cpu/stm32: added optional RS485 module to uart periph

[david-vankampen]

Contribution description

added optional RS485 module to uart periph, to leverage DE pin capabilities

Testing procedure

Updated a board that has a UART with the optional extra data, update the board's makefile to have FEATURES_REQUIRED += periph_uart_rs485 and verify that the DE pin toggles as expected using a logic analyzer.

Issues/PRs references

[david-vankampen]

@benpicco from the reference manual: this allows the MCU to interact with an RS-485 transceiver like this TI chip - the DE and RE inputs there can be tied to the DE line out of the MCU. DE means driver enable, RE I believe means Read Enable - basically allows for a half-duplex connection to the RS-485 bus. I will take a cut at adding the FEATURES_PROVIDED entry and HAS_PERIPH_RS485 as requested.

[david-vankampen]

@benpicco with the updates I just pushed, if I run the following command on tests/periph_uart: FEATURES_REQUIRED=periph_uart_rs485 make BOARD=nucleo-g070rb, and then use arm-none-eabi-objdump to check for the conditional code, I see it:

arm-none-eabi-objdump -S bin/nucleo-g070rb/tests_periph_uart.elf | grep de_pin
    if (uart_config[uart].de_pin != GPIO_UNDEF) {
        gpio_init(uart_config[uart].de_pin, GPIO_IN);
        gpio_init_af(uart_config[uart].de_pin, uart_config[uart].de_af);

whereas if I omit the FEATURES_REQUIRED variable from the call to make, I do not see those symbols included. I believe this should address your questions.

[david-vankampen]

@benpicco checking in - is anything still needed here?

[benpicco]

Can you add

FEATURES_OPTIONAL += periph_uart_rs485 

to tests/periph_uart so this is compile-tested?

And is the DE pin really all that's needed for RS485 operation?

[david-vankampen]

Added it to tests/periph_uart. and yes, the DE pin is enough to get the MCU communicating on RS-485, as long as it is routed to an RS485 transceiver, as linked above.

[fjmolinas]

@david-vankampen Kconfig needs an update to match the feature changes!

[david-vankampen]

@fjmolinas hows that?

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