UART sendbreak bug
There is a bug with the uart.sendbreak() method with V1.17 (haven't checked the latest)
The manual states the break signal lasts 13 bits. It actually is 11.
This has caused errors with external sensors that utilize the SDI-12 protocol. .The line needs to go for at least 12ms. Now, it is 9.167 ms. Even 13 bits is not long enough.
A good solution is to add a property where you can specify how many bits you can use.
SDI-12 is popular in agriculture and other industrial applications. (www.sdi-12.org)
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.