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Cortex M4 draining a lot of current.

Question asked by P_ldaru.Andres on Dec 24, 2016

I'm doing a motor controller project, where I was using  STM32F446RET6 to learn how to work with an ARM processor. I designed the board myself.

 

Without the microcontroller soldered the 3.3V LDO was working fine and the power led was on (current consumption < 10 mA). When I soldered on the microcontroller, the capacitors and the oscillator, the microcontroller starts draining current. I have put the current limit on the power supply I used up to 150 mA max and the microcontroller drains all of it, while the voltage drops to around 2.9 V after the LDO (it should be 3.3V but because of the current limiter setting on the power supply it went lower).

 

According to the data sheet here on pages 83-85, it shouldn't use more than 100 mA. I measured the resistance between all the MCU pins and ground and there didn't seem to be an anomaly, also checked for bridges between the pins under a microscope. Using an oscilloscope I measured that NRST was high and the input voltage wasn't oscillating. I couldn't detect a clock on the OSC pins, it seemed more like a 10 mV random noise

 

Also when I compared my schematic, attached below, to the hardware development guide here, it seems that my design should work.

 

So I was wondering if maybe someone can point out that there is something wrong/missing on my scheme or can confirm that maybe the 150 mA limit is too low and I should just try increasing the limit? Or maybe you can hint me as to what might be the problem. I was afraid of increasing the current limit because it seems a lot already and I thought there must be a problem and I might fry something.

 

In the schematic you can ignore the motor driver and the USB-UART chip on the second page, as these haven't been soldered. I have only soldered the LDO, the power LED, the three buttons, the microcontroller with its oscillator and the relevant capacitors/resistors. I have also soldered the USB socket and the USB ESD protection.

 

As a next step I thought about desoldering the oscillator and trying to run it with only the internal oscillator. Then I thought about desoldering the USB ESD protection, maybe it is interfering. But other than that, if anyone can't point out an error in my schematic, I should try to solder a new MCU because maybe I somehow fried it while soldering (but that's not likely I guess)?

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