/* p10_2.c: Stepper motor control
 *
 * This program controls a unipolar stepper motor 
 * using PB00, PB01, PB02, PB03.
 *
 * Tested with Atmel Studio 7 v7.0.1006 and Keil MDK-ARM v5.21a.
 */

#include "samd21.h"

void delayMs(int n);

int delay = 10;
int direction = 0;

int main(void) {
    const char steps[ ] = {0x9, 0x3, 0x6, 0xC};
    int i = 0;

    REG_PORT_DIRSET1 = 0x0000000F;      /* make PB03-PB00 output */

    while (1) {
        if (direction) {
            REG_PORT_OUTCLR1 = 0xF;     /* clear four control bits */
            REG_PORT_OUTSET1 = (steps[i++ & 3]); /* set control bits */
        } else {
            REG_PORT_OUTCLR1 = 0xF;     /* clear four control bits */
            REG_PORT_OUTSET1 = (steps[i-- & 3]); /* set control bits */
        }
        delayMs(delay);
    }
}

/* 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");
}