/* p4_1.c UART on SERCOM3 transmit at 9600 Baud
 *
 * Send a string "YES" to UART on SERCOM3.
 * UART3 is connected to EDBG virtual connection to the host PC COM port.
 * Use TeraTerm to see the message "YES" on a PC.
 *
 * By default, the clock is running at 1 MHz.
 *
 * Tested with Atmel Studio 7 v7.0.1006 
 * and Keil MDK-ARM v5.20 Device Family Pack v1.2.0.
 */

#include "samd21.h"

void UART3_init(void);
void UART3_write(char data);
void delayMs(int n);
unsigned char* ARRAY_PORT_PINCFG0 = (unsigned char*)&REG_PORT_PINCFG0;
unsigned char* ARRAY_PORT_PMUX0 = (unsigned char*)&REG_PORT_PMUX0;

int main (void) {
    UART3_init();

    while (1) {
        UART3_write('Y');
        UART3_write('e');
        UART3_write('s');
        delayMs(10); /* leave a gap between messages */
    }
}

/* initialize UART3 to transmit at 9600 Baud */
void UART3_init(void) {
    REG_PM_APBCMASK |= 0x00000020;  /* enable bus clock for SERCOM3 */
    REG_GCLK_CLKCTRL = 0x4017;      /* GCLK0 to SERCOM3 */

    REG_SERCOM3_USART_CTRLA |= 1;               /* reset SERCOM3 */
    while (REG_SERCOM3_USART_SYNCBUSY & 1) {}   /* wait for reset to complete */
    REG_SERCOM3_USART_CTRLA = 0x40106004;       /* LSB first, async, no parity,
        PAD[0]-Tx, BAUD uses fraction, 8x oversampling, internal clock */
    REG_SERCOM3_USART_CTRLB = 0x00010000;       /* enable Tx, one stop bit, 8 bit */
    REG_SERCOM3_USART_BAUD  = 13;               /* 1000000/8/9600 = 13.02 */
    REG_SERCOM3_USART_CTRLA |= 2;               /* enable SERCOM3 */
    while (REG_SERCOM3_USART_SYNCBUSY & 2) {}   /* wait for enable to complete */
        
    ARRAY_PORT_PINCFG0[22] |= 1;    /* allow pmux to set PA22 pin configuration */
    ARRAY_PORT_PMUX0[11] = 0x02;    /* PA22 = TxD */
}

void UART3_write(char data) {
    while(!(REG_SERCOM3_USART_INTFLAG & 1)) {}  /* wait for data register empty */
    REG_SERCOM3_USART_DATA = data;              /* send a char */
}

/* millisecond delay based on 1 MHz system clock */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 199; i++)
            __asm("nop");
}