AVR Freaks

Uwe60, I agree with Lars's posts #11 and #14.  I was going to reply to your post #10 but Lars wrote what I would have written.

The include files have nothing to do with Start or ASF in particular.  They contain the manufacturers definitions for the chip and formulate a standard.  Using your own defines only makes your code non-standard.  

If you were to use the include files your code could be used by anybody and would be standard.  Using the standard defines will also speed up your coding.

Code as you wish but please use the standard defines when you are sharing code, otherwise you are only causing more confusion for the OP when he tries to compare your code to others' code.

Also a bit of friendly advice, use spaces in your code, whitespace takes up no code space.  I find your code quite difficult to read, that makes comprehension of it all the more difficult.

EDIT: I admire your enthusiasm and effort, but much is missing:

No, please don't remove your code. That would make the thread context incomplete.  Lets try to make it better for the OP, I will try to help with that using your code as a base.  Consider it a learning opportunity for us all.

@maaply, Ran out of time to complete it, so this code for the SAM D20 Xplained Pro does not have a fifo or ring buffer and it does nothing more than echo the typed character back out to the PC, and as all good micro programs should, it blinks an LED.  Only receive interrupts are in place at this time, transmit is handled by Sercom3_putchar().   Baud rate is set at 115200.

This code does not use ASF, Start, or any other code foundation, it uses only that which is provided in the include files from the pack for the SAM D20.  The code is not elegant, that is Uwe's fault​, but it does show how to initialize the SERCOM3 USART on the SAM D20 Xplained Pro.  I would like to thank Uwe for providing the code for the SAM D10 version, that saved an enormous amount of time to migrate the code to the SAM D20, I learned a lot and the effort was worth every minute.

Create an AS7 C project, copy these files into it, study them, then run them.  I will leave it as an exercise for you to add the code for the buffer and any other features that you want.

main.c

/*
* D20USART.c - really main.c
*
* For use on a SAM D20 Xplained Pro
*
* Created: 3/15/2018 13:17:44
* Author : Larry larryvc based on code by Uwe Uwe60
*/

#include "sam.h"
#include "stdbool.h"

volatile uint8_t sercom3_errors, sercom3_char_received, sercom3_data;

void Sercom3_Init(void);
char Sercom3_putchar(char ch);
char Sercom3_getchar();

int main(void)
{
	REG_PORT_DIRSET0 = PORT_PA14;  // SAM D20 Xplained Pro LED

	// Initialize the SAM system

	SystemInit();

	// 8MHz oscillator - set to divide by 1
	// Allows for good BAUD rates up to 460800 - higher BAUD rates require a DFFL48 derived clock
	// Same as SYSCTRL->OSC8M.bit.PRESC = 0;
	REG_SYSCTRL_OSC8M &= ~SYSCTRL_OSC8M_PRESC_Msk;
	while ((REG_SYSCTRL_PCLKSR & SYSCTRL_PCLKSR_OSC8MRDY) == 0);

	Sercom3_Init();

	while (1) {

		if(sercom3_char_received ==  true) {
			if (sercom3_errors == false) {
				Sercom3_putchar(sercom3_data); // send received character
				sercom3_char_received = false;
				} else {
				// Do something about the error
				//while (1) { // trap for debug
				//	for(volatile uint32_t i = 0; i <= 10000; i++);
				//	REG_PORT_OUTTGL0 = PORT_PA14;
				sercom3_errors = false; // Ignored the char
			}
		}

		// Crude delay loop
		// Should use Systick for timing
		for(volatile uint32_t i = 0; i <= 100000; i++);
		REG_PORT_OUTTGL0 = PORT_PA14;
	}
}

sercom3usart.c

/*
* sercom3usart.c
*
* For use on a SAM D20 Xplained Pro
*
* Created: 3/17/2018 01:13:11
* Author : Larry larryvc based on code by Uwe Uwe60
*/

#include "sam.h"
#include "stdbool.h"

// BAUD calculation
#define CLOCKRATE 8000000
#define BAUDRATE 115200
#define BAUD 65536UL - ((uint64_t)65536 * 16 * BAUDRATE) / CLOCKRATE

#define REC_ERR 2

extern uint8_t sercom3_errors, sercom3_char_received, sercom3_data;

// Using Interrupt for receive only, Sercom3putchar for transmit
void SERCOM3_Handler()
{
	if(REG_SERCOM3_USART_INTFLAG & SERCOM_USART_INTFLAG_RXC)	//character received?
	{
		sercom3_char_received = true;
		sercom3_data = REG_SERCOM3_USART_DATA;	//fetch received character

		if(REG_SERCOM3_USART_STATUS & (SERCOM_USART_STATUS_FERR | SERCOM_USART_STATUS_BUFOVF))	//receive errors happened?
		{
			REG_SERCOM3_USART_STATUS = SERCOM_USART_STATUS_FERR | SERCOM_USART_STATUS_BUFOVF;	//reset errors (framing and buffer overflow error)
			sercom3_errors |= REC_ERR;	//set global rec error flag in my global error variable not to use the char
		}

	}
}

char Sercom3_putchar(char ch)
{
	while ((REG_SERCOM3_USART_INTFLAG & SERCOM_USART_INTFLAG_DRE) == 0);
	REG_SERCOM3_USART_DATA = ch;
	return ch;
}

char Sercom3_getchar(char ch)
{
	while ((REG_SERCOM3_USART_INTFLAG & SERCOM_USART_INTFLAG_RXC) == 0);
	return REG_SERCOM3_USART_DATA;
}

void Sercom3_Init(void)
{
	// PORT Setup

	// SERCOM3 PAD[2] PIN_PA24 is TXD, SERCOM3 PAD[3] PIN_PA25 is RXD
	// Set PAD[2] PIN_PA24 TXD to output
	// #define REG_PORT_DIRSET0           (*(RwReg  *)0x41004408UL) /**< \brief (PORT) Data Direction Set 0 */
	REG_PORT_DIRSET0 = PORT_PA24;

	// Set PMUXEN bit for both PINCFG24 PIN_PA24 TXD and PINCFG25 PIN_PA25 RXD
	// 24 8 bit registers offset to PINCFG24, pointer is 16 bit to handle int value, (32 - 24 * 2) = 12 16 bit pointer offset
	// Uses 4 instructions, 7 clock cycles, 12 bytes including pointer address
	// #define REG_PORT_PINCFG0           (*(RwReg  *)0x41004440UL) /**< \brief (PORT) Pin Configuration 0 */
	// Faster and shorter than
	// PORT->Group[0].PINCFG[24].reg |= PORT_PINCFG_PMUXEN;
	// PORT->Group[0].PINCFG[25].reg |= PORT_PINCFG_PMUXEN;
	*((uint16_t *)&REG_PORT_PINCFG0 + 12) = (PORT_PINCFG_PMUXEN) | (PORT_PINCFG_PMUXEN << 8);

	// Set PMUX12 to peripheral function C for PIN_PA24 TXD and PIN_PA25 RXD
	// 12 8 (4+4) bit registers offset to PMUX12
	// Uses 3 instructions, 5 clock cycles, 10 bytes including pointer address
	// #define REG_PORT_PMUX0             (*(RwReg  *)0x41004430UL) /**< \brief (PORT) Peripheral Multiplexing 0 */
	// Same result as PORT->Group[0].PMUX[12].reg = PORT_PMUX_PMUXO_C | PORT_PMUX_PMUXE_C;
	*((uint8_t *)&REG_PORT_PMUX0 + 12) = PORT_PMUX_PMUXO_C | PORT_PMUX_PMUXE_C;

	// Clock Setup

	// Set up GCLK - Generic Clock Controller
	// GCLK_CLKCTRL - GEN Generic Clock Generator is GCLKGEN0
	// GCLK_CLKCTRL - ID Generic Clock Selection ID is GCLK_SERCOM3_CORE
	// GCLK_CLKCTRL - CLKEN Clock Enable is enabled
	// #define REG_GCLK_CLKCTRL           (*(RwReg16*)0x40000C02UL) /**< \brief (GCLK) Generic Clock Control */
	REG_GCLK_CLKCTRL |= GCLK_CLKCTRL_GEN_GCLK0 | GCLK_CLKCTRL_ID_SERCOM3_CORE | GCLK_CLKCTRL_CLKEN;
	while(REG_GCLK_STATUS & GCLK_STATUS_SYNCBUSY);

	// Power Management

	// Enable peripheral clock for SERCOM3
	// APBC Mask SERCOM3 SERCOM3 APB Clock Enable is enabled
	// #define REG_PM_APBCMASK            (*(RwReg  *)0x40000420UL) /**< \brief (PM) APBC Mask */
	REG_PM_APBCMASK |= PM_APBCMASK_SERCOM3;

	// SERCOM3 Setup

	// SERCOM3 USART CTRLA register setup
	// Disables SERCOM3 registers Enable-Protection so we can write to all the SERCOM3 registers
	// SERCOM3_USART_CTRLA - Mode = USART with internal clock
	// SERCOM3_USART_CTRLA - RUNSTDBY = Generic clock is enabled in all sleep modes. Any interrupt can wake up the device.
	// SERCOM3_USART_CTRLA - TXPO assigned to PAD2
	// SERCOM3_USART_CTRLA - RXPO assigned to PAD3
	// SERCOM3_USART_CTRLA - DORD = LSB is transmitted first
	// #define REG_SERCOM3_USART_CTRLA    (*(RwReg  *)0x42001400UL) /**< \brief (SERCOM3) USART Control A */
	REG_SERCOM3_USART_CTRLA = SERCOM_USART_CTRLA_MODE_USART_INT_CLK | SERCOM_USART_CTRLA_RUNSTDBY | SERCOM_USART_CTRLA_TXPO_PAD2
							  | SERCOM_USART_CTRLA_RXPO_PAD3 | SERCOM_USART_CTRLA_DORD;
	while(REG_SERCOM3_USART_STATUS & SERCOM_USART_STATUS_SYNCBUSY);

	// SERCOM3 USART BAUD register setup
	// #define REG_SERCOM3_USART_BAUD     (*(RwReg16*)0x4200140AUL) /**< \brief (SERCOM3) USART Baud */
	REG_SERCOM3_USART_BAUD = BAUD;

	// SERCOM3 USART CTRLB register setup
	// TXEN is enabled
	// RXEN is enabled
	// DORD = LSB is transmitted first
	// #define REG_SERCOM3_USART_CTRLB    (*(RwReg  *)0x42001404UL) /**< \brief (SERCOM3) USART Control B */
	REG_SERCOM3_USART_CTRLB = SERCOM_USART_CTRLB_TXEN | SERCOM_USART_CTRLB_RXEN;
	while(REG_SERCOM3_USART_STATUS & SERCOM_USART_STATUS_SYNCBUSY);

	// Enable SERCOM3 interrupts
	// RXC Receive Complete Interrupt Enabled
	// #define REG_SERCOM3_USART_INTENSET (*(RwReg8 *)0x4200140DUL) /**< \brief (SERCOM3) USART Interrupt Enable Set */
	REG_SERCOM3_USART_INTENSET = SERCOM_USART_INTENSET_RXC;

	// Enable SERCOM3 NVIC Interrupt
	NVIC_EnableIRQ(SERCOM3_IRQn);

	// Enable SERCOM3
	// #define REG_SERCOM3_USART_CTRLA    (*(RwReg  *)0x42001400UL) /**< \brief (SERCOM3) USART Control A */
	REG_SERCOM3_USART_CTRLA |= SERCOM_USART_CTRLA_ENABLE;
	while(REG_SERCOM3_USART_STATUS & SERCOM_USART_STATUS_SYNCBUSY);
}

EDIT: Changed  the comments in block starting with "Set PMXEN bit for both PINCFG24 PIN_PA24 TXD and PINCFG PIN_PA25 RXD"