Recently bought a QAV250 kit with a PixHawk 4 Mini as my first step into quad and autopilot. Expected to recycle an older Spektrum DX5e with an AR6110e reciever, at least until I learned enough to make a different RC purchase. As I looked to connect the AR6110e to the PX4, I see the error of my ways.
The AR6110e RCVR appears to have only PWM outputs (THRO, AILE, ELEV, RUDD, etc.) and the PicHawk 4 normally wants a single channel, (PPM?), from the RC.
I suspect my only choice to use AR6110e RCVR is to buy a PWM Encoder board, such as the “Hobbypower PPM Encoder V2.0”.
Question here is: What are the four CAP pins on the PixHawk 4 Mini? The docs https://docs.px4.io/en/flight_controller/pixhawk4_mini.html#interfaces
say they are “PWM Input Capture”. It would seem I could jumper four PWM outputs of the AR6110e to the four CAP “S” pins and get four channel control. (+: 5V and GNDs)
However, running through the QGroundControl V3.4.4 Vehicle Setups, I saw no reference to such a configuration.
About 18 months late in replying! I was wondering the same thing and came across this question.
I did a little research - so I’ll summarize what I’ve found here in case anyone else ends up here too.
The CAP pins are included in the FMUv5 and later PX4 designs. So I looked at the pinout for the FMUv5, i.e. the design behind the Pixhawk 4, that’s linked to from the PX4 reference design page.
The pinout is a Google Docs spreadsheet - if you go to the All Pinouts sheet you’ll see CAP1 to CAP3 defined like so:
Pin
STM32F7 signal
FMU usage
Functional description
5
TIM2_CH1_IN
FMU_CAP1
Digital INPUT: Typicaly used for PWM input or edge capture
3
TIM2_CH2_IN
FMU_CAP2
Digital INPUT: Typicaly used for PWM input or edge capture
11
TIM2_CH4_IN
FMU_CAP3
Digital INPUT: Typicaly used for PWM input or edge capture
If you look further, you’ll see that the descriptions of CAP1 to CAP3 look very similar to FMU_CH1 to FMU_CH8:
Pin
STM32F7 signal
FMU usage
Functional description
10
TIM1_CH3
FMU_CH2
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
13
TIM4_CH2
FMU_CH5
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
14
TIM4_CH3
FMU_CH6
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
9
TIM1_CH1
FMU_CH4
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
11
TIM1_CH2
FMU_CH3
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
14
TIM1_CH4
FMU_CH1
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
6
TIM12_CH1
FMU_CH7
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
9
TIM12_CH2
FMU_CH8
Digital I/O: Output: PWM, GPIO, Input: PWM Input or capture
FMU_CH1 to FMU_CH8 have been part of PX4 designs since the start.
If you then switch to the FunctionTakeOff sheet you’ll see the TIM groups mentioned above described as follows:
STM32F7 signal
Usage
TIM1[4321],T4[23]
PWM to servos (6+)
TIM12[1 or 2]
Piezo / buzzer driver out
TIM2[124]
PWM input capture / timer capture
So it looks like FMU_CH1 to FMU_CH8 have a standard predefined output purpose but if unused can be repurposed as PWM/GPIO output pins or as PWM/capture input pins.
And the CAP1 to CAP3 pins appear to be a newer addition in FMUv5 and later designs which have no predefined purpose and can, for whatever reason, only be used as input pins.
If anyone has a good reference on how to use PWM/capture input pins, I’d be interested to see that. Are they only useable if writing your own autopilot C code for additional special-purpose functionality? Or can they be automatically routed for particular purposes in QGroundControl / Mission Planner?
