/* p6_5.c Toggle the LED0 using the SysTick interrupt
 *
 * This program sets up the SysTick to interrupt at 1 Hz.
 * The system clock is running at 1 MHz.
 * 1 sec/1 us = 1,000,000 for RELOAD register.
 * In the interrupt handler, the LED0 is toggled.
 *
 * Tested with Atmel Studio 7 v7.0.1006 and Keil MDK-ARM v5.21a 
 */

#include "samd21.h"

int main(void) {
    __disable_irq();
    REG_PORT_DIRSET1 = 0x40000000;  /* make PB30 output for LED */

    /* Configure SysTick */
    SysTick->LOAD = 1000000-1;      /* reload with number of clocks per second */
    SysTick->VAL = 1000000-1;
    SysTick->CTRL = 7;              /* enable SysTick interrupt, use system clock */
    __enable_irq();

    while (1) {
    }
}

void SysTick_Handler(void) {
    REG_PORT_OUTTGL1 = 0x40000000;  /* toggle LED0 */
}