How do I check the state of an output pin?

Forum 2005-2010 (read only) Hardware Interfacing
1

Let's say I've set an pin in OUTPUT mode either HIGH or LOW, and I don't want to have to keep track of what I've set it to myself, but I need to know later in my code what I've set it to so I don't try to turn it off over and over again.

Will a call to DigitalRead() return this state? And is doing that fast? IF not, is there some value in an array somewhere which I can read back the last state I set it to?

This is the code I'm having trouble with:

void updatearray() {

  int firstpin = 2;
  int dutycycle = 1785; // Dutycycle = 1000000 microseconds / 70hz update / groups.  1785 = 8 groups of 8 at 70hz.
  unsigned long delaystart;
  int timeelapsed;
  
  for (int x = 0; x < 64; x+=8) { // Simulate updating 64 leds.
    
    if (x < 8) { 
    
      for (int y = 0; y < 8; y++) {
        if (array[x+y] > 0) { 
          digitalWrite(firstpin + x + y, HIGH);
        }
      }        
          
      //delayMicroseconds(dutycycle);

      delaystart = micros(); 
      
      do {

        timeelapsed = micros()-delaystart; 
        
        for (int y = 0; y < 8; y++) {          
          
          // Divide the brightness value in the array by 255 to produce a number between 0 and 1.  Multiply dutycycle by this value to determine how long we should wait before turning the led off.
          if (timeelapsed > (dutycycle*(array[x+y] / 255.0))) { 
            digitalWrite(firstpin + x + y, LOW);
          }
          
        } 
        
      } while (timeelapsed < dutycycle); // Regardless of whether all LED's are off, wait for duty cycle to complete.
      
      for (int y = 0; y < 8; y++) {
        digitalWrite(firstpin + x + y, LOW);
      }
      
    }
    else {
      delayMicroseconds(dutycycle); // Removing this delay would make the leds brighter when fewer are lit, and could burn them out if we are pulsing them at higher than their rated current.
    }  
    
  }
  
}

It's a little confusing because I'm simulating how bright an 8x8 array would be with just 8 leds, so it just delays after the first column is rendered. But basically, the idea is that the values in my array should define brightness levels for each led, where a value of 255 should make the led light for the full 1785 microseconds, and a value of 1 would make it light for just 1 microsecond.

The problem is, while the leds I've set to the lowest values are dimmer, they're only slightly dimmer. A brightness of 1 isn't even CLOSE to being full off. It's more like 50% off. So I'm thinking maybe setting the ports off over and over again is a bad idea.

2

I think reading will work - unless digitalRead changes the pinMode which I doubt.

If you look at the code for digitalRead and read the ATmega datasheet you'll find the answer (though much simpler to just try it - a truly definitive answer!)

3

I did just try it, and I assume it's working, but I don't see any change in the brightness of my leds, so either it's always reading high, or reading is as slow as writing.