UART with the same pin for both RX and TX doesn't work on ESP32 variants
Port, board and/or hardware
esp32 port
MicroPython version
MicroPython v1.23.0 on 2024-06-02; LOLIN_C3_MINI with ESP32-C3FH4
MicroPython v1.23.0 on 2024-06-02; LOLIN_S2_MINI with ESP32-S2FN4R2
MicroPython v1.23.0-602.g403401490d on 2024-10-25; LOLIN S3 MINI with ESP32S3
MicroPython v1.23.0 on 2024-06-02; Generic ESP32S3 module with ESP32S3
Reproduction
from micropython import const
from machine import Pin, UART, mem32
PIN_NUM = const(2)
PIN_FUNC_REG = const(0x60004554 + 4 * PIN_NUM) # GPIO_FUNCx_OUT_SEL_CFG_REG
UART_NUM = const(1)
UART_BAUDRATE = const(1000000)
pin = Pin(PIN_NUM)
# pass the same `pin` to `tx` and `rx` doesn't work!
uart = UART(UART_NUM, UART_BAUDRATE, tx=pin, rx=pin)
uart.write(b'test') # UART signal doesn't routed to `PIN_NUM `.
print(uart.read()) # nothing gets printed..
# pass only `tx` (to get the `UART_SIG` number)
uart = UART(UART_NUM, UART_BAUDRATE, tx=pin)
UART_SIG = mem32[PIN_FUNC_REG] # see "Peripheral Signals via GPIO Matrix" in datasheet
# now try again, this time overwrite `PIN_FUNC_REG` with previous value
uart = UART(UART_NUM, UART_BAUDRATE, tx=pin, rx=pin)
mem32[PIN_FUNC_REG] = UART_SIG # ESP32-C3 is 6, ESP32-S3 is 12...
uart.write(b'test')
print(uart.read()) # b'test' is printed!
Expected behaviour
I expected:
uart = UART(UART_NUM, UART_BAUDRATE, tx=pin, rx=pin)
to work- but is seems to overwrite the TX output configuration and only the RX pin is configured correctly.
My workaround is to re-write the TX output configuration after the UART initialization:
uart = UART(UART_NUM, UART_BAUDRATE, tx=pin, rx=pin)
mem32[PIN_FUNC_REG] = UART_SIG
I think the UART class needs to support this case of a single pin for both TX and RX.
I use it to control some single line smart servo motors with open-drain UART bus...
The ESP32 support this common UART configuration (like RS485),
it also support echo-cancellation that may also be a nice option for the UART class..
Observed behaviour
Tested the provided sample code on ESP32-S2, ESP32-S3, ESP32-C3 -
All have the same bug and the same suggested workaround fix.
Additional Information
No, I've provided everything above.
Code of Conduct
Yes, I agree
docs/esp32/quickref.rst Document that a Pin can be passed to UART constructor on the ESP32 ports.
Summary
The UART() constructor has the tx= and rx= parameters to specify a pin number. On the ESP32 port, an initialized machine.Pin() object can also be passed. This is useful to specify parameters such as mode=Pin.OPEN_DRAIN or drive=Pin.DRIVE_3
I could not find the documentation for this neither on the ESP32 specific page (https://docs.micropython.org/en/latest/esp32/quickref.html) nor on the generic UART() page ((https://docs.micropython.org/en/latest/library/machine.UART.html)
Testing
This is a documentation change only. However, I tested on a ESP32-S3 that mode=Pin.OPEN_DRAIN and drive=Pin.DRIVE_3 alter the voltages at the UART pins accordingly:
txpin = Pin(tx_number, mode=Pin.OUT, drive=Pin.DRIVE_3)
rxpin = Pin(rx_number, mode=Pin.IN)
while True:
UART( 1, baudrate=baudrate, tx=txpin, rx=rxpin)
time.sleep(5)
UART( 1, baudrate=baudrate, tx=txpin, rx=rxpin, invert=UART.INV_TX)
time.sleep(5)
See also discussion #16959