Measuring the Output Resistance of an Arduino

I've needed to estimate the output resistance of an Arduino board, an Arduino Micro in this case.
Resullts: 50 ohms in output mode, 37.5 k ohms in pullup mode
Sketch:

int ledPin = 13;
int pin = 4;

void setup() {
  pinMode(ledPin, OUTPUT);
  //pinMode(pin, INPUT_PULLUP);
  pinMode(pin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);
  delay(1);
  digitalWrite(ledPin, LOW);
  
  //delay(9);
  digitalWrite(pin, HIGH);
  delay(9);
  //delayMicroseconds(8852);
  digitalWrite(pin, LOW);
  delay(10);
}

Setup Arduino Micro:



Oscilloscope Waveforms:



Arduino Micro Schematic: