I made a simple uC (8266) based schematic that controls some peripherals. It is powered via 12v ac-dc power supply connected to the lm2596:

PCB1: PCB 1.1 The uC has uart connector like that:

PCB1: PCB 1.2

This is the first device made on separate pcb.

I also made a second device on another pcb - it’s an usb/uart converter to connect the uC to the computer:

PCB2: PCB 2.1

And 3v3 power source on this converter's board:

PCB2: PCB 2.2

Physically both devices connected via 5pins pgm/prg connector.

And here the magic starts. When I connect 12v to the uC board and usb to the usb/uart adapter and them both to each other, my computer simply reboots almost immediately. But when I disconnect 12v from the uC, and power it from the usb-attached 3v3 power source, everything is working just fine. Please help - what I missed and what is my mistake? The only thing I suspect could influence, is the connected ground planes of the 12v power source and computer's usb port. Is it related to the ground plane at all? Have no idea what could it be.

Thank you.

One more update: just did a check, thanks to comments here. Connected the ground wire of the prg/pgm connector only. So, both devices shares the ground line only. Computer rebooted. So, basically all this schematic is irrelevant.

The question can be simplified (thanx to @Swedgin): why computer reboots when I connect USB ground to the 12v external power source ground?

  • \$\begingroup\$ what is VCC? it seems that VCC from USB and VCC from the 12 V regulator output are shorted. can you clarify? \$\endgroup\$
    – User323693
    Aug 16, 2019 at 9:35
  • \$\begingroup\$ There are 2 different devices connected by 5pins jst \$\endgroup\$
    – BUKTOP
    Aug 16, 2019 at 10:06
  • \$\begingroup\$ is LM2596 regulator 5V one or 3,3 V one? have you checked it operation? \$\endgroup\$
    – User323693
    Aug 16, 2019 at 10:21
  • \$\begingroup\$ @Umar it’s 3v3. I suppose it doesn’t matter since uc vcc is not connected to the usb/uart converter. And uc woks ok, there’s 3.3v \$\endgroup\$
    – BUKTOP
    Aug 16, 2019 at 10:22
  • \$\begingroup\$ for the USB connector the net name is also VCC. doesn't it mean they are shorted? \$\endgroup\$
    – User323693
    Aug 16, 2019 at 11:08


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