class UART – duplex serial communication bus

A universal asynchronous receiver-transmitter (UART) is an interface for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel.

See also https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter

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("A", 9600)                       # init on connector P3 with given baudrate
uart.init(9600, bits=8, parity=None, stop=1) # init with given parameters

On the Kookaberry 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.

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\n')   # write the 3 characters and a newline character

UART Constructors

class machine.UART(id, baudrate=9600, bits=8, parity=None, stop=1, \*, ...)

Construct a UART object of the given id.

id is a string which can be:

  • “A” (on connector P3) with the Rx pin on P3A / PA10 / J4, and the Tx pin on P3B / PA9 / J10

  • “B” (only on the edge connector) with the Rx pin on J7, and the Tx pin on J8

_images/uart_pins.png

Fig. 13 UART GPIO Pin Assignments

UART Methods

UART.init(baudrate=9600, bits=8, parity=None, stop=1, \*, ...)

Initialise the UART bus with the given parameters:

  • baudrate is the clock rate (default is 9600).

  • bits is the number of bits per character, 7, 8 (default) or 9.

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

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

Additional keyword-only parameters that may be supported by a port are:

  • 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.

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.

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.

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.

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.

UART.write(buf)

Write the buffer of bytes to the bus.

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

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, 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.

UART Constants

UART.RTS -- 256
UART.CTS -- 512
UART.IRQ_RXIDLE -- 16