.. 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.
- 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.