants

.. data:: UART.RTS
 UART.CTS

 to select the flow control type.

Flow Control

On Pyboards V1 and V1.1 UART(2) and UART(3) support RTS/CTS hardware flow control using the following pins:

• UART(2) is on: (TX, RX, nRTS, nCTS) = (X3, X4, X2, X1) = (PA2, PA3, PA1, PA0)
• UART(3) is on :(TX, RX, nRTS, nCTS) = (Y9, Y10, Y7, Y6) = (PB10, PB11, PB14, PB13)

On the Pyboard Lite only UART(2) supports flow control on these pins:

(TX, RX, nRTS, nCTS) = (X1, X2, X4, X3) = (PA2, PA3, PA1, PA0)

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

When the UART's 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, nCTS is an active low input with pullup enabled. To achieve flow control the Pyboard's nCTS signal should be connected to the target's nRTS and the Pyboard's nRTS to the target's nCTS.

CTS: target controls Pyboard transmitter

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

If the Pyboard'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: Pyboard 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.

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