feat(tx16smk3): add PC-controlled RGB ring feedback

[szokezoltan95]

Summary of changes:

Adds host-to-radio RGB feedback for the RadioMaster TX16S MK3 through the existing Advanced USB Joystick HID interface.

• Adds an 8-byte vendor-defined HID Feature Report to Advanced Joystick mode on the TX16S MK3.
• Supports setting one color across all 20 gimbal-ring LEDs, explicitly releasing PC control, and refreshing the override with a keepalive.
• Individual ring-LED control is not included.
• Automatically releases the PC override after five seconds without a valid set-color or keepalive command.
• Immediately releases the override when USB Joystick mode disconnects.
• Applies the PC color as a front-buffer overlay, allowing EdgeTX and Lua to continue updating their normal back-buffer state.
• Leaves the six customizable function-switch LEDs under EdgeTX control.
• Restores the current EdgeTX-controlled ring color immediately when the override is released.
• Keeps the Classic Joystick report descriptor unchanged.
• Limits the increased HID descriptor allocation to the TX16S MK3.

Motivation

Allow PC simulators and desktop applications to provide visual status feedback through the TX16S MK3's built-in gimbal-ring LEDs while using the radio as a USB joystick, without requiring separate LED hardware.

Feature Report payload

Byte	Meaning
0	Protocol version (1)
1	Command
2	Target (0 = all gimbal-ring LEDs)
3	Red
4	Green
5	Blue
6–7	Reserved

Commands

Value	Command
1	Set color and refresh the timeout
2	Release the override
3	Keepalive; refresh an active override without changing its color

Testing

• Built successfully for PCB=TX16SMK3 using the EdgeTX Docker development image.
• Flashed and tested on a RadioMaster TX16S MK3.
• Verified simultaneous Advanced Joystick axis/button input and RGB updates.
• Verified operation while the EdgeTX RGB LED special function is enabled, without color flickering.
• Verified explicit release restores the current EdgeTX color.
• Verified inactivity restores EdgeTX control after approximately five seconds.
• Verified USB disconnection restores EdgeTX control.
• Verified repeated delayed keepalives maintain the override.
• Verified the radio boots and operates normally after flashing.

Final local firmware build SHA-256:

abf0c64d300e40c9db5046df29f4d61d2c467de3a41dd589c333dcbc9da7ccaa

[philmoz]

Why?

[szokezoltan95]

The goal is to let a PC simulator or other desktop application provide visual feedback through the radio's built-in gimbal-ring LEDs while the TX16S MK3 continues operating as a USB joystick.

For example, a simulator could use the rings to indicate an aircraft or vehicle state, warning, mode, or other application-defined status without requiring separate external LED hardware. EdgeTX and Lua can currently control these LEDs locally, but the PC has no corresponding feedback path through the joystick connection.

I kept this initial implementation deliberately narrow: Advanced Joystick mode only, one color across the gimbal-ring LEDs, no change to Classic mode, and automatic release on timeout or USB disconnection. The six function-switch LEDs remain controlled by EdgeTX.

I built this primarily for my own simulator setup, but submitted it in case the capability is useful to others. I am open to changing the architecture or scope if a more general host-feedback interface would fit EdgeTX better.

[3djc]

One radio, a single user use case, should stay in its own fork imho

[szokezoltan95]

Understood. The gimbal-ring hardware itself appears to be specific to the TX16S MK3, although EdgeTX already contains the generic RGB infrastructure needed to operate it locally.

Would a generic host-feedback interface still be of interest, or is PC-driven control outside the intended scope of EdgeTX? If it is out of scope, I am happy to maintain this in my fork.

[gagarinlg]

Understood. The gimbal-ring hardware itself appears to be specific to the TX16S MK3, although EdgeTX already contains the generic RGB infrastructure needed to operate it locally.

That's not true. A couple of radios already support and have controllable LEDs, like the Flysky ST16, PA01, TX15, ...

[szokezoltan95]

That's not true. A couple of radios already support and have controllable LEDs, like the Flysky ST16, PA01, TX15, ...

Thank you for the info, I am just working with what I have physically available.

[pfeerick]

is PC-driven control outside the intended scope of EdgeTX

To be honest, I think so... it's primary scope is actual control / flight, PC simulator secondary... and even then, not niche cases. i.e. if one or more mainstream simulators actually supported / wanted to support this, then there would be reason to support it in the version of edgetx that everyone uses.