Is it possible to identify looking at the schematics whether a GPIO pin is configured as Input or Output pin for the case in following image?

PIN93 and PIN94 of a MCU are connected directly with connector J2 pins. It looks to me that these pins are in INPUT mode due to the capacitors on these pins. What are the chances that this is the other way round?

enter image description here

  • 4
    \$\begingroup\$ I'd expect those to be analog inputs, as I wouldn't expect those capacitors on a digital output. \$\endgroup\$ Feb 14, 2019 at 20:33
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    \$\begingroup\$ These seem to be analog inputs with low pass filters. \$\endgroup\$
    – Eugene Sh.
    Feb 14, 2019 at 20:33
  • 1
    \$\begingroup\$ Possibly digital inputs if the MCU in question has schmitt-trigger inputs. Of course, if the board designer wasn't thinking, you don't know what it means. \$\endgroup\$
    – TimWescott
    Feb 14, 2019 at 20:43
  • \$\begingroup\$ So its Input, either digital or analog.. but not Output \$\endgroup\$
    – scico111
    Feb 14, 2019 at 21:08

1 Answer 1


There is no good reason to wire a capacitor directly across an output. If, for example, the capacitor is discharged then it behaves as a momentary short-circuit when the output is turned on. This presents a momentary overload to the output. Similarly when charged, pulling it low will effectively present a momentary connection to V+.

On the other hand, as an input R and C behave as a low-pass filter. This could be used to debounce a digital input or stabilise an analog reading. In both cases it introduces a small delay in the response time.


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