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
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.
- 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.
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 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.
Turn off the UART bus.
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)
Read characters. If
nbytesis 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
Read bytes into the
nbytesis 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
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
Write the buffer of bytes to the bus.
Return value: number of bytes written or
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.
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
- 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
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.
- trigger can only be