Add PPM support

[ColinNiu]

Add PPM support.

Acceptable PPM channel value is 750~ 2250us but will be mapped to 1000~2000us with thresholding applied (750 ~1000us=>1000, 2000 ~2250us=>2000).

[kilrah]

The 1000-2000 clamp is a bit short, will cause clipping with common systems.
Common flight controllers actually need the whole 1000-2000 range by default, an it's common to set a channel beyond that for error detection. OpenTX/er9x radios put out 988-2012 by default, more with extended limits enabled.
Would recommend supporting something like 850-2150 unless there's something internal preventing it.

[ColinNiu]

Hi @kilrah , yes, already tried, but method rc_data_update_channel in rc/rc_data.c will crop the channel value into range RC_CHANNEL_MIN_VALUE ~ RC_CHANNEL_MAX_VALUE.

About using special ppm value to do error detection, I think it happens on the Rx end (as a failsafe indicator), so it may not be a big problem (for RC->Raven Tx)?

That's just my opinion, how do you think?

[ColinNiu]

or... maybe we can limit valid ppm channel values to a more reasonable range, beyond which we say it's in the failsafe state? Then what would it be?

[kilrah]

Best would be to figure out what scaling is applied between the CRSF protocol and PWM values, and allow as PPM in whatever's possible within RC_CHANNEL_MIN_VALUE and RC_CHANNEL_MAX_VALUE.

I doubt CRSF clips at 1000-2000 "equivalent" since IRC got so much flak before them for not having a bigger range on the EzUHF.

[ColinNiu]

CRSF uses 172 ~ 1811, and I found here, SBUS uses the same range (without endpoint change).
If mapped to PWM, CRSF value 0 would be PWM 895, and CRSF max possible value 2047 (unsigned 11bit) would be 2144 in PWM.
CRSF |0--[172------1811]--2047| => PWM |895--[1000------2000]--2144|

To support the extra range, the RC part of Raven system may need to be revised.

[fiam]

Fixes #13

[SJNASJNA]

Thank you for your attention to my question

[fiam]

Looks perfect except for those two minor cosmetic changes. Thanks!

[fiam]

Can you add explicit braces here?

[ColinNiu]

updated, braces added.

    if (input_ppm->pulseCountInQueue == 0)
    {
        return false;
    }