GoKawiilI was very excited to get the latest version of my PCB from the manufacturer. This new version had a magnetometer on it, so that I could more accurately track the yaw angle of my drone.
The first thing I did was start to program it over USB. I added code for initializing and reading the magnetometer, and I got back some data that seemed correct for the X and Y axes, but the Z axis would read a constant value. I did some light debugging, and learned, among other things, that the Z axis has slightly different internal circuity, but eventually I decided to put a pin in it and focus on other things.
Eventually I had to plug my drone into the battery that would power it in flight, and all of a sudden the magnetometer stopped working completely. I pivoted back to it in an effort to debug it, but nothing worked. I tried to reset the board, I tried to add the code to automatically reset and reinitialize the magnetometer if I hadn’t heard from it for a while, but no matter what I tried it would stay dead when the battery was plugged in.
When I would unplug the battery and plug the USB back in, the magnetometer would recover and work just like it had before. So right away I have to start thinking about what the 3.3V line which powers the magnetometer looks like under USB vs battery.
A Qwiic connector gives me easy access to the 3.3V bus. Here you can see it’s reading a pretty steady 3.3V
Under battery it similarly reads about 3.3V, and the reading on the multimeter is steady
So does that mean it’s not an issue with the 3.3V bus? Well, not exactly. The multimeter is going to give you a bulk reading, but it’s possible that the line is noisy even though the average reading is 3.3V. The way we get to 3.3V from the initial 5V/8V is through a SY8113IADC voltage regulator. This is a modern switching regulator which is very efficient, but it achieves that efficiency by, you guessed it, rapidly switching the input supply on and off to effectively drop it down to the desired voltage (as opposed to the older kind of regulator which would just dissipate the energy as heat).
This switching causes ripples in the voltage line, and if we hook up an oscilloscope to the line we should be able to see these ripples in detail.
This means the 3.3V line ranges from 3.14V to 3.7V. The BMM150 magnetometer is rated for up to 3.6V.
This means the 3.3V line ranges from 2.74V to 4.34V! This must be why the magnetometer simply won’t work on battery power
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