How can a UART communication burn an MCU?

[ALipk.1]

I am using the STM32f4-Discovery board, with the stm32f407vgt6 mcu. I am using it's USART peripherals at pins PB10 and PB11. Upon connecting another device to the USART communication something in the MCU failed, and now the board is not working properly.

The MCU is heating up when I power it up, the leds on the board don't light up.

I have used the other device with the board many time before, but never observed this problem.

I have measured the resistance of the pins relative to the ground and I found out the PB10 pin is shorted to the ground of the board.

Does anyone have an idea what can cause this failure? Or maybe knows where I can find schematics of the mentioned pins(inside the mcu).

Thanks, Alex

[waclawek.jan]

Always connect grounds first.

JW

[Ozone]

Better connect only in unpowered state.

The maximum for STM32 is 5 Volt tolerance (which usually applies to UART pins), above that the protection circuitry blows.

[waclawek.jan]

Fair point; but ground difference might/will blow the pins even if unpowered.

JW

[Tesla DeLorean]

Think of the silicon in the part as being a very rapid blowing fuse.

The IO pin cell is diagrammed in the Data Sheet, as I recall.

The pins are NOT compatible with RS232 levels. So applying -5 or -10V or whatever is going to have a quite deleterious effect.