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I have an BluePill board (Arm Cortex M3 Dev. Board). Also a a4988 stepper motor driver. BluePill is powered from usb of my PC. Stepper motor driver is powered by external power supply which is 220V -> 12V. Both my pc and the external power supply are connected to same plugs. The grounds are wired between a4988 and BluePill. The weird problem is, when I connect my PC13 pin to STEP pin. I can smoothly drive my stepper motor. When I connect to another pin such as A7, A6 my stepper motor turns randomly. Power supply and wires are too close to each other (approximately 10-15cm). I think that emf occurs on the wire and the noise causes to move motor randomly but why it doesn't happen when I connect it to PC13.

When I connect a wire to STEP pin and the other side of wire is unconnected, there is also random turns.

Added my setup as @laptop2d said. Here is my setup

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    \$\begingroup\$ Provide a diagram of your setup, we are not at your bench and its too hard to troubleshoot problems via telekinesis \$\endgroup\$ – Voltage Spike Nov 8 '18 at 18:12
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    \$\begingroup\$ @Ames: Please embed your image into your post. This has several benefits: all the information will be in one place; we can read the text and see the schematic at the same time; more people will respond since they don't have to follow links; the question will still be intelligible when the link dies. \$\endgroup\$ – Transistor Nov 10 '18 at 10:10
  • \$\begingroup\$ I could not because when i pressed img button, stackexchange adds with invalidated img url, so I added to my google drive and linked to post \$\endgroup\$ – arnes Nov 10 '18 at 10:14
  • \$\begingroup\$ guys any help ? \$\endgroup\$ – arnes Nov 17 '18 at 19:14
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On the BluePill board, PC13 sinks LED1 through a resistor. So it's 3v3 -> LED -> resistor-> PC13 -> step pin. Try enabling pull-up on the other pin you want to use eg A7.

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