No pullup resistor on pin 13 on Duemilanove?

I just got my first Duemilanove.

I noticed, though, that I don’t seem to be able to use the pullup resistor on pin 13 when using pin 13 as an input pin. Is this expected behavior, because of the LED, or is my Duemilanove a little funky?

Here’s a sketch that prints 1 for every pin, and 0 for pin 13 on my board.

void setup() {

  Serial.begin(9600);
  for(int index = 0; index < 19; index++) {
    pinMode(index, INPUT);
    digitalWrite(index, HIGH);
  }
  delay(1000);
  
}

void loop() {

  for(int index = 0; index < 19; index++) {
    Serial.print("Pin ");
    Serial.print(index, DEC);
    Serial.print(" ");
    Serial.println(digitalRead(index));
  }
  Serial.println();
  delay(1000);
}

The pin 13 LED and it’s serial current limiting resistor is acting like a pull down load. One would have to pull up with a pretty low value resistor to over come this, much lower then the ‘weak’ pullup that can be activated in software.

Pin 13 is best used as an output pin. To use it as an input pin the source sending the signal would have to be able to source at least a couple of milliamps at 5v as the LED and it’s resistor represents a load that otherwise a AVR high impedance input pin does not exhibit.

Of course one is free to just cut or unsolder one of the resistor legs to disconnect this pull down load.
Lefty