Second digitalRead() of PWM pin always HIGH?

Forum 2005-2010 (read only) Software Troubleshooting
1

Hi all,

I've just got my arduino and am very happy with it. I'm not really a hardware guy so it's nice to have something easy-to-use that I can understand! :slight_smile:

I've discovered a problem though. It seems that if you do a digitalRead() from a PWM pin (9, 10, or 11) configured as input, the second and subsequent digitalRead()s will always return HIGH, until the signal on the pin actually changes.

That is:

1 h = digitalRead(10); /* h gets correct value /
2 h = digitalRead(10); / h is HIGH regardless of value /
3 / Signal changes from LOW to HIGH or vice versa /
4 h = digitalRead(10); / h once again gets correct value*/

This wouldn't be a problem except that sometimes I get spurious HIGH values in line 1: ie h gets HIGH regardless.

If I move the input to one of the other pins (not PWM), everything works fine.

Has anyone else experienced this problem?

2

When digitalRead() mistakenly returns HIGH, is the pin actually connected to a low voltage? If it's floating (not-connected), you'll get random readings (e.g. all HIGHs). If so, please post your code.

I ran the following program, and with pin 10 connected directly to ground, the digitalRead() always returned LOW.

void setup()
{
  Serial.begin(9600);
  pinMode(10, INPUT);
}

void loop()
{
  Serial.println(digitalRead(10));
  delay(300);
}
3

Thanks for your answer.

I did some more testing and got the same results as you, so I think perhaps I was just silly and had a bug the first time (I'm interfacing with another device and I think my timing was slightly wrong so I actually was reading wrong values). Or the pin could have been floating temporarily.

Either way it works now. Thanks again for the suggestions!