How can I digitalRead a pin that is in pinMode OUTPUT?

Question

I have a very simple test sketch in which I'm trying to set a pin to HIGH and then read its state with digitalRead. Here is my sketch.

void setup() {     Serial.begin(9600); }  void loop() {     delay(1000);      pinMode(3, OUTPUT);     digitalWrite(3, HIGH);     delay(1000);      pinMode(3, INPUT);     Serial.println(digitalRead(3)); } 

Serial monitor result:

0 0 0 0 

I have come to understand that changing the pinMode will stop it from being HIGH. So setting a pin to HIGH in OUTPUT mode and then changing to INPUT mode will change it to LOW. So the digitalRead will always return 0. If I don't change the pinMode it won't be able to read the pin. So how can I read the current setting of a pin that is in OUTPUT mode without losing the value?

Answer 1

In this case you just want to access the data register itself.

PORTB and PORTD registers contain the pin data you are looking for. I finally got access to an Arduino to figure it out. You want to use bitRead(PORTD, pin).

Serial.println(bitRead(PORTD,3)); //Reads bit 3 of register PORTD which contains the current state (high/low) of pin 3. 

Reference Bit Read Operation for more information.

Answer 2

Your sketch should be

void setup() {     Serial.begin(9600); }  void loop() {     delay(1000);      pinMode(3, OUTPUT);     digitalWrite(3, HIGH);     delay(1000);      // pinMode(3, INPUT); // get rid of this line     Serial.println(digitalRead(3)); } 

That's all. Then it reads the pin's state which in your case is "HIGH". If you set the pinMode to input it will read the input depending on what is connected. If you are writing "HIGH" to an input pin the internal pullup will be activated. It does not matter if you write HIGH before setting it to input mode or after setting it to input mode. Unless of course you are driving a load that is to high for the output pin (e.g. a switch to ground). Then this would probably kill the pin.

If you have written a low and set the pin to low it might float which may lead to any kind of unpredictable behaviour.