Input only normal with pin13

Using Arduino IDE 1.x
1

Hello everyone. I have got a strang thing:
With this schema:


and this code:

/*
* Switch test program
*/
int switchPin = 2; // Switch connected to digital pin 2
void setup() // run once, when the sketch starts
{

  • Serial.begin(9600); // set up Serial library at 9600 bps*
  • pinMode(switchPin, INPUT); // sets the digital pin as input to read switch*
    }
    void loop() // run over and over again
    {
  • Serial.print("Read switch input: ");*
  • Serial.println(digitalRead(switchPin)); // Read the pin and display the value*
  • delay(100);*
    }

I connect all things.

but when I read analoge input, I get 0...1....0...1...,the button doesn't work at all. After manipulation, I found out if I put the resistor connected the pin2 even without connecting the breadboard, the alternation 0&1 occurs. What happens here?

Moreover, I have tried other pins, only pin13 works with this schema and code. Why?

Thanks.

2

You need this after the pinMode statement to turn on the internal pullup

digitalWrite (switchPin, HIGH);

Then it will read High until you ground the pin to make it read Low.

3

Pin 13 has an LED to resistor to ground, I imagine you see that read as a consisent 0?

4

COOL, you are so right CrossRoads, that answer all my questions.

Thank you very very much again.

5

But I have another question:
Why it alternate with 0 and 1 with my first code?

6

CrossRoads:
Pin 13 has an LED to resistor to ground, I imagine you see that read as a consisent 0?

I too would caution that with a ? As the led would not be forwarded biased when wired to a 'floating' input pin, I'm not convienced it would indeed act like a pull-down, but rather most likely a open circuit and as such might have no effect on the pin and the pin would remain in a 'floating' input state.

Lefty

7

"Why it alternate with 0 and 1 with my first code?"
Pin 2 would read inconsistently if not connected to +5 or to Gnd because it would be "floating"- the pin when in input mode requires very little current flow to make it high or low, with nothing connected it can drift back & forth, with just a finger touch often being enough to make it change.

Maybe the LED/resister wouldn't seem enough to make it read low - but if you are seeing a steady low, then maybe it is.

I don't have any bootload arduino's on hand at the moment, need to get a couple programmed so I could try this myself.

8

Ok guys, lets the alternation aside for the moment.
I have another problem: digitalWrite (switchPin, HIGH); makes it "read High", but when I wirte : digitalWrite (switchPin, LOW); I can't make it read low until I connect the 5V to make it read high. How come?

9

Why would you write your pin that is setup as an input Low?
Writing it high enables the internal pullup resister. Writing it low turns it off - then you are back to a floating input again. Then you need to pull it low externally to make it read low.
Do you have anything connected to this pin?

10

Have a read of this:-
http://www.thebox.myzen.co.uk/Tutorial/Inputs.html

11