C program to Control the Speed of a Motor

Here is an outline of a C program to control the speed of a motor. Note that it is better to use the DBug12 printf function rather than the C printf function. The reason for this is that, to use the C printf, we would need to define a putchar() function to tell C how to send a byte to the screen. It is easier to just use the DBug13 printf, and convert the floating point number to an integer before printing. 
#include "hc12.h"
#include "DBug12.h"

#define TRUE  1
#define FALSE 0

volatile unsigned int period;
volatile int calc_dc_flag;

main()
{

    float speed;      /* Actual speed, measured using TIC1 */
    float desired;    /* Desired speed from pot */
    float m;          /* Slope to calculate desired speed */
    float b;          /* Intercept to calculate desired speed */
    float fdc;        /* Fractional duty cycle (0.1 to 1.0) */
    float k;          /* Control proportionality constant */
    int i;            /* Temp variable */

    /* Setup stuff 
     * Enable A/D converter, start continuous conversions
     * Enable Timer Subsystem, enable TIC1 interrupt,
     * Enable RTI interrupt 
     * Enable PWM Channel 0, set PWPER0 to 249
     */

    enable();

    PWDTY0 = 124;    /* Start with 50 % duty cycle */

    /* Wait for 10 seconds (160 RTI periods) for motor to come to speed */

    i = 0;
    while (i < 160)
    {
        if (calc_dc_flag)
        {
            i = i + 1;
            calc_dc_flag = FALSE;
        }
    }

    while(TRUE)
    {
        if (calc_dc_flag)
        {
            speed = 1.5e6/((float) period);
            desired = m * ((float) ADR0H) + b;
            fdc = fdc + k * (desired - speed);
            if (fdc < 0.1) fdc = 0.1;
            if (fdc > 1.0) fdc = 1.0;
            PWDTY0 = fdc * (float) (PWPER0 - 1);
            calc_dc_flag = FALSE;
            DBug12FNP->printf("Desired Speed = %f\n\r",(unsigned int) desired);
            DBug12FNP->printf("Actual Speed  = %f\n\r",(unsigned int) speed);
        }
    }
}

@interrupt void rti_isr(void)
{
    calc_dc_flag = TRUE;
    RTIFLG = 0x80;
}

@interrupt void tic1_isr(void)
{

//    Find difference between TC1 now and TC1 last time
    period = ...;
    TFLG1 = xxxx;
}