How to tell if your flight controller MCU / CPU is dead, or has just been bricked
How to tell if your flight controller MCU / CPU is dead, or has just been bricked
Always check before soldering
Chances are you are looking at this page because you have an issue with your flight controller, hopefully you have already followed our suggestions on what to check before you solder your FC. As part of a flight controller's manufacturing process, it needs to be flashed with the firmware it will ship with and if this fails (due to a damaged MCU, it will not pass quality control checks.
Therefore, to avoid any issues with warranty claims It is always important to test your flight controller via USB before you solder anything to it. If you have an issue with the MCU before soldering it is much faster and easier to get a replacement under warranty since the manufacturer cannot claim that your soldering, or incorrect wiring has caused damage.
Quick Test - Shorted MCU, or damaged 3V regulator.
This guide is targeted towards betaflight but is applicable to just about any other flight controller that uses a MCU. The symptoms between a not working fried CPU, and a CPU that is missing firmware are almost identical, with the exception that it is impossible to flash firmware to a dead CPU. While uncommon with reputable manufacturers, CPUs may arrive from the manufacture dead, die when it first powers on do to a manufacturing defect. More common is for the flight controller to die if you wire up your board incorrectly (e.g. connect the 5v rail or battery power to a UART port). When a CPU dies, you may or may not see magic smoke.
The STM32 chipset requires 3.3v of power to operate, and so on-board dedicated voltage regulators are required to provide stable power to the CPU during operation. Since this is usually the only thing on the board requiring this voltage, you can determine if the CPU is fried using the following two methods:
Use a multimeter to measure the resistance between the 3.3v pad on your flight controller, and any ground pin on the board. The resistance should be in the mid to high kilo-ohm range if the CPU is operational. The resistance will be near zero (1-3 ohms) if the CPU is dead.
If the above step shows normal resistance, and your FC doesn't work, the issue could also be due to something being shorted on the MCU itself, (due to a voltage spike as an example). To test the MCU itself please refer to our specific guide here - Has my FC been fried or shorted? Flight controller processor (MCU) pinouts.
If you do not have a multimeter available (you should really get one), find the 3.3v voltage regulator on your flight controller, and connect it to power (i.e. USB). If the temperature remains low (about 30C), then the flight controller is probably okay. If temperatures rise quickly (between 80-150C), then the CPU is probably dead.
The video below helps show this process.
FC Does Not Turn On Properly - Powers via USB Only Or Battery Only, but not the other
This is normally caused by the 5V Schottky Barrier Diode on your flight controller. While this is slightly more technical to diagnose and find, fortunately Dronemesh has made a nice easy to follow video that goes over this process, however in summary here are the steps you need to follow:
Locate the 5V Schottky diode using your multimeter probes on the 5V rail (its usually one of 3 on a typical flight controller)
Test each input of the diode to check it is working or not.
If it does not work, it will need to be replaced, but DroneMesh offers a quick hack solution if you are in a rush to get things working. Please note that if your board is still under warranty please do not attempt to bypass, or repair the diode before contacting us first.
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