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 machine.UART(id, ...)¶
Construct a UART object of the given id.
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, 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.
0will not invert lines (idle state of both lines is logic high).UART.INV_TXwill invert TX line (idle state of TX line now logic low).UART.INV_RXwill invert RX line (idle state of RX line now logic low).UART.INV_TX | UART.INV_RXwill invert both lines (idle state at logic low).
flow specifies which hardware flow control signals to use. The value is a bitmask.
0will ignore hardware flow control signals.UART.RTSwill enable receive flow control by using the RTS output pin to signal if the receive FIFO has sufficient space to accept more data.UART.CTSwill 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.CTSwill 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 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 calldeinit()as it will prevent callinginit()again.
- UART.deinit()¶
Turn off the UART bus.
Note
You will not be able to call
init()on the object afterdeinit(). A new instance needs to be created in that case.
- 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
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
Noneon timeout.
- UART.readinto(buf[, nbytes])¶
Read bytes into the
buf. Ifnbytesis specified then read at most that many bytes. Otherwise, read at mostlen(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
buforNoneon 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
Noneon timeout.
- UART.write(buf)¶
Write the buffer of bytes to the bus.
Return value: number of bytes written or
Noneon 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.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
- 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 returnsFalse.Note
For the esp8266 and nrf ports the call may return
Trueeven 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
- 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
UARTinstance.trigger configures the event(s) which can generate an interrupt. Possible values are a mask of one or more of the following:
UART.IRQ_RXIDLEinterrupt after receiving at least one character and then the RX line goes idle.UART.IRQ_RXinterrupt after each received character.UART.IRQ_TXIDLEinterrupt after or while the last character(s) of a message are or have been sent.UART.IRQ_BREAKinterrupt 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 Writing interrupt handlers.
Returns an irq object.
Due to limitations of the hardware not all trigger events are available on all ports.
Availability of triggers¶ 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.