Because you do not explain that in words or in code how are we supposed to know that, by guessing and making assumptions as we always seem to have to do here.
If you need help, then spend time in writing, detailing exactly in clear and understandable terms what you have and what you need to achieve.
Otherwise we will go around and around in circles guessing.
In your code you have, albeit commented out the following, which lead me to 'assume' you were using a hardware communications port:
/* while (!(UCSR0A & (1 << UDRE0))) // Wait for empty transmit buffer
UCSR0A |= 1 << TXC0; // mark transmission not complete
while (!(UCSR0A & (1 << TXC0)));*/ // Wait for the transmission to complete
Do you see how that might be confusing for people ?
Paul - VK7KPA