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PIC 16f690 MATH Problem.....

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vortexmc

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PIC 16f690 MATH Problem......
Hi there guys. I am currently working on a distance measurement application using the sharp rangefinder. I am using a PIC 16F690 microcontroller to read in the analogue voltage signal from the rangefinder. The maximum distance measured by this sensor is 30cm. The problem I am have is writing the math routine to convert the adc value too distance. For this project, I must use PIC assembly language. The equation for the conversion is :

volts = analogRead(IRpin)*0.0048828125
distance = 12.21*exp(volts -1.15)

I am a newbie at PIC assembly, so any help would help me greatly assist me. The ADC is 10 bit. The exp is short for exponent.
 

I must use PIC assembly language
Click to expand...

a not-so-clever way is to check out the floating point routines by microchip;

a slightly-more-clever way is to figure out a way to do it with fixed point math.

the smarter way is to use C.
 

are you sure the distance is 12.21 * e^(v-1.15) ???

in your case, i would go and get some linear device! (something without the 'e' )
 

Just make a lookup table (ADC value to distance). No complex math is required in PIC assembly language.
To access table by 'Flash Program Read'.
To generate table in your favorite language.
EX: JavaScript
Code: 
<html>
<head><script type="text/javascript">
function distance()
{
   var volts;
   var dist; // unit: mm
   var adcV; // ADC value

   // build distance table via ADC value
   document.write( "<pre>adcV2DIST:<br />" );
   for( adcV = 0; adcV < 1024; adcV++ ) {
      volts = adcV * 0.0048828125;
      dist = Math.floor( 12.21 * Math.exp( volts - 1.15 ) * 10 );
      if( (adcV % 16) == 0 ) { document.write( "   dw   " ); }
      str = "" + dist + ","; len = str.length;
      spc = ""; for( i=0; i<6-len; i++) { spc += " "; } // leading space
      document.write( spc,str );
      if( (adcV % 16) == 15 ) { document.write( "<br />" ); }
   }
   document.write( "</pre>" );
}
</script></head>
<body>
   <input type="button" onclick="distance()" value="Generate Table" />
</body>
</html>
 

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