Drive TX line high before switching to input

[SteveCEvans]

This PR attempts to fix the issues tackled by #13095.

The proposed solution mentioned at #13095 (comment) of only disabling the TX output once at 100ms have passed since a received character, resulted in the original issue resurfacing which was the FC dropping into DFU mode when a connected WS VTX was powered off.

The PR drives the TX line high with a pull-up before switching to an input with a pull-up. For #13095 to have any positive effect it must be either preventing the TX being sampled low, or the TX line is actually being pulled down by the attached device. This PR addresses the former.

@dxs94 could you please test this PR and report back.

[github-actions]

Do you want to test this code? You can flash it directly from Betaflight Configurator:

• Simply put #13098 (this pull request number) in the Select commit field of the Configurator firmware flasher tab (you need to Enable expert mode, Show release candidates and Development).

WARNING: It may be unstable. Use only for testing!

[blckmn]

AUTOMERGE: (FAIL)

• github identifies PR as mergeable -> FAIL
• assigned to a milestone -> PASS
• cooling off period lapsed -> PASS
• commit count less or equal to three -> PASS
• Don't merge label NOT found -> FAIL
• at least one RN: label found -> PASS
• Tested label found -> FAIL
• assigned to an approver -> PASS
• approver count at least three -> FAIL

[SteveCEvans]

The unit tests are working for me running under Linux.

CC version: clang version 10.0.0-4ubuntu1  Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin
CXX version: clang version 10.0.0-4ubuntu1  Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin

Whereas the cloud build is:

CC version: Ubuntu clang version 14.0.0-1ubuntu1.1 Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin
CXX version: Ubuntu clang version 14.0.0-1ubuntu1.1 Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: ```

[SteveCEvans]

@haslinghuis As you'll see I've made a number of commits to fix some, but not all of the unit tests. Something is broken around the definition of STATIC_UNIT_TESTED I suspect. Has the unit test script been changed to undefine UNIT_TEST - see build_config.h.

[haslinghuis]

Clang versions has been updated in the workflow image: actions/runner-images#8329

EDIT: should be fixed using #13099

[dxs94]

Tested with BF configurator 10.10.0RC1; selected BF 4.5.0.Z.
Commit #13098 is NOT working (no stick movement - no crossfire telemetry).
Same conditions, commit #13095 shows stick inputs.

[SteveCEvans]

Unit test changes backed out as this is not the place to fix that issue.

[SteveCEvans]

@dxs94 Thanks for testing. I've just pushed a commit which adds a new debug mode, UART_TX.

Could you please log with that. I presume this is still on an F4?

This will log the following:

Debug	Value
debug[0]	Count of times the UART 2 TX line's state is read
debug[1]	Count of times the UART 2 TX line is found to be pulled low
debug[2]	Count of times the UART 2 TX line's is switched to an input

If we see debug[1] incrementing with everything powered on... that's bad.

[dxs94]

Hi yes still the same F411 board from GEPRC P16 as in #12902. Unfortunately no black box on this one so no logging :(

[SteveCEvans]

In light of testing there are evidently issues with some receivers so closing this and moving to #13100.