Black Solo Cube: Two questions to experts on powering

(reposted from here thx to a comment by Matt_C)

I would need help of an expert, familiar with the Black Cube.

I’m in the course of building/designing a Cube carrier board, and have a scheme which I believe is fully working. However, I observe two points which I just can’t figure out. I would much appreciate input as regards these points.

FIRST SOME BACKGROUND (a bit lengthy):

I have three Cubes for my tests, which I will call Cube0, Cube2, and Cube3:

picture1 in next post since as a new user I’m not allowed to post more than one

Cube0 and Cube2 appear to be identical hardware-wise, and their visible PCB matches FMUv3_REV_D. Cube3 however is a different hardware revision. It neither matches FMUv3_REV_D, FMUv3_REV_G, nor FMUv3_REV_1B. From the serial number one might conclude it’s an earlier revision than Cube0 and Cube2.

In order to establish a comparable test situation I have flashed all three cubes first with ArduPlane to wipe out the parameters, then flashed them with ArduCopter 3.6.7 (or BetaCopter but this doesn’t matter here), and set “my default” parameters by loading one and the same .param file into MissionPlanner and writing the params to the cubes. I use the Nuttx v3 firmware versions (ArduCopter-v3.px4).

I have two test environments, first my home-made cube test carrier board and second a Solo motherboard with the IMX6 removed and a battery plug for powering:

picture2 in 2nd-next post since as a new user I’m not allowed to post more than one

On the Cube test carrier board I can power the Cube from a 5V3 powerbrick source or from the USB plug. I also have connected the PC to the Serial5 debug console to see what’s going on. I can change some things by use of some pin headers.

I have waded through what I believe are all publicly available electrical schemes for the Cube (FMU), PSM, and CB, as well as through the relevant pieces of code in AP and the bootloader.

The result of my research into the electrical schemes and codes is that only the connections VDD-5V-IN-PROT and VBUS on the cube’s 80-pin header (and of course GND) are relevant for the following. All other ‘power-related’ pins, VDD-SERVO, VDD-SERVO-IN, IO-VDD-3V3, VDD-3V3-SPEKT-EN, VDD-5V-PERIPH-EN, VDD-BRICK-VALID, VDD-BACKUP-VALID, VBUS-VALID, VDD-5V-PERIPH-OC, and VDD-5V-HIPOWER-OC, are irrelevant for the following (for different reasons, but irrelevant).

The VDD-5V-IN-PROT is the Cube’s main 5V power supply. VBUS is connected to the Cube’s internal USB plug, protected by a Z-diode, and connected to the PA9 VBUS pin of the STM32F437. This is the part is encircled by the blue line in the scheme drawing below.

The PA9 VBUS pin is used in the ArduPilot code, but also in the bootloader: It’s state is read and if high the bootloader jumps directly to the app, and if low the bootloader waits 5 secs before it jumps to the app:


Four test conditions will be considered:

A) Cube test carrier with Scheme2

B) Cube test carrier with Scheme1

C) Solo motherboard

D) Cube test carrier with Scheme3

The three schemes are these:

picture3 in 3rd-next post since as a new user I’m not allowed to post more than one


The following deals with what happens after powering up the cube/system. The chain of events can be broadly distinguished into two phases:

  • bootloader delay phase: The bootloader waits for 5 secs for some activity on the USB before it jumps to the app. The Cube’s orange LEDs blink quickly. If a USB is connected the USB is enumerated and de-enumerated (the PC’s soundspeaker pings twice).
  • AP initialization: The app starts. The Cube’s LEDs flicker and eventially go off. Debug messages are send to Serial5. The beeper makes noises. If a USB is connected it is enumerated by the PC. It is possible to connect with MissionPlanner to the Cube. And so on and forth.

Observations for test condition A:

Everything works fine. All three cubes undergo the bootloader delay phase, start up and behave just fine.

Observations for test condition B:

Here the behavior is very different for Cube0/Cube2 and Cube3.

For Cube0/Cube2 everything works fine. Both cubes undergo the bootloader delay phase, start up and behave just fine, irrespective on how they are powered, through the 5V3 powerbrick or the USB.

However, Cube3 undergoes the bootloader delay phase, starts up and behaves just fine ONLY if powered by the USB! If powered from the 5V3 powerbrick it undergoes the bootloader delay phase, but then gets stuck and does NOT jump to the AP initialization! The orange LEDs are then on constantly, and the Cube appears to be just “dead”. There is absolutely NO output on the debug console Serial5, so the AP initialization phase wasn’t even started.

Note that the electric connections in Scheme2 are supposed to mimic the original PSM, which instead of the diode OR-ing has a power switch, but otherwise I can’t see any electrically relevant difference.

Note also that the problem exists for only Cube3, which appears to be an earlier hardware revision than Cube0&Cube2.

Observations for test condition C:

Everythings works fine. All three cubes start up and behave just fine.

However: The cubes do NOT undergo the bootloader delay phase! Instead they are only briefly in the bootloader and then jump directly to the app i.e. AP initialization.

Observations for test condition D:

In the Scheme3 used here the VBUS line and therefore the voltage at PA9 is forced to low by connecting it to ground (accordingly here the system must not be powered by USB but just from the powerbrick).

The behavior is exactly as that for test condition B, when powered from the 5V3 powerbrick. Cube0/Cube2 just work fine. Cube3 undergoes the bootloader delay phase, but then gets stuck and does not jump to the app.


There are two questions from the above, first why Cube3 behaves differently to Cube0/Cube2, and second why the bootloader delay phase is not skipped except when using the Solo motherboard.

Why does Cube3 behave differently to Cube0/Cube2?

The only difference I can see between ‘powered by USB’ and ‘powered by the powerbrick’ is the voltage on PA9. So I think the question can also be stated as: Why does Cube3 not start the AP app when PA9 is low, but gets stuck and fails to start?

Note that the difference of Scheme1 as compared to Scheme2 is that it forces PA9 to high in all conditions, which seems to confirm the conclusion that it’s the voltage level on PA9 which is relevant.

It is strange though that Cube3 behaves just fine on the Solo motherboard. As mentioned, Scheme1 used in test condition B is supposed to mimic the PSM used on the Solo motherboard. So the question is actually twofold: Why doesn’t Scheme1 work for Cube3 and what is so magic about the Solo motherboard/PSM that it here works?

Why is the bootloader delay phase not skipped except when using the Solo motherboard?

This observation, that the bootloader delay phase is skipped with the Solo motherboard, really caught me by surprise. I’ve not observed that for all my other boards (pixracer, AUAVX2, Pixhawk1). They all undergo the bootloader delay phase, and this so even though their electrical schemes are identical in that here too PA9 is connected to VBUS. Test condition D appears top confirm this, i.e. that the bootloader delay phase is undergone independent on the voltage level on PA9.

This however is appears to be in contradiction with the bootloader code. And it raises the question what is so magic about the Solo motherboard/PSM that here the bootloader delay phase is skipped?

If you made it to here you really deserve my deepest thanks :slight_smile:

Cheers, Olli


All 3 of them are solo cube’s the ones for sale look more like this. I edited your post to reflect this.

As mentioned, they are cubes extracted from 3DROBOTICS Solo drones.

They have a different bootloader to assist in how it talks to the companion computer.

They are 2.0 cubes.

The 2.1 was designed for standalone use and has a normal bootloader.

Depending on when in the release cycle of Solo each was programmed, there were different behaviours.

Updating the bootloader to the current Ardupilot bootloader May help

Looks like cube 3 is a very early Cube with the 4th corner on the PCB.

I wouldn’t use that one for anything, it’s as good as junk.

ah, it seems I can post/edit now, still just one pic per post, so here the next pic



yes, these are cubes from/for Solo. They are very attractive from the price point, and there are still many out there :slight_smile:

you will understand that I don’t consider myself responsible for the cube naming confusion … it was called a black cube for a long time :wink:

ah, didn’t know this or considered this possibility. Thx for clarifying!

LOL :smiley:

btw, just to report, in case anyone cares, I made the observation that when I replace the USB S-diode in the above schemes by an ideal diode then Cube0&Cube2 and also Cube3 behave like on the Solo motherboard, i.e. no problem with Cube3, and the cubes skip the bootloader delay phase when powered from the powerbrick. This at least clarifies the “magic” of the motherboard/PSM. One maybe can understand that the bootloader delay phase isn’t skipped, but why a S diode would make Cube3 get stuck after the bootloader is still strange. Anyway, since I planed to go with an ideal diode anyhow, I guess I leave that to the unresolved mysteries chapter.

Many thx for the responses :slight_smile:

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Have you considered using the PSM as per the original design?

It manages all of this for you with the ideal diode etc.

You can get the pre made PSM module from Hex (via distributors etc)

Or you can make your own.

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I’m afraid I have to say I did not, and this on purpose. The PSM is certainly a nice and proven piece of hardware, but I think it’s too sophisticated for what I think I need and want (and I also think that it makes all existing cube carriers expensive, from my perspective). Pl note that I’m all DIY&hobby, and thus prefer to go with a minimal BOM, when it does the job.

Even though it’s a bit OT, I’d like to also admit that the main reason I have looked into this was a significant dissatisfaction with the available cube carriers. I mean, again, they certainly are nice and proven pieces of hardware, but some aspects, mostly of the practical kind, I just didn’t liked. Doing my own cube carrier, albeit admittedly very basic and simple when compared to e.g. yours, also allows me to do “nonsense” I couldn’t do otherwise.

I hope you find that acceptable and not inappropriate. I’m just trying to have fun with the ArduPilot stuff. :slight_smile:


No worries! I absolutely support as many carrier boards as possible, that’s why I designed it to be modular :slight_smile:

Keep up the good work!

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great :sunny:

it will probably be the simplest and cheapest and easiest to build one

Please just refer people to the registration form if you put out your drawing as a reference please :slight_smile:

I understand what you’re saying and am happy to do so but I’m not sure what exactly you’d like me see to do.
Should it be a link to here,, or to here, If a link, where should it be placed, in the Eagle design file? The github repo? With what “key words”, etc…

Ideally, could you give me an example or suggestion I could use?

“The Cube footprint and hole pattern are protected under a Patent, ProfiCNC offers the patent for your free, non exclusive, perpetual use on carrier board design, to gain this license, please follow the instructions at, Thanks”

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many thx for the suggestion.
I guess I won’t put it up in that wording however, since honestly I don’t think that this is a valid legal claim (unless you or anyone proves me wrong) and I’m the sort of guy who cares about the statements she/he is making in public material, but I certainly fully support your intention. I’ll find a proper wording which will certainly include the link. Since I have seen that sentence also on the schematics, I guess you’re ok with me putting it in the schematics, right.
thx for your kind help,

btw, just in case anyone should happen to be interested, this is there this is currently standing at, and heading too:

to conclude the thread I’d like to mention that I finally have released the UC4H Cube Carrier board mentioned and shown in the above (

It actually works and flies, a demonstration you can find in this video at 6:30:

I also did two videos discussing the carrier in some more detail:

Thanks for bringing The Cube to the community !!!

Have fun, Olli


Thank you Olli!