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I'm encountering a very strange problem at the moment. In the course of my school project I need to drive a NEMA17 stepper motor with a DRV8825 stepper driver, which is soldered with its breakout on my PCB-Prototype. For the power supply, I have three parallel Li-ion-batteries (around 4.2V when fully charged), with a MT3608 boost converter converting the voltage to 24V which directly goes to the Vmot pin of the DRV8825-driver.

Problem is the following: the stepper doesn't run smoothly, it changes direction and runs pretty random. I'm already familiar with the typical symptoms of this (like wrong calibration of the current restriction or wrong wiring of the stepper coils). However, I already tried to fix these issues, without any success.

Now comes the interesting part: When I connect a separate board intended for the DRV8825 (DollaTek 3D Printer 42 Stepper Motor Driver Expansion Board 8825/A4988) and wire the male connection pins of the board with my female connection pins on my own PCB-Board, it works just fine. Even after controlling all signals with an oscilloscope, I couldn't find any noise on the logic pins or the coil pins. Even after soldering a decoupling capacitor between Vdd and Gnd of the DRV8825 pins, nothing has changed. A decoupling capacitor between Vmot and Gnd is already on the PCB Prototype.

I am running the stepper with the tone()-function, sending a rectangle-wave signal with a frequency of 1000Hz to the driver. I'm also of course setting the sleep pin to high, with a short delay after waking up the driver until the first step (as recommended on the datasheet). So no complicated software, which of course is already running smoothly on the separate Amazon-board.

It really is a mistery to me why this is not working, any ideas? You can find the eagle layout below.

You can find the driver on the top right (A4988 as decription is wrong)

Eagle layout, stepper driver below on the right

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The answer to this problem was simply putting two 4700µF filtering capacitors across the boost converter (in- and output).

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