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 pyb import UART

# init with the given baudrate
uart = UART(3, 9600, timeout_char=1000)

# init with explicit parameters
uart.init(9600, bits=8, parity=None, stop=1, timeout_char=1000)

Bits can be 7, 8 or 9. Parity can be None, 0 (even) or 1 (odd). Stop can be 1 or 2.

Note: 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')   # write the 3 characters

Individual characters can be read/written using:

uart.readchar()     # read 1 character and returns it as an integer
uart.writechar(42)  # write 1 character

To check if there is anything to be read, use:

uart.any()          # returns the number of characters waiting

Note: The stream functions read, write, etc. are new in MicroPython v1.3.4. Earlier versions use uart.send and uart.recv.

Constructors¶

class pyb.UART(bus: int | str, *args, **kwargs)¶

Construct a UART object on the given bus (an integer peripheral index, e.g. 3 for UART3). With no additional parameters the object is created but not initialised (it retains the previous bus settings, if any); if extra arguments are given the bus is initialised. See init() for the available parameters.

UART(3) is wired to the same header pins on every STM32 OpenMV Cam:

Signal	Header pin
`TX`	`P4`
`RX`	`P5`

Additional UART buses are available on some boards:

Bus	TX pin	RX pin	Available on
`UART(1)`	`P1`	`P0`	OpenMV Cam M7 / H7 / H7 Plus / Pure Thermal
`UART(4)`	`P2`	`P3`	OpenMV Cam N6
`UART(7)`	`P14`	`P13`	OpenMV Cam N6

Methods¶

init(baudrate: int, bits: int = 8, parity: int | None = None, stop: int = 1, *, timeout: int = 1000, flow: int = 0, timeout_char: int = 0, read_buf_len: int = 64) → None¶

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.

flow sets the flow control type. Can be 0, UART.RTS, UART.CTS or UART.RTS | UART.CTS.

timeout is the timeout in milliseconds to wait for writing/reading the first character.

timeout_char is the timeout in milliseconds to wait between characters while writing or reading.

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

This method will raise an exception if the baudrate could not be set within 5% of the desired value.

Note: with parity=None, only 8 and 9 bits are supported. With parity enabled, only 7 and 8 bits are supported.

deinit() → None¶: Turn off the UART bus.

any() → int¶: Returns the number of bytes waiting (may be 0).

read(nbytes: int | None = None) → bytes | None¶

Read characters. If nbytes is specified then read at most that many bytes. If nbytes are available in the buffer, returns immediately, otherwise returns when sufficient characters arrive or the timeout elapses.

If nbytes is not given then the method reads as much data as possible. It returns after the timeout has elapsed.

Note: for 9 bit characters each character takes two bytes, nbytes must be even, and the number of characters is nbytes/2.

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

readchar() → int¶

Receive a single character on the bus.

Return value: The character read, as an integer. Returns -1 on timeout.

readinto(buf: bytearray, nbytes: int | None = None) → int | None¶

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.

readline() → bytes | None¶

Read a line, ending in a newline character. If such a line exists, return is immediate. If the timeout elapses, all available data is returned regardless of whether a newline exists.

Return value: the line read or None on timeout if no data is available.

write(buf: bytes | bytearray | str) → int | None¶

Write the buffer of bytes to the bus. If characters are 7 or 8 bits wide then each byte is one character. If characters are 9 bits wide then two bytes are used for each character (little endian), and buf must contain an even number of bytes.

Return value: number of bytes written. If a timeout occurs and no bytes were written returns None.

writechar(char: int) → None¶: Write a single character on the bus. char is an integer to write. See the CTS flow control section below for blocking semantics when CTS flow control is enabled.

sendbreak() → None¶: Send a break condition on the bus. This drives the bus low for a duration of 13 bits.

Constants¶

RTS: int¶: Bit flag for the flow argument of init(); enables RTS (request-to-send) hardware flow control on the receive path.

CTS: int¶: Bit flag for the flow argument of init(); enables CTS (clear-to-send) hardware flow control on the transmit path. May be OR-ed with RTS to enable both directions.

Flow Control¶

UART(3) supports RTS/CTS hardware flow control. On the OpenMV Cam M7, H7, H7 Plus and Pure Thermal the flow-control pins are:

(TX, RX, nRTS, nCTS) = (P4, P5, P1, P2)

On the OpenMV Cam N6 only nRTS is exposed (on header pin P7); nCTS is not routed to the I/O header.

In the following paragraphs the term “target” refers to the device connected to the UART.

When the UART’s init() method is called with flow set to one or both of UART.RTS and UART.CTS, the relevant flow-control pins are configured. nRTS is an active-low output and nCTS is an active-low input with pull-up enabled. To wire up flow control, connect the OpenMV Cam’s nCTS to the target’s nRTS and the OpenMV Cam’s nRTS to the target’s nCTS.

CTS: target controls OpenMV Cam transmitter¶

If CTS flow control is enabled the write behaviour is as follows:

If the OpenMV Cam’s UART.write(buf) method is called, transmission will stall for any periods when nCTS is False. This will result in a timeout if the entire buffer was not transmitted in the timeout period. The method returns the number of bytes written, enabling the user to write the remainder of the data if required. In the event of a timeout, a character will remain in the UART pending nCTS. The number of bytes composing this character will be included in the return value.

If UART.writechar() is called when nCTS is False the method will time out unless the target asserts nCTS in time. If it times out OSError 116 will be raised. The character will be transmitted as soon as the target asserts nCTS.

RTS: OpenMV Cam controls target’s transmitter¶

If RTS flow control is enabled, behaviour is as follows:

If buffered input is used (read_buf_len > 0), incoming characters are buffered. If the buffer becomes full, the next character to arrive will cause nRTS to go False: the target should cease transmission. nRTS will go True when characters are read from the buffer.

Note that the any() method returns the number of bytes in the buffer. Assume a buffer length of N bytes. If the buffer becomes full, and another character arrives, nRTS will be set False, and any() will return the count N. When characters are read the additional character will be placed in the buffer and will be included in the result of a subsequent any() call.

If buffered input is not used (read_buf_len == 0) the arrival of a character will cause nRTS to go False until the character is read.