[Forwarded from mlat 🏳️‍🌈]
ISR(USART_RXC_vect) {
    char r = UDR;

    switch (uart.state) {
        case UART_IDLE: //payload start
            while (UCSRA & (1 << UDRE)) {}
            if (r < BUFSIZE && r > 0) {
                uart.len = r; //set payload length
                uart.state = UART_PAYLOAD;
                UDR = 0x00;
            } else if (r == 0) {
                UDR = 0xf0;
            } else if (r >= BUFSIZE) {
                UDR = 0xf9;
            }
            while (UCSRA & (1 << UDRE)) {}
            break;
        case UART_PAYLOAD:
            uart.buffer[uart.idx] = r;
            uart.crc = _crc8_ccitt_update(uart.crc, r);
            uart.idx++;
            if (uart.len == uart.idx) {
                uart.state = UART_CRC;
            }
            break;
        case UART_CRC:
            while (UCSRA & (1 << UDRE)) {}
            if (_crc8_ccitt_update(uart.crc, r)) {
                UDR = 0;
            } else {
                UDR = 0xff;
            }
            while (UCSRA & (1 << UDRE)) {}
            uart.state = UART_IDLE;
            break;
        case UART_CONFIRM:
            uart.conf_ret = r;
            uart.state = UART_IDLE;
        case UART_LOCK:
            while (UCSRA & (1 << UDRE)) {}
            UDR = 0xff;
            while (UCSRA & (1 << UDRE)) {}
            break;
    }
}