Why FRSky CPPM signal is so disappointing. [UPDATED]

[ Update at the end ]

You bought a brand new FRSky with 8 channel and a promise of a helpful PPM Sum output.

Be warned that you cannot use this with 8 channels. Only 6 channels could be used with some risks (only 5 for real safety). [Note #1 at the end] It's really disappointing.

Yes. FRSky's CPPM signal has a BIG problem: It has a period of only 18 milliseconds. What does that means? Here we go:

A PPM Sum signal usually has a period of 20ms. As each channel uses up to 2ms so you need 16ms to fully accommodate the data from 8 channels (8 * 2 = 16ms).

Now comes an important element: the Sync Pulse. It needs to be wider than all other ones  to indicate the start of a new PPM train. Any 8 channels system based on a 20ms has room for a 4ms pulse (16 + 4 = 20ms). Even with all the channels at 100% a system like that still gives you a perfect sync pulse.


That's the BIG problem with FRSKy CPPM. If you start using some switches and knobs you are pretty much risking to lose the sync on your autopilot. Because the sync pulse is squeezed until having the same size of any channel.


I hope they can fix that with a firmware update sooner, because I believe it's not acceptable.

Until that, you cannot use it on your autopilot without risks. [Note #1 below]

[ NOTE #1 ]

This is far away from the ideal, but there is a cheat to eliminate the risks when using just 6 channels by suppressing CH7 and CH8 from CPPM.

At least on a ER9X or ERSky9X radios there is this way:
By changing your model's setup to use a Proto PPM 6CH it will not output CH7 and CH8.
(The frame space (300uSec) though is just ignored. I did not see any changes.)
I've verified on Oscope. It works!!! The CPPM was outputed from RX without CH7 and CH8.
Is still a shame and disappointing using just 6 from 8 channels. But that seems to eliminate the risk.

[ NOTE #2 ]

Jani, from jDrones took this issue to FRSky's GM/CEO and got a response from them. They are now baking a solution to release a new firmware. Probably the new CPPM frame period would be 27ms (the next available number dictated by the hardware's clock division).


Yesterday (28th September)  Jani came up with a beta firmware from FRSky. It does output CPPM frames at 27ms. So far we have some positive feedback from DIYDrones dev team's tests. I'll not update this post anymore. A new post instead will show some results followed by a mini updating tutorial for those CPPM capable receivers. Stay tuned! ;)


This post shows how to fix it.


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Comment by Sandro Benigno on October 7, 2012 at 3:00am

@John: Those FRSky's TF Series are full compatible with FASST 2.4G Systems and they have a PPM framing at 20ms. So, you're free of this problem.

Comment by Sandro Benigno on October 7, 2012 at 5:32am

@Oliver: The advantage of using CPPM on APM is due to use just one cable for all the 8 channels and also on having access to the RSSI output signal (which is a pwm pulse running at 9.2 microseconds period i.e. around 109kHz).

D8R-XP firmware has pwm RSSI output from factory. D8R-SP doesn't have it. However they share the same FCC ID: "XYFD8TD8R". So, by updating any SP with a XP firmware it would remap the SP pins to XP's layout allowing you to enable RSSI or CPPM at the same way.

PWM at 18ms has more resolution in terms of time (more updates per second) and also has no problem of space between the channels so you can safely increase the wide of channels (more resolution in terms of steps between min and max values from sticks, knobs, etc).

The big difference is when you compare CPPM periods:

CPPM at 27ms has more space between the data from channels(less updates per second). However, it allows you to safely extends the wide of the pulses of all channels (more resolution in terms of steps from the sticks, knobs, etc).

CPPM at 18ms has less space between the data from channels i.e. more resolution in terms of time (more updates per second). However, there is no space for full 8 channels neither for expanding some of them safely.

Comment by Oliver Sumpton on October 9, 2012 at 10:15pm

@Sandro: Thank you for the detailed info. I really appreciate it.

Do you know if it will be possible to flash a D8R-II+ with the upcoming XP firmware? Currently Hobbyking only has the D8R-II combo with either the D8R-II+ or V8RF-II.

Also, should the RSSI output of the XP firmware be an acceptable signal for use with minimOSC-extras as per the following instructions http://code.google.com/p/minimosd-extra/wiki/RSSI ?

Thanks again!

Comment by Jani Hirvinen on October 10, 2012 at 5:29am

Status update to all: After several days we gave green light for FrSky for this new custom 27ms software and I am happy to let you all know that software is now publicly available for download.

So if you need to have 27ms and 8 channel output from your PPM capable receivers. Point your browser to FrSky pages and get latest software upload.

For people who did download test files from jDrones website, old files will be now removed and there are only one official software.

FrSky download page for official software is: http://www.frsky-rc.com/download.asp?id=23

Files are under names of:

CPPM 27ms for D8R-XP

CPPM 27ms for D4R-II

Comment by Sandro Benigno on October 10, 2012 at 5:37am

That's great! =)
Time to upload the tutorial in few mins.

Comment by Sandro Benigno on October 10, 2012 at 6:02am

@Oliver: No, unfortunately the D8R-II isn't on their list and I really don't know about its hardware.

About the RSSI: It's not compatible as it is. Like I said the RSSI signal from those RX is a PWM at 9.2us period. So, you need to add a "Low Pass Filter" before feeding an APM's analog port. I'm running tests right now with some different filters ("faster but rippling" and also "slower but cleanest"). It will become a tutorial with lots of infos, waveforms, graphics, etc.

Comment by Rob_Lefebvre on October 10, 2012 at 7:05am

I asked them about the older receivers, and they basically said that maybe you could put the firmware for the D8R-XP into a D8R-II, but they don't recommend it and don't know what will happen.

I guess I'm going to have to buy some new receivers, because I only have 1 D4R-II, and no XP's.  I've got an SP, a D8R-II and a D8R and a D4FR.

Comment by Oliver Sumpton on October 10, 2012 at 11:38am

@Jani: awesome!
**anyone looking for the now posted XP firmware, check Jani's comment (4 up)!

@Sandro: you rock. I look forward to the tutorial!

side note: what about running the RSSI signal first through an Arduino via pulseIn() and then.. ok nm same problem, PWM output right? (I use a 5v Arduino Pro Mini on my hexa for camera control and lighting).

@R_Lefebvre: do you know if the hardware of the XP and II are at all different?

Comment by Rob_Lefebvre on October 10, 2012 at 12:26pm

No, I have no idea, sorry.

Would they have changed the model number if the hardware was the same?  Just different firmware?  Not sure.

Comment by Sandro Benigno on October 10, 2012 at 1:03pm

@Oliver: The one-pole LPF is just a capacitor and a resistor. We could use an MCU to read pulses and report through i2c or SPI... but it's kinda too much... I guess.

@Robert: The D8R-SP and D8R-XP has even the same FCC ID: "XYFD8TD8R". So, yes... looks like it's just a different firmware and the sticker showing the new pin mapping. But I can't tell the same for the D8R-II. I don't have one here.


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