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Problem int32 over flow

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raman00084

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Im having the problem in reading the adc for the ac signal
it is working for the 800mv above the 800mv
im reading only 800mv.

i think the problem in int32 overflow due to the squaring the int16 adc value.
im using ccs c complier for the pic18f
is there any option to use 64bit multiplication in ccs for 8 bit microcontroller

following is the code.


Code: 
float read_adc_AC(unsigned char adc_inChannel,int16 samples,unsigned int16 in_usADCzero)
{
   unsigned int16 usADCValue = 0 ;// usAccADC = 0;
   unsigned int16 usLoop = 0 ;
    int32 ulAccuADCvalue = 0;
    int32 tmpadc = 0;
   float ulTemp = 0;
   float ftemp = 0;

   set_adc_channel( adc_inChannel);
 for(usLoop = 0 ; usLoop <samples; usLoop ++)
   {
     
      usADCValue = read_adc() - in_usADCzero ;
    ulAccuADCvalue += (int32)(usADCValue*usADCValue);
   delay_us(155);

   }
  ulTemp = sqrt(ulAccuADCvalue /samples);
 
//!   return (ulTemp*(4.880)) ; 
return (ulTemp) ; 
  
}
 

the above code is used for the ac current measurement on hall effect sensor acs712/20A
 

I assume, the ADC values have only 12 significant bits, so you won't need 64 bit arithmetic.

Your code has however a "popular" fault. It's squaring a 16-bit value with 16 bit result. Review a C programming text book about type conversion rules in case of doubt.
Code: 
(int32)(usADCValue*usADCValue);
must be changed into
Code: 
((int32)usADCValue*usADCValue);
 
Hi,

An 32 bit overflow doesn't result in a fixed valud, it is wrapped around instead.

So I don't think it is an 32 bit overflow.

Klaus
 
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