Pixhawk Mini; PWM on Pin 3 is a constant 1500 us (Resolved)

Hi - According to my oscilloscope, when the Pixhawk Mini is fully armed, there is just a constant 1500 us width pulse on channel 3 (which is for the throttle in the Pixhawk Mini airplane configuration). Since the signal is a constant 1500 us, the ESC never arms etc, which is irrelevant anyway (since the the pulse width never changes).

The servos work, everything else is as expected (except for an annoying general flakiness of the airspeed sensor calibration, which should have been taken care of by the new generation airspeed sensor with supposedly corrected 5V/3.3V issues). But I’ll try to debug the I2C bus later - does anyone know why pin 3 just has two states, which is off, and constant 1500 us pulse width?

If there weren’t any signal on the pin, then I would assume the board is bad or something, but this smells like a weird configuration issue.

(yes, most recent firmware; checked all the obvious things). Any ideas or hints would be much appreciated!


This problem has been resolved. According to 3DR tech support, for some fixed wing configurations, the PWM Throttle signal actually goes to pin 4, and not pin 3. This means that the current online documentation for the Pixhawk mini pinout is wrong. If you have problems, use a scope to figure out where your PWM throttle signal is going; it’s not guaranteed to be pin 3.

This is actually set by the airframe configuration, and somewhat unfortunately the throttle position isn’t always in the same spot.

Hi Dagar - Yup. Someone should clarify the ‘quickstart’ documentation (https://docs.px4.io/en/assembly/quick_start_pixhawk_mini.html) which currently says MAIN 3 = Throttle. One sentence such as “Outputs are set in the airframe configuration; planes typically use either 3 or 4” would do the trick.

@jan, I’ve added a PR to comment on this at top of Motors here - does this address your concern?

Someone should clarify the ‘quickstart’ documentation

Fair enough. Note that the docs are completely open so you can be that someone :slight_smile: There is a little icon that looks like a pen/notepad at the top of every page on the docs to take you to the page on github where you can make a quick change.

Yes, thanks, will do - I’ve been keeping track of potential edits and I’ll start adding those on GitHub.

Much appreciated.

I have changed this section (again) as shown here. Essentially I have removed all the “typical configuration” examples (because they aren’t) and made it clear that the airframe reference is the canonical place to find out connections and motor direction diagrams.

As part of this change I also remove several other “standard configuration” and “motor map” docs. The airframes reference does the job of all these docs.

Yes - perfect. That’s the real solution!

Hi - well, for me it is of little help. I’m new to using autopilots so a lot of this terminology is foreign to me.
I’m using Pixhawk Mini. My airframe is a Single Copter, Frame type: Plus.

Trying to make the Pixhawk work is taking me longer than it took to build the copter. Instructions for Single Copter say throttle at CH3. I connected everything properly but the motor would not run for the reason stated at the beginning of this thread. Took me a week to diagnose that the signal is incorrect. I guessed that perhaps throttle may be on a different channel. I tried all channels but there is no throttle anywhere. I do need CH1, 2, and 4 for flaps. They work properly when attached to those channels (MAIN). Being new to autopilots I really don’t know what else to do, how to change the configuration, how to reassign output channels or make the assigned channels work.
Any advice for a novice like myself?

Correction: throttle output from Pixhawk Mini in my airframe is supposed to be on channel 5. Channel 3 is my Rx channel for the throttle.
So I’m using channels 1-4 for flaps and ch5 for motor and have this buzzing noise in ch5.

Hi andrew

I know that it is tempting to piggy back off closed issues, but please don’t. If you think issues are closely related, then link to them.

You won’t find a throttle on a copter because there is no throttle output for copter - there is for fixed wing because the thust motor is the main output. The outputs you get will depend on the airframe that you have selected - one of these: https://docs.px4.io/en/airframes/airframe_reference.html#copter