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
esp32/machine_uart: Change default UART(1) pins on ESP32 with SPIRAM.
Summary
SPIRAM usually uses GPIO 9 and 10 on ESP32, so don't use them as default UART pins.
Fixes issue #18544.
Testing
Tested on ESP32 with SPIRAM. Creating a machine.UART(1) (with default pins) no longer crashes the device.
Trade-offs and Alternatives
The alternative is to just document this, saying that you must always specify the tx/rx pins when SPIRAM is installed (on an ESP32-OG). But from a user's point of view I think it's nicer if the default pins don't conflict with SPIRAM.