Support configurable pull down on Smart Audio VTXs

[robertlacroix]

This adds configurable support for TBS Unify Nano VTXs that require a pull low.

Thanks to @DieHertz for the approach.

Addresses #5329

Legal disclaimer: I am making my contributions/submissions to this project solely in my personal capacity and am not conveying any rights to any intellectual property of any third parties.

[DieHertz]

It's incomplete, we didn't want to adopt this workaround in it's current state, there in fact was a similar PR from me before.
I just had no time to implement it the way I planned to yet.

[robertlacroix]

@DieHertz what's missing? Do you want to take care of it? If you want I can help.

[controlflow]

Any news? :/

[hubertott]

I have tested this by patching the 3.3.3-maintenance branch with these changes and compiling it for my MATEKF405-CTR.

I am using a hardware UART

Setting CLI values:
vtx_pulldown = ON
vtx_halfduplex = ON

And for the first time I can now control the VTX via the FC/OSD!

Thank you!

[gajewski-tomasz]

I'm also waiting for it. I had to downgrade to 3.2.4.... any chance of seeing it on master soon?

[fredericmorin]

Bump @mikeller, @DieHertz

Same issue here. signal seems to be floating. Touching wire with damp finger make SA work on Unify Nano.

edit: 20k resistor hack did it. software pull down is a must still.

[gajewski-tomasz]

Cherry-picked this onto 3.4.1, works like a charm. Xrotor F4 (OMNIBUSF4SD) + TBS Unify Pro Nano.

[ArcaneFPV]

Hi guys, sorry to pester but I'm struggling to get SA to work on a new betaflight 3.5 build, I'm usually pretty good at tracking down a fix but this stuff has got me boggled! I see folks above talking about things like 'pull downs on smart audio vtx's' and installing resistors, any direction would be immensely appreciated!

[gajewski-tomasz]

@ArcaneFPV let's keep this discussion related to the pull request, try to ask your questions on facebook groups, e.g. here: https://www.facebook.com/groups/betaflightgroup/

[robertlacroix]

Fixed in #7603