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How can GPIO pins output voltage be equal to VDD/VDDA or 0V(GND)?

Question asked by kong.kevin on Apr 8, 2015
Latest reply on Apr 9, 2015 by kong.kevin
Hi all,

I want to use STM32L100's two  GPIO pins to output high voltage as equal to VDD/VDDA and low voltage as equal to 0V(GND) for, say, two series resistors R1 and R2, and use ADC to measure one's voltage. However, I noticed that in STM32L100's datasheet, it mentioned that GPIO output voltage may not reach as high as VDD/VDDA, and not as low as GND, especially when outputting high current. This definitely will influence the voltage measured by ADC. Could anyone help to point out how GPIO can output a voltage as high as VDD/VDDA, and as low as GND? I looked up the datasheet and user manual, but found no clear answer. Thanks a lot for the help!

Best regards,
Kevin Kong