Why would an OUTPUT pin need a pulldown resistor?

Development Boards Arduino
1

I have connected an HC-11 wireless serial module to a MEGA board. The SET pin needs to be low to program the module. If I just connect the MEGA pin 13 (and digitalWrite LOW) to the module SET pin then I am unable to program the module (even though the pin 13 led is off). If I include a pulldown resistor then everything is okay.

I am only just starting with this stuff. Am I missing something?

This code works with the pulldown resistor, does not without.

int pinSet = 13;
int incomingByte = 0; 
void setup() {                
  pinMode(pinSet, OUTPUT);   
  digitalWrite(pinSet, LOW);
 Serial.begin(9600);  
 Serial1.begin(9600);  
 Serial1.println("AT");
 delay(1000) ; 
}
void loop() {
        if (Serial1.available() > 0) {
                // read the incoming byte:
                incomingByte = Serial1.read();

                // say what you got:
                Serial.print("I received: ");
                Serial.println(incomingByte, DEC);
        }

}
2

I don’t know how quickly this HC-11 unit boots up, and for how long this SET pin must be low. But when the AVR chip in the Mega resets after turning on, it’s pins get set to high impedance mode. Or very little current is sourced or drained through it’s output pins in that state. This means that in this period, until the program sets the pin mode to digital out low, the SET pin floats to whatever level aditional circuitry and induction/capacitance drives it to. This may prevent the desired operation of your HC-11 module if the bootloader or initial code takes too long.

3

Thank you. This was the answer.