// reverse the pair blinking

#include <stdint.h>

#define DDRA  (*(volatile uint8_t*)0x21)
#define PORTA (*(volatile uint8_t*)0x22)

#define DDRB  (*(volatile uint8_t*)0x24)
#define PORTB (*(volatile uint8_t*)0x25)

void delay1sec(void){
    TCNT1  = 0;
    TCCR1A = 0x00;
    TCCR1B = 0x05;        // prescaler 1024
    while (TCNT1 < 15625);
    TCCR1B = 0x00;
}

void setup() {
    DDRA = 0xFF;          // Row 1 output
    DDRB = 0xFF;          // Row 2 output
}

void loop() {

    for (int8_t i = 6; i >= 0; i -= 2) {
        uint8_t mask = (0x03 << i);   // pair mask

        PORTA = mask;    // Row 1
        PORTB = mask;    // Row 2

        delay1sec();
    }
}