#ifndef F_CPU
#define F_CPU 20000000UL // 20 MHz clock speed
#endif

#include <avr/io.h>

#define UBRR_VAL (( F_CPU / (16UL * 250000UL)) - 1)

int main(void) {
  // Initialize USART0
  // baud
  UBRR0H = (uint8_t) (UBRR_VAL >> 8);
  UBRR0L = (uint8_t) UBRR_VAL;

  // frame format: 8n2
  UCSR0C = (1 << USBS0) | (1 << UCSZ01) | (1 << UCSZ00);
  UCSR0B = (1 << TXEN0);

  // start sending stuff
  uint8_t value = 0x00;

  while (1) {
    // wait send buffer empty
    while (!(UCSR0A & _BV(UDRE0))) {
    }

    // send next byte
    UDR0 = value++;
  }
}