AVR Freaks

// lux = ((KA * adc * adc) + (KB * adc)) / SCALE 

#define KA    82      // Coeficient of X^2
#define KB    1487    // Coeficient of X
#define SCALE 10000   // Scale factor 

static int adcval;    // ADC reads put it value here
char sLux[7];   // String with the result of conversion
char sLux2[7];   // String with the result of conversion 

void setup()
{
  Serial.begin(19200);
}
void loop() {
  Serial.println("adcval  calculate   modifiedcalc");
  for (adcval=1; adcval < 1024; adcval+=50) {
    Serial.print(adcval);
    Serial.print("     ");
    Calculate(adcval, KA, KB, SCALE, sLux);   // sLux string seems to have a problem
    Serial.print(sLux); 
    Calculate2(adcval, KA, KB, SCALE, sLux2);   // sLux string seems to have a problem
    Serial.print("         ");
    Serial.println(sLux2);
  }
  delay(10000);
}


void Calculate(int value, int a, unsigned int b, long scale, char *str)
{
  static unsigned long value2;               // value^2
  static unsigned int var;                   // result
  //
  value2 = (unsigned long)(value * value);         // Get value^2
  //
  var = (((unsigned long)(a) * value2) + ((unsigned long)(b) * value)) / scale;
  //
  ultoa(var, str, 10);        // Convert to ASCII string
} 

void Calculate2(int value, int a, unsigned int b, long scale, char *str)
{
  static unsigned long value2;               // value^2
  static unsigned int var;                   // result
  //
  value2 = (unsigned long)value;  // Convert
  value2 *= value2;  // square
  value2 *= (unsigned long)a; // Add a
  value2 += ((unsigned long)(b) * value);
  value2 /= scale;
  var = value2;  // back to 16bits
  ultoa(var, str, 10);        // Convert to ASCII string
} 

adcval  calculate   modifiedcalc
1     0         0
51     28         28
101     98         98
151     209         209
201     36104         361
251     16         553
301     250         787
351     36268         1062
401     303         1378
451     122         1734
501     36263         2132
551     36165         2571
601     36107         3051
651     347         3571
701     371         4133
751     36180         4736
801     6         5380
851     153         6064
901     342         6790
951     33         7557
1001     304         8365

#define KA    82UL      // Coeficient of X^2
#define KB    1487UL    // Coeficient of X
#define SCALE 10000UL   // Scale factor
int lux,adc_value;
char tempstr[8];

//--------------------------------------------------

void Calculate(int adc, char *str)
{
   lux = ((KA * adc * adc) + (KB * adc)) / SCALE ;  
   utoa(lux, str, 10);    // Convert to ASCII string
} 

//--------------------------------------------------

int main(void) 
{                               
   Calculate(adc_value,tempstr);