This is a discussion re the bad Accel, Gyro and Baro values that we're seeing with ArduCopter-3.1.  The increase in the SPI bus speed from 500khz to 8Mhz has exposed a hardware problem on some boards.  That hardware problem is that the 3.3V regulator has been blown so all sensors are running at 5V instead of the intended 3.3V.


How have these regulators been burnt out?

  • Attaching a radio receiver or MinimOSD to the APM while the APM is only powered through the USB (see video below)

  • Some clone boards seem to come from the factory with blown regulators.  3DR boards might also come with blown regulators although they do a specific check of the regulator as part of the regular QA process.
  • It is not (as far as we know) actually caused by the AC3.1 software itself, it just exposes the problem.  You could prove this to yourself by checking the 3.3V regulator (see video above) before and after the upgrade.


How can we fix the regulator?

Option #1:  If it's a new board (so that it's less likely you burned it out yourself) you could report the problem to the retailer that sold you the board and ask for an replacement.  If it's 3DR it's called an "RMA".

Option #2: if you're handy with a soldering iron you can replace the regulator yourself.  On the APM2.5.2 (and higher) boards it's not that difficult.  On the APM2.5 it's far more difficult.

For APM2.5.2 : TPS79133DBVR

For APM 2.5: MIC5219-3.3YML TR

How can I stop it from happening again?

Do not connect any devices such as a radio receiver, MinimOSD, GPS, etc while the APM is powered especially while powered only through the USB cable.

Attaching a 100uF capacitor across any of the APM's radio input's 5V and GND pins will stop the regulator from being blown by plugging in a receiver.  video here!

There are very few reports of regulators being blown twice and no reports of it ever failing in flight.


Below are some graphs of the types of values that we are seeing on these boards.

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Sorry i should have said that it's important for manufacturers to deliver good quality boards and for sure they are doing their best to do it.

3DR has strict quality checks so they are a premium source.

Every manufacturer can have a small quantity of faulty boards that were not detected during the quality assurance checks.

Anyway let's come back to the problem. The important point is to find the root cause of the errors we can see on your boards.

I did try the test firmware on two boards, a 2.5.1 and a 2.5.2. The 2.5.1 was connected through USB to my computer, the 2.5.2 is the one installed on my 3DR Y6 with a GPS + Compass module + power module + 3DR radio + Jeti receiver connected in PPM.

Those two boards did not show any errors at all neither fallback to 512 KHz in the command line. ErrCnt did stay at 0.

I did try to arm, move the transmitter sticks, move the frame. I had a 3D fix so the GPS was communicating with the APM at the same time.

This did not produce any SPI errors.

This is confirming my first low level SPI tests, no errors at all on the SPI bus for my APM boards. I think that if we had an electrical design problem on the APM, i would have seen some errors appearing at least at 8 MHz.

I tried to put my fingers everywhere on the board to modify tracks impedances, and this did not produce SPI errors neither.

This narrow down a bit the problem : it could be a hardware problem or powering problem on your board, or a software problem caused by something specific in your setup.

But i don't think that it is a global problem. Let's wait for some more reports.

And thanks a lot for your detailed reports. This is very useful.


So, here comes the first surprise: My 3.3V is not 3.3V on my faulty board. The green channel is the 5V taken of the input header. The yellow line should be the 3V3 taken at the solder pad C22/23 & U3. All together at the moment I plugin my USB port. The yellow line goes all the way up to 5V in this case!

I don't think its a general hardware or software problem, otherwise we would see that failure with allot more ppl trying 3.1.

Whatever this is causing, Bruce and my board have a similar out of spec hardware problem. And I am pretty sure its on the board. Because I swap boards on my quad and all works fine with 8MHz. I put both boards stripped on the same USB port as power supply and one works and the other won't. So it can not be any hardware around the APM board.

See my last post with the scope shot. My 3.3V is actually 5V. Maybe Bruce can take a multimeter and just verify on his board the 3.3V Voltage. If his has the same issue with the 3.3V being somehow shorted or the regulator shot and equals 5V board voltage then maybe that's also the reason why his goes more crazy when he plugs in the Battery and his board voltage jumps from 4.6V USB port supply to 5.0V from the BEC??

Meaning his should be 3.3V go from actual 4.6V to 5V assuming his 3.3V regulator is shot or the 5V and 3.3V lines are somehow connected/shortened.

Bruce, can you measure that? 


I'll be putting a meter to the board this weekend and have a 3.3v reg to try if needed. I'll follow up Saturday afternoon.

I am afraid if I unsolder that 3.3V regulator now the 5V will not disappear. I am thinking more the 5V and 3.3V tracks are somewhere connected by some unclean soldering or bad board.

Hahaha should I wait till you make the hardware fix beta tester??

By the way, do you stock those TPS79133 at home? Or how do you get from tonight a regulator on Saturday?

I have some 3.3v regs leftover from building a 5.8GHz video transmitter. I'll "wire it in" to test. If that works, I'll get the spec'd part. I'll probably do some touch work on the SMDs with a fine tip. It'll be a blast! Haha.

Oh I see, I was just wondering hahaha I think the regulator should actually be solderable with a conventional tool. I hate SMD like the pest :)

Much more important I need to solder some plugs on some new batteries and get those charged for the weekend so my autotuned quad can finally go test flying. The APM board has to wait or I just knock that regulator off while I solder my plugs.

That's a nice news !

I think that we should do a small hardware modification on the APM boards.

It would be important to measure the 3V3 power line so that we can check the 3V3 health in the pre-arm checks, and report it in the logs.

It would be only a new track from the 3V3 net to a free ADC converter input on the AVR2560.

At the same time routing this 3V3 net to a PIN for easy voltage measurement would be nice as well.

In the mean time for actual boards, it's possible to do that with a custom cable connected one side to the i2c DF13 connector, the other side to an ADC input pin.

I think that the i2c DF13 connector is the only one with 3V3 on it.


I think if Bruce reports similar readings on his 3.3V power supply then we can assume that this is the cause for our problems. And to be sure if we can manage to get it back to 3.3V (if its not a short circuit between 5V and 3.3V in the board somewhere) and the failures disappear then this would be 100% proof.


Sure, but the MPU6000 and or MS5611 could have been partially damaged by the over voltage and exhibit out of specifications noise values, or even have digital damages. So be careful to not trust this board too much if you can repair it.

I think if Bruce reports similar readings on his 3.3V power supply then we can assume that this is the cause for our problems. And to be sure if we can manage to get it back to 3.3V (if its not a short circuit between 5V and 3.3V in the board somewhere) and the failures disappear then this would be 100% proof.

I should have the readings this evening.  

Oliver, your concerns are right and I would agree that 5V on the 3.3V supply does not do any good but then on the other hand me and Bruce have both been flying with this situation.

Mine is flying since a couple months with this condition since newly put in (I assume it had that issue from the beginning since it wouldn't boot without my special power start-up sequence). But all that time with the 3.0.1 release I never noticed any weird flight behavior that I would have blamed on the APM messing up.

Weird flight maneuvers only came from me being on the sticks :)


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