/* p5_4.c Toggle LED0 on SAMD21 Xplained Pro using TC3
 *		
 * This program uses TC3 to toggle PB30 at 4 Hz. The LED0 on board
 * will blink at 2 Hz.
 * TC3 is configured to run continuously in 8-bit mode. TC3 counter 
 * clock is derived from 1 MHz main clock. The prescaler divides the
 * GCLK_TC3 by 1024. The TOP count is set to 244-1. The counter
 * overflows at 1 MHz / 1024 / 244 = 4. Every time the counter
 * overflows, PB30 is toggled. The yellow LED0 on board is connected
 * to PB30.
 *
 * Tested with Atmel Studio 7 v7.0.1006 and Keil MDK-ARM v5.21a.
 */

#include "samd21.h"

int main (void) {
    REG_GCLK_CLKCTRL = 0x401B;      /* GCLK0 -> TCC2/TC3 */
    REG_PM_APBCMASK |= 0x00000800;  /* enable TC3 Clock in PM */
    REG_TC3_CTRLA = 1;              /* reset */
    while (REG_TC3_CTRLA & 1) {}    /* wait till out of reset */
    REG_TC3_CTRLA = 0x0704;         /* prescaler /1024, 8-bit mode, NFRQ */
    REG_TC3_COUNT8_PER = 244 - 1;   /* maximum count value in 8-bit mode */
    REG_TC3_CTRLA |= 2;             /* enable TC3 */
    REG_PORT_DIR1 |= 0x40000000;    /* make PB30 output */

    while(1) {
        if (REG_TC3_INTFLAG & 1) {  /* wait until OVF is set */
            REG_TC3_INTFLAG = 1;    /* clear OVF flag */
            REG_PORT_OUT1 ^= 0x40000000;    /* toggle LED0 */
        }
    }
}