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 different boards.

On the RI5, it appears that no UARTs are actually available - at least I couldn’t find an ID that would allow me to create one. So available parameters are unknown and the rest of this section is a little light on RI5-specific detail as it’s difficult to do experiments when you can’t create the base object! It’s based on the STM32 port code instead since that’ll probably be quite similar…

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 machine.UART(id, ...)

Construct a UART object of the given id. Any additional parameters are passed on to the init function.

Methods

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. Can be 8 or 9.

  • parity is the parity, None, 0 (even) or 1 (odd).

  • stop is the number of stop bits, 1 or 2.

  • timeout is the timeout in milliseconds to wait for the first character.

  • timeout_char is the timeout in milliseconds to wait between characters.

  • flow is RTS | CTS where RTS == 256, CTS == 512

  • read_buf_len is the character length of the read buffer (0 to disable).

UART.deinit()

Turn off the UART bus.

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)
UART.read([nbytes])

Read characters. If nbytes is specified then read at most that many bytes, otherwise read as much data as possible.

Return value: a bytes object containing the bytes read in. Returns None on timeout.

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.

UART.readline()

Read a line, ending in a newline character.

Return value: the line read or None on timeout.

UART.readchar()

Receive a single character, and return it as an integer (or -1 on timeout).

UART.write(buf)

Write the buffer of bytes to the bus.

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

UART.writechar(char)

Write a single character to the bus. char is the integer to write. Returns None.

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.

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

Create a callback to be triggered when data is received on the UART.

  • trigger can only be UART.IRQ_RXIDLE

  • hard seems to specify whether it’s a hardware or software interrupt?

  • handler an optional function to be called when new characters arrive.

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.

Constants

UART.RTS = 256

Flow parameter setting for initialization.

UART.CTS = 512

Flow parameter setting for initialization.

UART.IRQ_RXIDLE = 16

IRQ flag “idle”. (Replaces UART.RX_ANY from the base documentation.)