fix(f4): limit CRSF external speed to 3.75Mbps

[3djc]

Based in conversation between ELRS and ETX teams, it has been determined that CRSF external module speed needs to be limited to 3.75 Mbps for F4 radios.

While plenty of users THINK they have a very well working setup, since no obvious error message is present, there are however a significant number of invisible CRC errors leading to telemetry frames been lost, without any way for the user to be aware of that.

Analysis by ELRS team:

This is very present for F4 radio, but much less so for newer H7 radio, so the decision has been made to limit it just for F4 class radio

This PR will prevent 5.25 to be selected on those radio, and also limits the value to 3.75Mbps for existing models.

@elecpower you might want a companion side for it too, for consistency (albeit the radio will limit speed even if a model was edited to 5.25 with companion).

PS: don't shoot the messenger !!!!

[elecpower]

@elecpower you might want a companion side for it too, for consistency (albeit the radio will limit speed even if a model was edited to 5.25 with companion).

@3djc you wish is my command

[CapnBry]

It is true. For external modules, 3.75M baud has an incoming (to handset) error rate in the ballpark of roughly 1 in 1M, where at 5.25M it is around 1 in 50K. They're all missing bytes, not bit errors or otherwise corrupted data, and the UART register overflow error bit is set.

For internal modules, 5.25M works fantastically, with an error rate orders of magnitude lower than even the external at 3.75M. I'm not even sure you'll even see 1 error in a full day of data at the fastest ELRS link rate.

[3djc]

@elecpower you might want a companion side for it too, for consistency (albeit the radio will limit speed even if a model was edited to 5.25 with companion).

@3djc you wish is my command

Thanks a lot !

[pfeerick]

Code and rationale looks good, haven't tried it yet though.

PS: If you have an issue with the messenger, you gotta go through me first! :-P

But seriously, we can't allow for known bad settings to be selectable... just like the default hardware related settings should generally "just work" for everyone where possible.

[richardclli]

What radios are tested for this problem? I am thinking we may need to do more analysis to check for the root cause first before doing some quick fix change.

[gismo2004]

I am running 3.75 on my V16 since quite some time without issues. Setting it higher leads to problems with elrs.lua and also telemetry issues from time to time.

So V16 can be seen as tested I think.

[richardclli]

I am running 3.75 on my V16 since quite some time without issues. Setting it higher leads to problems with elrs.lua and also telemetry issues from time to time.

So V16 can be seen as tested I think.

Did you tested with Internal ELRS module or external ELRS module? I spot that many radios did not have RX DMA enabled for external S.Port, and this maybe the root cause of unstable ELRS telemetry instead.

[gismo2004]

Yes, my understanding was that this is only a problem with external Moduls. However I must admit, that I haven't tested with older firmware as initially when I got my V16 I can't think of problems in that regards and I always was running the highest rate back then. But this could also be an always present problem but not visible for some reason

[richardclli]

Yes, my understanding was that this is only a problem with external Moduls. However I must admit, that I haven't tested with older firmware as initially when I got my V16 I can't think of problems in that regards and I always was running the highest rate back then. But this could also be an always present problem but not visible for some reason

Then the RX DMA maybe a root cause then, in 2.11, the horus target, only the X12S has setup the TELEMETRY RX DMA properly. Let me see if it is possible to setup the RX DMA for external module first.

[richardclli]

Checked, v16 did not have telemetry rx dma enabled because the dma1 stream5 is occupied by the dac sound.

However, Flysky nv14/el18/pl18u did not use dac for sound, so should be ok to use telemetry dma rx.