/* p4_4.c UART on SERCOM4 echo at 9600 Baud
 *
 * This program receives a character from UART on SERCOM4 receiver
 * then sends it back through UART on SERCOM4 transmitter.
 * UART on SERCOM4 is connected to PB08-Tx and PB09-Rx.
 * A 3.3V signal level to USB cable is used to connect PB08/PB09
 * to the host PC COM port.
 * Use TeraTerm on the host PC to send keystrokes and observe the display
 * of the characters echoed.
 *
 * 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 UART4_init(void);
void UART4_write(char data);
char UART4_read(void);

unsigned char* ARRAY_PORT_PINCFG1 = (unsigned char*)&REG_PORT_PINCFG1;
unsigned char* ARRAY_PORT_PMUX1 = (unsigned char*)&REG_PORT_PMUX1;

int main (void) {
    char c;
    UART4_init();

    while (1) {
        c = UART4_read();
        UART4_write(c);
    }
}

/* initialize UART4 to transmit at 9600 Baud */
void UART4_init(void) {
    REG_PM_APBCMASK |= 0x00000040;  /* enable bus clock for SERCOM4 */
    REG_GCLK_CLKCTRL = 0x4018;      /* GCLK0 to SERCOM4 */

    REG_SERCOM4_USART_CTRLA |= 1;               /* reset SERCOM4 */
    while (REG_SERCOM4_USART_SYNCBUSY & 1) {}   /* wait for reset to complete */
    REG_SERCOM4_USART_CTRLA = 0x40106004;       /* LSB first, async, no parity,
        PAD[1]-Rx, PAD[0]-Tx, BAUD uses fraction, 8x oversampling, internal clock */
    REG_SERCOM4_USART_CTRLB = 0x00030000;    /* enable Tx, Rx, one stop bit, 8 bit */
    REG_SERCOM4_USART_BAUD  = 13;               /* 1000000 / 8 / 9600 = 13.02 */
    REG_SERCOM4_USART_CTRLA |= 2;               /* enable SERCOM4 */
    while (REG_SERCOM4_USART_SYNCBUSY & 2) {}   /* wait for enable to complete */

    ARRAY_PORT_PINCFG1[8] |= 1;     /* allow pmux to set PB08 pin configuration */
    ARRAY_PORT_PINCFG1[9] |= 1;     /* allow pmux to set PB09 pin configuration */
    ARRAY_PORT_PMUX1[4] = 0x33;     /* PB08 = TxD, PB09 = RxD */
}

void UART4_write(char data) {
    while(!(REG_SERCOM4_USART_INTFLAG & 1)) {}  /* wait for data register empty */
    REG_SERCOM4_USART_DATA = data;              /* send a char */
}

char UART4_read(void) {
        while(!(REG_SERCOM4_USART_INTFLAG & 4)) {}  /* wait until receive complete */
        return REG_SERCOM4_USART_DATA;       /* read the receive char and return it */
}