#include <avr/io.h>
#include <avr/interrupt.h>
#include <stdint.h>

volatile char c;

ISR(USART_RXC_vect) {
    if (UCSRA & (1<<FE | 1<<DOR | 1<<PE)) {
        c = UDR;
        c = '?';
    }
    else {
        c = UDR;
    }
    UCSRB &= ~(1<<RXCIE);
    UCSRB |= (1<<UDRIE);
}

ISR(USART_UDRE_vect) {
    static uint8_t echo = 0;
    UDR = c;
    echo++;
    if (echo == 2) {
        echo = 0;
        UCSRB |= (1<<RXCIE);
        UCSRB &= ~(1<<UDRIE);
    }
}

int main(void) {
    // 9600-7-E-1
    UCSRA = 0x00;
    UCSRB = 0x98;
    UCSRC = 0xA4;
    UBRRH = 0;
    UBRRL = 23;
    sei();
    while (1);
    return 0;
}