====== Serial Interface ======

This library provides AVR asynchronous serial interface usage functions.

===== Data Types =====

  * **//usart//** \\ Data type to hold USART inteface control, status and data register addresses. //usart// typed variables should be initialized at the beginning of the program code. For initializations, there is a macro-functions //USART//, whose only parameter is the index of interface (0 or 1).

  * **//usart_databits//** \\ Data bits count enumeration data type. Options:
    * //USART_DATABITS_5// - 5 data bits.
    * //USART_DATABITS_6// - 6 data bits.
    * //USART_DATABITS_7// - 7 data bits.
    * //USART_DATABITS_8// - 8 data bits.
    * //USART_DATABITS_9// - 9 data bits.

  * **//usart_stopbits//** \\ Stop bits count enumeration data type. Options:
    * //USART_STOPBITS_ONE// - One stop bit.
    * //USART_STOPBITS_TWO// - Two stop bits.

  * **//usart_parity//** \\ Parity mode enumeration data type. Options:
    * //USART_PARITY_NONE// - Disabled.
    * //USART_PARITY_EVEN// - Even parity.
    * //USART_PARITY_ODD// - Odd parity.

===== Functions =====

  * **//USART_BAUDRATE_ASYNC(baud)//** \\
    Macro function to calculate USART baud rate register value in asynchronous mode. Parameters:
    * //baud// - Desired baud rate.
    * Returns baud rate register value.

  * **//void usart_init_async(usart port, usart_databits data_bits, usart_stopbits stop_bits, usart_parity parity, usart_baudrate baudrate)//** \\
    Initializes asynchronous USART. Parameters:
    * //port// - USART interface variable.
    * //data_bits// - Data bits.
    * //stop_bits// - Stop bits.
    * //parity// - Parity mode.
    * //baudrate// - Baud rate register value (can be calculated with //USART_BAUDRATE_ASYNC// macro function).

  * **//void usart_send_char(usart port, char symbol)//** \\
    Blocking character transmission. Functions wait until transmit buffer empties before writing a character to the buffer. Parameters:
    * //port// - USART interface variable.
    * //symbol// - Character to be sent.

  * **//void usart_send_string(usart port, char *text)//** \\
    Blocking string transmission. Parameters:
    * //port// - USART interface variable.
    * //text// - Pointer to strin (char array). String has to end with null character.

  * **//bool usart_has_data(usart port)//** \\
    Checks for data in receive buffer. Parameters:
    * //port// - USART interface variable.
    * Returns //true// when there is a character in receive buffer, and //false// when not.

  * **//char usart_read_char(usart port)//** \\
    Reads a character from receive buffer. Before reading user must check if there actually is  a received character. Parameters: 
    * //port// - USART interface variable.
    * Returns character.

  * **//bool usart_try_read_char(usart port, char *symbol)//** \\
    Reads a character from receive buffer if there is any. Parameters:
    * //port// - USART interface variable.
    * //symbol// - Pointer to character variable. If there is a character in receive buffer, it is written to the pointed variable.
    * Returns //true// when there was a character in receive buffer, and //false// when not.

===== Example =====

USART interface is configured to use 8 data bits, one stop bit, 9600 bps baud rate and no parity mode. Program sends a string, waits until some character is received and then reads it out.

<code c>
#include <homelab/usart.h>

// Use USART interface 0.
usart port = USART(0);

int main(void)
{	
	char c;

	// Interface initialization.
	usart_init_async(port,
		USART_DATABITS_8,
		USART_STOPBITS_ONE,
		USART_PARITY_NONE,
		USART_BAUDRATE_ASYNC(9600));

	// String sending.
	usart_send_string(port, "Hello\n");

	// Waiting for incoming data.
	while (!usart_has_data(port)) {}

	// Read out the received character.
	c = usart_read_char(port);
}
</code>