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External input source is not detected in GPIO.pulldown mode

Question asked by Salahuddin Asjad on Sep 25, 2017
Latest reply on Sep 26, 2017 by waclawek.jan


I am currently working on a project that involves a STM32 F4 Discovery board where I am trying to use an external input source as the input to the GPIO pin. The external input source is a power supply set to 3.3V. The pins are set to mode GPIO_PULLDOWN such that the input is grounded when no particular value is assigned. However I am unable to change the input value once it is grounded. When I set the power supply to give 3.3V to the input pin, it still stays grounded. But when I connect a 3.3V pin on the board to the input pin, it gets 3.3V as input. I have noticed one thing, which I find a bit strange: when I use a multimeter to measure the input pin, the multimeter measures 3.3V on the input pin when I use GND from the power supply and the VCC from the input pin. But when I use the GND pin from microcontroller and VCC from the input pin, it outputs 0V. 


Any idea how I can solve this? Thank you in advance.