GPIO Sinking Resistance

[edwardsbobg]

I am powering an LED / current limiting resistor by sinking current into an IO pin. Where can I find the resistance of the IOs when sinking current?

[HertzSoGood]

Hey I'm a team member of edwardsbobg and wanted to followup on this question.  We're running an RGB LED and sinking the current into the nrf52832 using 100, 470, and 100 Ohm resistors respectively.  For simplicity let's just look at the red - it's powered with 3.3V, drops 1.92V over the LED, then drops 0.517V over the 100 Ohm resistor.  Thus the voltage at the pin is 0.863V.  The voltage drop over the resistor calculates to a current of 5mA.  

The 0.863V was surprising to me because I figured the GPIO to have close to zero resistance and thus the voltage would be close to 0V.  But this voltage is not negligible, which to me suggests I need to factor in this voltage when calculating the external resistor value.  Do people know why this 0.863V would be higher than 0.4V?

[Turbo J]

Did you remember to put the pin into high drive mode?

Standard drive is only about 4 mA, explaining your high voltage drop.