16.8.2.1.2 Code

Add to the main application source file, outside of any functions:

struct usart_module usart_instance;

#define MAX_RX_BUFFER_LENGTH   5

volatile uint8_t rx_buffer[MAX_RX_BUFFER_LENGTH];

Copy-paste the following callback function code to your user application:

void usart_read_callback(struct usart_module *const usart_module)
{
    usart_write_buffer_job(&usart_instance,
            (uint8_t *)rx_buffer, MAX_RX_BUFFER_LENGTH);
}

void usart_write_callback(struct usart_module *const usart_module)
{
    port_pin_toggle_output_level(LED_0_PIN);
}

Copy-paste the following setup code to your user application:

void configure_usart(void)
{
    struct usart_config config_usart;
    usart_get_config_defaults(&config_usart);

    config_usart.baudrate    = 9600;
    config_usart.mux_setting = EDBG_CDC_SERCOM_MUX_SETTING;
    config_usart.pinmux_pad0 = EDBG_CDC_SERCOM_PINMUX_PAD0;
    config_usart.pinmux_pad1 = EDBG_CDC_SERCOM_PINMUX_PAD1;
    config_usart.pinmux_pad2 = EDBG_CDC_SERCOM_PINMUX_PAD2;
    config_usart.pinmux_pad3 = EDBG_CDC_SERCOM_PINMUX_PAD3;

    while (usart_init(&usart_instance,
            EDBG_CDC_MODULE, &config_usart) != STATUS_OK) {
    }

    usart_enable(&usart_instance);
}

void configure_usart_callbacks(void)
{
    usart_register_callback(&usart_instance,
            usart_write_callback, USART_CALLBACK_BUFFER_TRANSMITTED);
    usart_register_callback(&usart_instance,
            usart_read_callback, USART_CALLBACK_BUFFER_RECEIVED);

    usart_enable_callback(&usart_instance, USART_CALLBACK_BUFFER_TRANSMITTED);
    usart_enable_callback(&usart_instance, USART_CALLBACK_BUFFER_RECEIVED);
}

Add to user application initialization (typically the start of main()):

configure_usart();
configure_usart_callbacks();