/* p4_2.c UART on SERCOM3 Receive at 115200 Baud
 *
 * Receive key strokes from terminal emulator (TeraTerm) of the
 * host PC to the UART on SERCOM3 of SAMD21 Xplained board. 
 * The UART3 is connected to EDBG virtual connection to the host PC COM port.
 * Launch TeraTerm on a PC and hit any key.
 * The LED program from P3_5 of Chapter 3 is used to blink the yellow LED0
 * according to the key received. 
 * You need to wait till the blinking stops before hitting another key.
 * Received key is not echoed back to the terminal, so you will not set
 * the character displayed.
 *
 * 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);
char UART3_read(void);
void LED_blink(int value);
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) {
    char c;
    UART3_init();
    
    /* initialize PB30 for LED0 */
    REG_PORT_DIRSET1 = 0x40000000;  /* initialize PB30 for LED0 */
    REG_PORT_OUTSET1 = 0x40000000;  /* turn off LED0 */

    while (1) {
        c = UART3_read();           /* wait for a character received */
        LED_blink(c);               /* blink the LED0 */
    }
}

/* initialize UART3 to receive at 115200 Baud */
void UART3_init(void) {
    REG_GCLK_CLKCTRL = 0x4017;      /* GCLK0 to SERCOM3 */
    REG_PM_APBCMASK |= 0x00000020;  /* enable bus clock for SERCOM3 */

    REG_SERCOM3_USART_CTRLA |= 1;               /* reset SERCOM3 */
    while (REG_SERCOM3_USART_SYNCBUSY & 1) {}   /* wait for reset to complete */
    REG_SERCOM3_USART_CTRLA = 0x40104004;       /* LSB first, async, no parity,
     PAD[1]-Rx, BAUD uses no fraction, 8x oversampling, internal clock */
    REG_SERCOM3_USART_CTRLB = 0x00020000;       /* enable Rx, one stop bit, 8 bit */
    REG_SERCOM3_USART_BAUD = 5138;              /* use arithmetic baud rate generation */
    REG_SERCOM3_USART_CTRLA |= 2;               /* enable SERCOM3 */
    while (REG_SERCOM3_USART_SYNCBUSY & 2) {}   /* wait for enable to complete */

    ARRAY_PORT_PINCFG0[23] |= 1;    /* allow pmux to set PA23 pin configuration */
    ARRAY_PORT_PMUX0[11] = 0x20;    /* PA23 = RxD */
}

char UART3_read(void) {
    while(!(REG_SERCOM3_USART_INTFLAG & 4)) {} /* wait until receive complete */
    return REG_SERCOM3_USART_DATA;          /* read the receive char and return it */
}

/* turn on or off the LEDs according to the value */
void LED_blink(int value) {
    value %= 16;                        /* cap the max count at 15 */

    for (; value > 0; value--) {
        REG_PORT_OUTCLR1 = 0x40000000;  /* turn on LED0 */
        delayMs(200);
        REG_PORT_OUTSET1 = 0x40000000;  /* turn off LED0 */
        delayMs(200);
    }
    delayMs(800);
}

/* 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");
}