drivers/periph/uart: add periph_uart_tx_ondemand feature

[benpicco]

Contribution description

We have a one-wire bus where the idle level is LOW:

If two sensors pull the bus high at the same time, no-one will be able to send anything.
To make this work with standard UART where idle is LOW, we employ a little hack in which we de-mux the UART TX pin when we are not sending and only mux it to TX when we want to send a frame.

This generates a spurious 0 byte after the de-muxing, but since our DOSE frames are well delimited, this noise outside the frame is no issue.

Testing procedure

This bus is really slow (2400 Baud) but it works:

2023-11-22 18:03:53,026 - INFO # > ping -i 5000 fe80::a844:d9ff:fe07:5dc6
2023-11-22 18:03:53,026 - INFO # 
2023-11-22 18:03:54,465 - INFO # 12 bytes from fe80::a844:d9ff:fe07:5dc6%5: icmp_seq=0 ttl=64 time=1417.622 ms
2023-11-22 18:03:59,459 - INFO # 12 bytes from fe80::a844:d9ff:fe07:5dc6%5: icmp_seq=1 ttl=64 time=1412.560 ms
2023-11-22 18:04:04,499 - INFO # 12 bytes from fe80::a844:d9ff:fe07:5dc6%5: icmp_seq=2 ttl=64 time=1433.686 ms
2023-11-22 18:04:04,499 - INFO # 
2023-11-22 18:04:04,501 - INFO # --- fe80::a844:d9ff:fe07:5dc6 PING statistics ---
2023-11-22 18:04:04,513 - INFO # 3 packets transmitted, 3 packets received, 0% packet loss
2023-11-22 18:04:04,515 - INFO # round-trip min/avg/max = 1412.560/1421.289/1433.686 ms

Issues/PRs references

previous attempt at this was #14300

[benpicco]

This is not exactly related to this PR, but without it SOFT_UART_PARAM_xx set in board.h would not get applied.

I first thought of using the software UART to invert the line polarity, but then I found I can use a real UART instead if I re-configure the TX pin.

[riot-ci]

Murdock results

✔️ PASSED

9cde80e drivers_dose: enable TX UART on-demand

Success	Failures	Total	Runtime
8081	0	8082	10m:04s

Artifacts

• Documentation preview