AVR Freaks

Hello!

This is my first post in ARM. I have some experience with AVR and now I'm trying for the first time an ARM Cortex M0 (ATSAMD20J18). 

I started a new project with ASF and added sercom usart in callback mode. This is my code:

void init_uart_dbg(void)
{

	struct usart_config config_usart;
	usart_get_config_defaults(&config_usart);
	config_usart.baudrate    = 115200;
	config_usart.mux_setting = USART_RX_0_TX_2_XCK_3;
	config_usart.pinmux_pad0 = PINMUX_PB12C_SERCOM4_PAD0;
	config_usart.pinmux_pad1 = PINMUX_UNUSED;
	config_usart.pinmux_pad2 = PINMUX_PB14C_SERCOM4_PAD2;
	config_usart.pinmux_pad3 = PINMUX_UNUSED;
	while (usart_init(&usart_dbg,SERCOM4, &config_usart) != STATUS_OK) {}
	usart_enable(&usart_dbg);
	usart_register_callback(&usart_dbg,	usart_write_callback_dbg, USART_CALLBACK_BUFFER_TRANSMITTED);
	usart_register_callback(&usart_dbg,	usart_read_callback_dbg, USART_CALLBACK_BUFFER_RECEIVED);
	usart_enable_callback(&usart_dbg, USART_CALLBACK_BUFFER_TRANSMITTED);
	usart_enable_callback(&usart_dbg, USART_CALLBACK_BUFFER_RECEIVED);

}

void usart_read_callback_dbg(struct usart_module *const usart_module)
{
	ioport_set_pin_level(LED_BLINK,1);
}
void usart_write_callback_dbg(struct usart_module *const usart_module)
{
	ioport_set_pin_level(LED_BLINK,1);
}

and 

int main (void)
{
	system_init();
	ioport_init();
	delay_init();

	ioport_set_pin_level(GSM_POWER,1);

	init_uart_dbg();

	system_interrupt_enable_global();

	usart_write_buffer_wait(&usart_dbg,"\n\n---------------RESET!!!-----------\n",38);

	while(1)
	{
		delay_ms(100);
		usart_write_wait(&usart_dbg,'A');
	}

}

The "A"s prints in my terminal but when I try to send a character, the interrupt doesn't fire (led should go on). The PCB is OK, I tested with a scope on RX pin of the uC (pin 25, PB12) and the signal is OK.

I don't know what am I missing. I want to echo back the received character.

thanks!