---conf_board.h

#define CONF_BOARD_ENABLE_USARTD0
#define USART_RX_Vect                   USARTD0_RXC_vect

---conf_sleepmgr.h

#define CONFIG_SLEEPMGR_ENABLE

---

#define USART_SERIAL_EXAMPLE            &USARTD0
#define USART_SERIAL_EXAMPLE_BAUDRATE   115200
#define USART_SERIAL_CHAR_LENGTH        USART_CHSIZE_8BIT_gc
#define USART_SERIAL_PARITY             USART_PMODE_DISABLED_gc
#define USART_SERIAL_STOP_BIT           false

---main

#include <conf_usart_example.h>
#include <asf.h>
#define LED0_GPIO                       IOPORT_CREATE_PIN(PORTC, 0)
#define LED1_GPIO                       IOPORT_CREATE_PIN(PORTC, 1)
#define LED2_GPIO                       IOPORT_CREATE_PIN(PORTC, 2)

#define LED0                            LED0_GPIO

ISR(USART_RX_Vect)
{
    usart_putchar(USART_SERIAL_EXAMPLE,
    usart_getchar(USART_SERIAL_EXAMPLE));
    while (!usart_tx_is_complete(USART_SERIAL_EXAMPLE)) {
    }
    usart_clear_tx_complete(USART_SERIAL_EXAMPLE);
}

int main(void)
{
    uint8_t tx_buf[] = "\n\rHello AVR world ! : ";
    uint8_t tx_length = 22;
    uint8_t received_byte;
    uint8_t i;

    board_init();
    sysclk_init();
    pmic_init();
    cpu_irq_enable();
    sleepmgr_init();
    sleepmgr_lock_mode(SLEEPMGR_IDLE);//SLEEPMGR_STDBY);

    ioport_configure_pin(LED0_GPIO, IOPORT_DIR_OUTPUT | IOPORT_INIT_HIGH);
    ioport_configure_pin(LED1_GPIO, IOPORT_DIR_OUTPUT | IOPORT_INIT_HIGH);
    ioport_configure_pin(LED2_GPIO, IOPORT_DIR_OUTPUT | IOPORT_INIT_HIGH);
   
    gpio_set_pin_high(LED0);


    // USART options.
    static usart_rs232_options_t USART_SERIAL_OPTIONS = {
        .baudrate = USART_SERIAL_EXAMPLE_BAUDRATE,
        .charlength = USART_SERIAL_CHAR_LENGTH,
        .paritytype = USART_SERIAL_PARITY,
        .stopbits = USART_SERIAL_STOP_BIT
    };

    // Initialize usart driver in RS232 mode
    usart_init_rs232(USART_SERIAL_EXAMPLE, &USART_SERIAL_OPTIONS);

    usart_set_rx_interrupt_level(USART_SERIAL_EXAMPLE, USART_INT_LVL_LO);
   
    for (i = 0; i < sizeof(tx_buf); i++) {
        usart_putchar(USART_SERIAL_EXAMPLE, tx_buf[i]);
        while (!usart_tx_is_complete(USART_SERIAL_EXAMPLE)) {
        }
        usart_clear_tx_complete(USART_SERIAL_EXAMPLE);
    }
   
    while (true) {
        sleepmgr_enter_sleep();
        gpio_toggle_pin(LED0);
    }

}