/* Four RGB LEDs

   David Johnson-Davies - www.technoblogy.com - 17th May 2016
   ATtiny85 @ 8 MHz (internal oscillator; BOD disabled)
   
   CC BY 4.0
   Licensed under a Creative Commons Attribution 4.0 International license: 
   http://creativecommons.org/licenses/by/4.0/
*/

// Colour of each LED; hex digits represent GBR
volatile int Buffer[4] = { 0x000, 0x000, 0x000, 0x000 };

// Display multiplexer **********************************************

void DisplaySetup () {
  // Set up Timer/Counter1 to multiplex the display
  TCCR1 = 1<<CTC1 | 7<<CS10;    // CTC mode; divide by 64
  OCR1C = 24;                   // Divide by 25 -> 78Hz
  TIMSK = TIMSK | 1<<OCIE1A;    // Enable overflow interrupt
} 
  
void DisplayNextRow() {
  static int cycle = 0;
  DDRB = DDRB & ~(1<<(cycle & 0x03));
  cycle = (cycle + 1) & 0x3F;   // 64 cycles
  int led = cycle & 0x03;
  int count = cycle>>2;
  int rgb = Buffer[led];
  int r = rgb & 0x0F;
  int b = rgb>>4 & 0x0F;
  int g = rgb>>8 & 0x0F;
  int bits = (count < r) | (count < b)<<1 | (count < g)<<2;
  bits = bits + (bits & 0x07<<led);
  DDRB = (DDRB & 0xF0) | bits;
  PORTB = (PORTB & 0xF0) | bits;
  DDRB = DDRB | 1<<led;
}

// Timer/Counter1 interrupt - multiplexes display
ISR(TIM1_COMPA_vect) {
  DisplayNextRow();
}

// Setup **********************************************

void setup () {
  DisplaySetup();
}

// Light show demo **********************************************

int Step = 0;

int red (int x) {
  int y = x % 48;
  if (y > 15) y = 31 - y;
  return max(y, 0);
}

int green (int x) { return red(x + 32); }
int blue (int x) { return red(x + 64); }

void loop () {
  for (int i=0; i<4; i++) {
    Buffer[i] = green(Step + i*12)<<8 | blue(Step + i*12)<<4 | red(Step + i*12);
  }
  Step++;
  delay(200);
}