Esp32: Add explicit "wait_tx_done" method to UART
Hello folks,
Faced with issue, I cannot control time when UART TX done from MicroPython.
Use case is next:
- I've tried to communicate by raw RS485 and I should know when exactly disable TX pin.
Currently, there are couple workarounds (judged by sources and undocumented). In these methods called uart_wait_tx_done():
- Via
init()method. E.g. send bytes and callinit(). But if implementation will be changed in future such method stopped worked. Especially this method is very strange to call in the middle of the UART operation. - Via
sendbreak()method. But also undocumented plus it always sends\x00
Proposal:
- I think it would be good to add explicit method for waiting UART TX done.
esp32/machine_uart: Change `sendbreak()` implementation to simply pull the pin low for the break period
Summary
Currently, UART.sendbreak() on esp32 will reconfigure the UART to a slower baudrate and send out a null byte, to synthesise a break condition. That's not great because it changes the baudrate of the RX path as well, which could miss incoming bytes while sending the break.
This PR changes the sendbreak implementation to just reconfigure the TX pin as GPIO in output mode, and hold the pin low for the required duration.
Testing
Existing test is updated (simplified) and works well on ESP32_GENERIC.
Also tested that RPI_PICO_W still passes this test.
Trade-offs and Alternatives
There's uart_write_bytes_with_break() but that requires at least 1 byte to be sent before the break, so we can't use that.
There's also the lower level uart_hal_tx_break() but I'm not sure how to use that properly in conjunction with the higher level driver.