Arduino 3x3 Row Column Scan LED matrix with PWM

/* Arduino UNO with 3x3 LED Array using row column scanning with PWM and only using built in ports.
 *
 * Author: Andrew Tuline
 *
 * Date: July 14, 2012
 *
 * This does not use any 3rd party chips, thus easy to use in a Lilypad.
 *
 * Uses synchronized PWM to support LED brightness.
 *
 * Neither the Arduino's analogWrite, nor the softPWM library worked for me with row/column scanning.
 * The LED's blinked, so I created a synchronized PWM capability in order to minimize blinking.
 *
 * The default for cathodes is HIGH and LOW for anodes. We will be applying PWM to the cathodes,
 * which lends itself nicely to a common anode configuration for RGB LED's later on.
 *
 * Seems to crash every so often, so I'm trying to be careful with usage of variables.
 *
 */


int mydelay = 500;                  // used to determine how often to change LED values
int oldmillis = 0;                  // a counter to support the delay value above
const int row[] = {3, 5, 6};        // rows are anodes. Consider them 'common' anodes
const int col[] = {11, 10, 9};      // columns are cathodes and will have PWM applied to them
int pixels[3][3];                   // 2-dimensional array of pixels;
int mycounter = 0;                 // used for PWM on each LED

void setup () {
  for (int i=0; i<3; i++) {
    pinMode(row[i], OUTPUT);                   // initialize Arduino ports as OUTPUT for the rows and columns
    pinMode(col[i], OUTPUT);
    digitalWrite(col[i], HIGH);                // turn off pixel to start by setting the cathode HIGH
  } // for

  randomSeed(analogRead(0));                  // randomizer

  Serial.begin(9600);                        // console setup, only used for debugging

} // setup()

void loop() {
  readSensors();
  refreshScreen();
} // loop()


void readSensors() {                           // this routine changes the value of the LED's, but only after a delay time

  if (int(millis()/mydelay) == oldmillis) {    // only change values if we've not done so in 'mydelay' milliseconds
      ++oldmillis;                             // increase the counter used to count the time between changing LED values
      mycounter = 0;                           // reset the refresh counter that's used for PWM so it doesn't cause the 9999 value below to strobe occasionally

    for (int x=0; x<3; x++) {                  // change the values for ALL of the LED's
      for (int y=0; y<3; y++) {
        pixels[x][y] = int(random(0, 5)*20);        // this is a PWM value, the higher the number, the fewer the times the LED will be pulsed
        if (pixels[x][y] == 80) {              // even a long duration of PWM is > 0 pulses, so let's represent OFF somehow
          pixels[x][y] = 9999;                 // this value represents an LED that is turned off. Testing has shown that 'mycounter' doesn't go this high
        }
      }
    }
  }
} // readSensors()


void refreshScreen() {
  mycounter++;                                    // use a counter that will be bitwise AND'ed with the pixel value to determine if a LED is turned on or not

  for (int thisRow = 0; thisRow<3; thisRow++) {   // enable a row by setting the anode to high
    digitalWrite(row[thisRow], HIGH);
    for(int thisCol = 0; thisCol<3; thisCol++) {  // now we go column by column
      int thisPixel = pixels[thisRow][thisCol];

      if ((mycounter & thisPixel) == thisPixel) {  // depending on the PWM value we will turn on the LED . .
        digitalWrite(col[thisCol], LOW);           // by setting the cathode LOW
        digitalWrite(col[thisCol], HIGH);          // now, set the column high again to turn if off
        }
    } // for thisCol
   digitalWrite(row[thisRow], LOW);                // once the columns are done, turn off the row (anode)
  } // for thisRow
} // refreshScreen()