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xisco
xisco
Senior
September 16, 2023
Solved

Input signal when MCU is powered off

  • September 16, 2023
  • 9 replies
  • 4599 views

Hello,

I will make some probes for reading electrical values (AC, Analog, digital,...), but I'm not sure what happens if I insert a signal to a GPIO when MCU is powered off.

I read time ago that all GPIOs are in high impedance state when MCU is not powered but I don't see this information in datasheets. It's the most logical, but I need to be sure for the safety of MCUs.

The MCUs that I will use are

STM32F446 (Nucleo board)

STM32G030 (Homemade boards)

STM32F103 (Blue Pill)

 

Thanks

 

This topic has been closed for replies.
Best answer by MasterT

If uCPU power is off, than no lines with voltage should be connected to inputs. Exception is only for high impedance signal sources, or you can intentionaly insert resistors in series with inputs. Injection current specified in the DS, usualy < 1mA, but better option would be optoisolation, there are analog galvanic isolation IC as well as digital

9 replies

TDK
TDK
September 16, 2023

They're mostly in high impedance mode, but the protection diodes are still present.

Look at the "Voltage characteristics" table in the relevant datasheets. Generally an input voltage of 3.3V is fine. More than that can be problematic. Depends on the exact pins you're using.

TDK_0-1694827342929.png

Also look at the GPIO port structure in the reference manual for understanding.

TDK_1-1694827681644.png

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    MasterT
    MasterTBest answer
    Lead II
    September 16, 2023

    If uCPU power is off, than no lines with voltage should be connected to inputs. Exception is only for high impedance signal sources, or you can intentionaly insert resistors in series with inputs. Injection current specified in the DS, usualy < 1mA, but better option would be optoisolation, there are analog galvanic isolation IC as well as digital

      xisco
      xiscoAuthor
      Senior
      September 16, 2023

      Thanks for your answer

      For analog inputs, I will reduce the maximum current to less than 1 mA, avoiding loss of accuracy in galvanic isolation.

      For digital inputs, I will use optocouplers.

       

      TDK
      TDK
      September 18, 2023

      > Obvious, if this diode were present, will be impossible to input a voltage >VDD+0.6V.

      It's certainly possible if the protection diode for FT pins is a 4 V zener, which the evidence strongly suggests.

      "If you feel a post has answered your question, please click ""Accept as Solution""."
      xisco
      xiscoAuthor
      Senior
      September 18, 2023

      Yes, it looks like a zener diode in series with a rectifier diode to avoid the forward polarization current of the zener diode.

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