AVR Freaks

Good morning everyone, First, introduce me in the forum! I am a Spanish electrical engineer, with great experience in the design of PCBs and in the programming of microcontrollers, but this is the first time that I have been involved in an ATMM ARM and I am having some problem. The first thing is that I do not like to work with ASF libraries that are already incorporated in the atmel studio. Therefore, I am programming the micro through registers. My problems, specifically with the USART, given that for some reason, I do not know if configuration or to address the interruciones generated by this peripheral with send me data through the port, I attach code for if you see any erratum. The first doubt I have is how to set the transmission speed to be the usual one; 9600 without parity and stop bit, and also how to manage the interrupt routines. As you can see in the code for now I only sent the ASCII 'A' character, but I can not complete the transmission successfully. Any ideas?

Thanks in advance.

#include "sam.h"

uint16_t i;

int main(void)
{
    SystemInit();
	GCLK->PCHCTRL[22].bit.GEN=0;
	GCLK->PCHCTRL[22].bit.CHEN=1;
	//REG_GCLK_PCHCTRL22= GCLK_PCHCTRL_GEN_GCLK0 | GCLK_PCHCTRL_CHEN; 
	MCLK->APBCMASK.bit.SERCOM3_=1;
	MCLK->APBAMASK.bit.OSCCTRL_=1;
	
	//PA22 UART TX SERCOM3
	//PA23 UART RX
	//PA24 CAN0TX
	//PA25 CAN0RX
	//Led PA15
	//Button PA28
	
	PORT->Group[0].DIRSET.reg=PORT_PA22;//TX as Output
	PORT->Group[0].DIRCLR.reg=PORT_PA23;//RX as Input
	PORT->Group[0].PINCFG[22].bit.INEN=1; //Tx input avialable
	PORT->Group[0].PINCFG[23].bit.INEN=1; //Rx input avialable
	PORT->Group[0].PINCFG[22].bit.PMUXEN=1;//Enable multiplexing
	PORT->Group[0].PINCFG[23].bit.PMUXEN=1;//Enable multiplexing
	PORT->Group[0].PMUX[11].bit.PMUXE=0x02;//Type D
	PORT->Group[0].PMUX[11].bit.PMUXO=0x02;//Type D
	
	
    SERCOM3->USART.CTRLA.bit.ENABLE = 0; //Disable USART for config
     
    SERCOM3->USART.CTRLA.bit.MODE = 1; //Set clock mode to internal clock
    SERCOM3->USART.CTRLA.bit.CMODE = 0; //Set clock to be asynchronous with bus clk
    SERCOM3->USART.CTRLA.bit.RXPO = 1; //Select Rx pin to be PAD[1]
    SERCOM3->USART.CTRLA.bit.TXPO = 0; //Select Tx pin to be PAD[0]
    SERCOM3->USART.CTRLA.bit.DORD = 1; //Set data order (MSB first)
    SERCOM3->USART.CTRLA.bit.FORM = 1; //Frame settings enable
	SERCOM3->USART.CTRLA.bit.SAMPR=0;
	
	SERCOM3->USART.BAUD.bit.BAUD = 64563;
	
    SERCOM3->USART.CTRLB.bit.CHSIZE = 0; //Config the char size field to 8 bit
    SERCOM3->USART.CTRLB.bit.RXEN = 1; //Enable Receiver
    SERCOM3->USART.CTRLB.bit.TXEN = 1; //Enable transmitter
    while (SERCOM3->USART.SYNCBUSY.bit.CTRLB); //wait
	
	NVIC_EnableIRQ(SERCOM3_IRQn);
	SERCOM3->USART.INTENSET.reg = SERCOM_USART_INTFLAG_RXC;
	SERCOM3->USART.CTRLA.reg |= SERCOM_USART_CTRLA_ENABLE;
	while(SERCOM3->USART.SYNCBUSY.reg & SERCOM_USART_SYNCBUSY_ENABLE);
     
	PORT->Group[0].PINCFG[28].bit.PULLEN=1;
	PORT->Group[0].PINCFG[28].bit.INEN=1;
	PORT->Group[0].DIR.reg=(0<<28);
	PORT->Group[0].OUT.reg=PORT_PA28;
	
	PORT->Group[0].DIRSET.reg=PORT_PA15;//Config as output
	while(1){
		PORT->Group[0].OUTSET.reg=PORT_PA15;//Put pin as HIGH
		for(i=0;i<50000;i++);
		PORT->Group[0].OUTCLR.reg=PORT_PA15;//Put pin as LOW
		for(i=0;i<50000;i++);
		while(!SERCOM3->USART.INTFLAG.bit.DRE);
		SERCOM3->USART.DATA.reg = 'A';
		while(!SERCOM3->USART.INTFLAG.bit.TXC);	
	}
}
void SERCOM3_Handler(){
	/*if (SERCOM3->USART.INTFLAG.bit.TXC)
	{
		SERCOM3->USART.INTENCLR.bit.TXC=1;
	}*/
}