Charlieplexing 6 LEDs on ATTiny85

CharlieplexingTiny85.c

#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>

void setLED(uint8_t LED);

uint8_t currentLED;
uint8_t times = 0;

/*
    LEDs connected to PB0, PB1 and PB2
    Truth table for LED matrix

    Led    | Pin High | Pin Low | Pin Disc. | PORTB | DDRB |
    0      | 0        | 1       | 2         | 0x01  | xFB  |
    1      | 1        | 0       | 2         | 0x02  | xFB  |
    2      | 1        | 2       | 0         | 0x02  | xFE  |
    3      | 2        | 1       | 0         | 0x04  | xFE  |
    4      | 0        | 2       | 1         | 0x02  | xFD  |
    5      | 2        | 0       | 1         | 0x04  | xFD  |

*/

void main()
{
  LED_PORT &= 0xF8;            // Set lower 3 bits to 0, i.e. off
  LED_DDR |= 0x07;             // Set lower 3 bits to 1, i.e. output

  // Set up timer prescalar to FCpu/64 @ 1MHz
  TCCR0B |= (_BV(CS00) | _BV(CS02));

  // Set timer to CTC mode
  TCCR0A |= _BV(WGM01);
  // Enable CTC interrupt
  TIMSK |= _BV(OCIE0A);
  // Enable global interrupts
  sei();

  // Set timer to compare 1/4 second
  OCR0A = 243;

  while(1)
  {
    //Turn on an LED
    setLED(currentLED);
  }
}

// Handle timer interrupt
// This triggers the interrupt every 1/2th of a second
// ATTiny85's Timer0 is 8 bit and therefore cannot store OCR0A values higher than 255
// This is a quick and dirty way to get the 2Hz refresh rate
ISR(TIMER0_COMPA_vect)
{
  int i;

  times++;

  if (times >= 2)
  {
    // Get random number for i
    i = rand() % 14;

    if (i != currentLED)
    {
      currentLED = i;
    }
    else
    {
      i = rand() % 14;
    }

    // Reset clock repeats
    times = 0;
  }
}

// Switch the state of the LED from on to off or from off to on
void setLED(uint8_t LED)
{
  //Clear existing status, we can only drive one LED at a time
  PORTB &= 0xF8;    // Set the lower 3 bits to 0, i.e. off
  DDRB |= 0x07;   // Set the lower 3 bits to 1, i.e. output

  switch (LED)
  {
    case 0: 
      DDRB  &= 0xFB;  // Disconnect the nr. 2 pin
      PORTB |= 0x01;  // Set pin 0 to high and pin 1 to low
      break;
    
    case 1: 
      DDRB  &= 0xFB;  // Disconnect the nr. 2 pin
      PORTB |= 0x02;  // Set pin 1 to high and pin 0 to low
      break;
    
    case 2: 
      DDRB  &= 0xFE;  // Disconnect the nr. 0 pin
      PORTB |= 0x02;  // Set pin 1 to high and pin 2 to low
      break;        

    case 3: 
      DDRB  &= 0xFE;  // Disconnect the nr. 0 pin
      PORTB |= 0x04;  // Set pin 2 to high and pin 1 to low
      break;              
            
    case 4: 
      DDRB  &= 0xFD;  // Disconnect the nr. 1 pin
      PORTB |= 0x01;  // Set pin 0 to high and pin 2 to low
      break;
    
    case 5: 
      DDRB  &= 0xFD;  // Disconnect the nr. 1 pin
      PORTB |= 0x04;  // Set pin 2 to high and pin 0 to low
      break;

    default: 
      break;
  }
}