/* p5_1.c Toggle LED0 on SAMD21 Xplained Pro using SysTick

 * This program configures SysTick to be a 24-bit free-running
 * down-counter.
 * Bit 20 of SysTick current value is written to the LED0
 * (PB30) continuously.
 * SysTick is based on system clock running at 1 MHz. 
 * So bit 20 of SysTick current value toggles about 1 Hz
 * (1,000,000 Hz / 2^20 = 0.9537 Hz).
 *
 * Tested with Atmel Studio 7 v7.0.1006 and Keil MDK-ARM v5.21a.
 */

#include "samd21.h"

int main (void) {
    REG_PORT_DIRSET1 = 0x40000000; /* make PB30 output for LED */
    
    /* Configure SysTick */
    SysTick->LOAD = 0xFFFFFF;   /* reload with max value */
    SysTick->CTRL = 5;     /* enable it, no interrupt, use system clock */

    while (1) {
        /* take bit 20 of SysTick current value and shift it to bit 30 
           then write it to PortB */
         REG_PORT_OUT1 = (SysTick->VAL & 0x100000) << (30-20);
    }
}