[SOLVED] Overflow Error , RMS value shown wrong .

[Raady Here]

Hi ,

I am measuring AC Voltage and I am calculating rms value for ADC.

I have an array which is having 128 samples of my signal.
while squaring number i get error.

        unsigned long int calval=0;
	unsigned int loop;
	float adcbufval;
	for(loop=0;loop<128;loop++)
	{
		printf("adcval %d = %d\t ",loop,adc_temp[loop]);
		calval = (adc_temp[loop])*(adc_temp[loop]);
		printf("\t %ld \n",calval);


               // calval += (adc_temp[loop])*(adc_temp[loop]);
               // adcbufval =(((sqrt(calval/samples))/4096)*OPV);
	       //printf("\n\r %f",adcbufval);
               //mesuredvoltage= adcbufval * fluke;        // fluke =249.62 as calculated
               // Printf("Volts = %f",measuredvolts);        // shown on UART.
	}

output :

adcval 1 = 168 28224
adcval 2 = 32 1024
adcval 3 = -88 7744
adcval 4 = -211 44521 // sqr(211) 44521 , it fine here
adcval 5 = -314 33060 // sqr(314) 98596-65536 = 33060 instead of 98596.
adcval 6 = -416 41984
adcval 7 = -522 10340
adcval 8 = -655 35809
adcval 9 = -773 7705
adcval 10 = -889 3889


Though I defined 'calval' as unsigned long int (range 0-4,294,967,295), it get over flowed at 65536 value.
In normal c complier its working fine.
Any suggestions, help.

Edit :
using dsPIC30f5011 / MPLAB 8.8x

Regards,
Raady.

 

[FvM]

You missed to tell the type of adc_temp[]. I guess it's signed int (16-Bit), which pretty explaines the problem.

Refer to a C text book regarding implicite type conversion. The type of a product is defined by the largest type involved in the multiplication. You need to type cast one multiplicand to 32-bit before.

 

[Raady Here]

Yes i have used 'adc_temp' as signed int ,
I have done as you mentioned.

for(loop=0;loop<128;loop++)
	{
		printf("adcval %d = %d\t\t",loop,adc_temp[loop]);
		calval = ((unsigned long int)(adc_temp[loop])*(adc_temp[loop]));
		printf("\t\t\t %ld \n",calval);

The output shows fine for positive numbers but negative numbers take the 2's complement,

adcval 88 = -47 -6158175
adcval 89 = -151 -19769071
adcval 90 = -240 -31399680
adcval 91 = -331 -43275271

Let me know where I was actually wrong !

 

[FvM]

The error is typecasting negative number to unsigned long, should be signed long.