USART on the MCU ATmega328p.

usart.h

#ifndef USART_H
#define USART_H

#include <avr/io.h>

/** 
 * Initialize the USART registers for async operation with 8N1 data frames.
 * 
 * Ref: 7810D–AVR–01/15, p. 159 - 165
 */
void usart_init(uint16_t baud_rate)
{
    uint16_t bdrr = (F_CPU / 16 / baud_rate - 1);
    
    UBRR0  = bdrr & 0xfff;
    UCSR0A = 0b00100000;
    UCSR0B = 0b00011000;
    UCSR0C = 0b00000110;
}

/**
 * Send one byte of data.
 * 
 * Ref: 7810D–AVR–01/15, p. 150
 */
void usart_transmit(uint8_t data)
{
    // Wait for empty transmit buffer.
    while (!(UCSR0A & (1 << UDRE0)));

    UDR0 = data;
}

/**
 * Send a string of data.
 */
void usart_transmit_string(char *data)
{
    for (; *data; data++)
        usart_transmit((uint8_t) *data);
}

/**
 * Receive a byte of data.
 * 
 * Ref: 7810D–AVR–01/15, p. 152.
 */
uint8_t usart_receive(void)
{
    // Wait for data.
    while (!(UCSR0A & (1 << RXC0)));

    return UDR0;
}

/** 
 * Flush the data buffer.
 * 
 * Ref: 7810D–AVR–01/15, p. 155
 */
void usart_flush(void)
{
    uint8_t _;
    while (UCSR0A & (1 << RXC0))
        _ = UDR0;
}

#endif