/**
 * Input change notification interrupt, triggered by the
 * Hall sensor. Used to determine what the motor speed
 * is, so we are able to display the image properly. */
void __ISR(_CHANGE_NOTICE_VECTOR, ipl7) ChangeNotice_Handler(void) {

    /* Time variables used to count clock ticks */
    uint32_t period = 0;
    uint32_t dummy;
    uint8_t hall;

    /* First thing, read the port, so no further
     * interrupts will be generated! */
    hall = HALL;

    /* Clear interrupt flag */
    IFS1bits.CNIF = 0;

    /* We only want rising-edge interrupts, so ignore
     * all others */
    if (hall != 0)
        return;

    /* Calculate how much has passed since last interrupt */
    period = readCounter();

    /* Count again for the next interrupt */
    startCounter();

    /* We must configure a timer to trigger at every column of pixels */
    dummy = period / IMAGE_WIDTH;
    initTimer(dummy);

    /* Reset the column index for the LEDs */
    column = START_POINT; // defined as 0
}