gistfile1.txt

#include <avr/io.h>
#include <avr/power.h>
#include <util/delay.h>

#define LED      PB0
#define LED_DDR  DDRB
#define LED_PORT PORTB

#define USART_BAUDRATE 115200
#define UBRR_VALUE     (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

#define set_bit(sfr, bit) (_SFR_BYTE(sfr) |= (1 << bit))
#define clear_bit(sfr, bit) (_SFR_BYTE(sfr) &= ~(1 << bit))
#define toggle_bit(sfr, bit) (_SFR_BYTE(sfr) ^= (1 << bit))

void usart_init() {
  UBRR0 = UBRR_VALUE;
  UCSR0B = (1 << RXEN0) | (1 << TXEN0);
  UCSR0C = (1 << UCSZ00) | (1 << UCSZ01); // 8 bits, no parity, 1 stop bit
}

void usart_send(uint8_t ch) {
  set_bit(LED_PORT, LED);
  while (!(UCSR0A & (1 << UDRE0)));
  UDR0 = ch;
  clear_bit(LED_PORT, LED);
}

uint8_t usart_receive(void) {
  while(!(UCSR0A & (1 << RXC0)));
  return UDR0;
}

void send_chars(uint16_t count) {
  uint8_t ch = 'a';

  while (count != 0) {
    usart_send(ch);

    if (ch == 'z') {
      ch = 'a';
    } else {
      ++ch;
    }

    --count;
  }
}

int main(void) {
  clock_prescale_set(clock_div_1); // 8Mhz clock
  OSCCAL = 45;
  usart_init();

  set_bit(LED_DDR, LED); // set LED pin for output

  while (1) {
    uint8_t command = usart_receive();

    switch (command) {
      case 'o':
        send_chars(1024);
        break;

      case 't':
        send_chars(10240);
        break;

      case 'b':
      default:
        usart_send(command);
        break;
    }
  }
}