Pixhawk 4 Mini has intermitent unstable PWM output on CAP 1,2,3

ArduPlane Plane 4.0
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I am currently building my second QuadPlane, this time using tilt on the front 2 motors. I had a very successful build based on ranger 2000 but ended up crashing due to a very odd situation that caused instability in the VTOL. In both I decided to use the HolyBro Pixhawk 4 Mini and am running the stable release of Plane 4.0.5. In the first design that was based on the Ranger 2000 I was using DSHOT150 for the 4 motors and in the Pix4 Mini DSHOT is only capable in a few groups of GPIO. I used the primary output of 4, 5, and then CAP9 and CAP10. DSHOT on the CAP pins seem to work fine. But in my second build because it is a whole lot more complicated using Tilt I decided to assign the servo outputs as follows -
(1-8 Primary outputs)
1 - Aileron 1
2 - Aileron 2
3 - Rudder 1 (I have a twin tail boom)
4 - Rudder 2
5 - Motor -1 (DSHOT150)
6 - Motor -2 (DSHOT150)
7 - Tilt Servo -1
8 - Tilt Servo -2
(CAP1 - CAP3 PWM and DSHOT capable outputs)
9 - (CAP1) - Motor -3 (DSHOT150)
10 - (CAP2) - Motor -4 (DSHOT150)
11 - (CAP3) - Elevator

CAP4 - Standard GPIO not PWM capable. (Used for Camera Trigger)

According to all the documentation that I can see this should work. I have discovered that for some reason I could not get the elevator servo to work. It just frantically starts oscillating back and forth and no input via SBUS on the elevator channel drives the servo reliably. I think some of the signal is making it but it is very unstable. I ended up moving the Elevator to one of the ports 1-8 and it works very stable and reliable on these primary ports. I also moved it to CAP1 and CAP2 to see if it was just a bad port but all the CAP ports act the same with PWM driving a servo. I ran some tests to the motors and it looks like DSHOT is reliable but I don’t know for sure. I now am starting to wonder if maybe there is a bug that caused the crash on the first Quadplane.

Hopefully someone who uses a Pix4 Mini can see if this can be replicated in their board. It is possible I have a bad board but I have not been able to pull my other Pix4 from the wrecked Quad yet.

I did update to the latest nightly build just to see if anything changed but it acts the same.

Here is a link to my set params -
https://1drv.ms/u/s!AnKjVxm8UzokidA03kmcs2k2NOFnmQ

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UPDATE – I just discovered that while DSHOT or OneShot of any kind is enabled on the board basic PWM out on all CAP interfaces are unreliable and unusable. I ended up moving motor PWM back to default and everything is working with the pin layout I have. I have been using ArduCopter for many years and have built countless numbers of Quads all running Ardupilot and to be honest I have always used PWM to the motors and have never had issues of any kind. The first build of a Quadplane came after about a year playing with Betaflight and I decided to try DSHOT with Ardupilot. I think it was a mistake. I am definitely going to stay away from anything other than Basic PWM and deal with ESC calibration for a while.

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Hi there Tim!

I’m just posting because I am also working on a quad plane. But I cannot seem to get the CAP pins to output.

Everything else outputs fine,
the SERVOn_FUNCTIONs are all set. I have set it to an aileron. But I am not getting anything.

Is there something else I am missing? Do I need to place a jumper cable to enable the CAP pins to be output? I can’t find info on it anywhere…

Cheers for any help!

Charlie.