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I have a microcontroller (dsPIC33EP64GS506, 64-pin, TQFP, link to its datasheet), which doesn't seem to work on 2 GPIO pins (pins 4:RC0 and 11:RC11), whereas all other pins are functional. The problem with these two pins is that they are directly connected to GND, even when the MCU is not powered. I tested all neighboring pins with continuity check to check for possible short circuits (bridges between pins), using a Fluke 179 multimeter. None of the neighboring pins are short-circuited.

Now, MCU pins are 3.3 V tolerant, and some of them are 5 V tolerant. In the multimeter's User manual, it says that the open circuit test voltage is <8.0 V DC, and the short circuit current is <1.1 mA. Is it possible that I fried the two MCU pins using this continuity test? I have two equal boards, the other one seems to work just fine.

Is it possible that I fried these pins while soldering? I've soldered the MCU on 300 °C with a 2.2 mm tip.

I found the error. It was due to the bad PCB manufacturing - via is connected to the ground plane, and it shouldn't be. See attached image. I've fixed this by cutting the ground plane around the problematic via.

enter image description here

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    \$\begingroup\$ It seems unlikely that either of these things would damage the micro. It would help if you updated your question with a link to the datasheet and a note of which pins are the problem. \$\endgroup\$
    – Jack B
    Commented Nov 18, 2016 at 15:16
  • \$\begingroup\$ It is dsPIC33EP64GS506 MCU, and pins are 4:RC0 and 11:RC11. I've edited the question. It seems that these two pins are directly connected to GND, even when the MCU is not powered. \$\endgroup\$
    – Marko Gulin
    Commented Nov 18, 2016 at 15:20

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Both scenarios (damage from using a multimeter or damage of a particular pin from soldering temperature) are very unlikely.

My guess is that you either have a firmware problem (some sharing of peripherals that have to be disabled) or you have damaged them in some other way such as too much voltage from ESD or an ungrounded soldering iron tip.

Since you have a board that is working (presumably? with the same firmware), it would have to be something that is undefined during startup to be a firmware problem.

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  • \$\begingroup\$ I forgot to mention: The problem with these two pins is that they are directly connected to GND. Even when MCU is now powered. \$\endgroup\$
    – Marko Gulin
    Commented Nov 18, 2016 at 15:17
  • \$\begingroup\$ Short(s) on the PCB would definitely be my suspicion (etch or solder). I always pay for 100% test so as to eliminate the first possibility. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 18, 2016 at 15:25
  • \$\begingroup\$ Ok, I will desolder the MCU now, and check the circuit again. Thanks! \$\endgroup\$
    – Marko Gulin
    Commented Nov 18, 2016 at 15:27
  • \$\begingroup\$ I managed to find a bug. It was due to the bad PCB manufacturing. Please see my first post. Thank you for your help! \$\endgroup\$
    – Marko Gulin
    Commented Nov 19, 2016 at 11:37
  • \$\begingroup\$ Good.. You might want to increase the clearance around your ground pour. Now the real question- which supplier sent you those boards? \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 19, 2016 at 11:40

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