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So I designed my first complex PCB containing a STM32F105.

I have a solder jumper connected to a GPIO pin and ground pulled upto 3.3v via a 10k resistor.

Everything was working fine until I closed the solder jumper. The board started using 600mA and the MCU was too hot to touch.

It turned out that the pull up resistor was a dead short, either a bad resistor or a solder bridge, when measured with a DMM it was 0ohm. (All other pull ups on other jumpers measured correctly 10k) So as soon as I connected the jumper the GPIO was connected to both VDD and VSS.

After I remove the jumper the current returns to normal and I can connect to the MCU via ST-Link which identifies it fine.

But the board doesn't seem to do anything else, interact with any user inputs or anything else it is supposed to.

Have I fried the MCU, if so how can it connect to ST-Link? Could the MCU be fine and its just a coincidence that nothings working?

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  • \$\begingroup\$ There's not really enough information here to know what is wrong. If you can connect with an stlink you can try to bring up a debugger and explore the chip. But your description of what happened does not seem quite right. If a pin were connected to both Vdd and Vss then current probably wouldn't be flowing through the chip but past it, so the chip wouldn't heat up. A logic (vs power) chip that gets hot is more likely either one that has been subjected to out of range voltages, or one that has internal biases reversed and is conducting in ways it shouldn't. \$\endgroup\$ – Chris Stratton Aug 6 '16 at 18:31
  • \$\begingroup\$ Shorting a GPIO output to a rail is out of spec, but often survivable. Are you sure you don't mean 5v rather than Vdd? \$\endgroup\$ – Chris Stratton Aug 6 '16 at 18:31
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    \$\begingroup\$ Was that GPIO an input or an output? \$\endgroup\$ – CL. Aug 6 '16 at 18:31
  • \$\begingroup\$ I have edited the question to make it slightly clearer. I did exactly what I have explained. I cannot understand either why the chip got hot. I soldered the jumper and noticed the chip was very hot when I picked the board up. \$\endgroup\$ – Terry Gould Aug 6 '16 at 18:52
  • \$\begingroup\$ Did you solder the jumper while the power was on? That is a very bad idea. \$\endgroup\$ – Turbo J Aug 6 '16 at 20:10
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The chip getting very hot is a typical symptom of latch-up. It can be induced by a test probe slipping or something like that. A common mistake is to apply signal from a signal generator with large output capability before the circuit is powered. It will survive this, typically, until you turn the power on, then you get the sizzle.

Modern IC chips are resistant to relatively large errant input currents that would in the past cause latchup (tens of mA or more) but at some point you are going to trigger the giant parasitic SCR that lives inside your CMOS chip and then the current is limited only by the power supply capability and series resistance.

We also see this in space applications where radiation can cause latchup.

At 600mA you probably fried the chip to some degree and irreparably damaged it .

Sometimes if you set your power supply to a lower maximum current limit (not much more than your circuit actually requires) you can survive this kind of incident. In such cases, cycling the power is still required to get the SCR to turn off.

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The chip wasn't damaged.

I still don't fully understand how the chip got so hot.

After the short was removed the circuit worked fined, it just happened to be a ground issue in my wiring causing the lack of function.

The chip still runs warm to the touch when powered but it seams to function correctly, I've only lost function of the pin that the short was across.

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