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A math formula in AD

 
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sf



Joined: 02 Jul 2007
Posts: 9

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A math formula in AD
PostPosted: Tue Sep 28, 2010 9:19 am     Reply with quote

Hi,

Today isn´t a real good day for me, i have been stucked in a math formula, that i can´t realise, well vacations is need........

But if anybody can help me.....thanks in advance.

The formula i need is the next :

Based on an ADC read at 10 bits.
Having the values "MAX" and "MIN".
I need to "translate" the AD read´s to value of "MIN" to "MAX".

For example:
The "MAX" and "MIN" are values that i change any time.
ADC values from 1 to 1024.
Min=50 and Max=300

My result sould be:
for adc=1 -> result=50
for adc=1024 -> result=300

thanks in advance.
sf
Gabriel



Joined: 03 Aug 2009
Posts: 1067
Location: Panama

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PostPosted: Tue Sep 28, 2010 9:35 am     Reply with quote

Code:
input min;
input max;

diff=max-min;
step=diff/1024;

result=adc_value*step;
result+=min;


this is very off the top of my head... i believe that should doit...

use interger math to make it quicker.... its going to take some ram...

and 16-32 bit vars.

hope that helps... or at least puts you on the right path...

sorry for the crude Pseudo code...


Gabriel.
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CCS PCM 5.078 & CCS PCH 5.093
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Tue Sep 28, 2010 9:42 am     Reply with quote

Are you sure about your ADC values?.
The normal ADC return for a 10bit ADC, would be 0 to 1023, not 1 to 1024....
Range though is 1023 counts in each case.

Code:

int32 adcval; //note int32 to avoid arithmetic wrap


    //code to get the value here

    if (adcval<MIN) adcval=MIN;
    else {
        adcval=((adcval*(MAX-MIN)/1023)+MIN;
    }


So for half way (512), 512*250 (MAX-MIN) = 128000
Divided by 1023 (in integer)=125
Plus 50 = 175

Half way between MAX and MIN as required. Same applies for the whole range.

Best Wishes
sf



Joined: 02 Jul 2007
Posts: 9

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PostPosted: Tue Sep 28, 2010 10:17 am     Reply with quote

Smile Thank you both for this great help.

@Ttelmah
The adc is from 0 to 1023 Embarassed I´m thinking in another value.
My mistake is when i put 1 to 1024, i´m was thinking to add 1 to the adc reading´s for make easier other values.

It´s another way to see that i need vacations very fast.........


Thank you all.
SF
Gabriel



Joined: 03 Aug 2009
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PostPosted: Tue Sep 28, 2010 10:26 am     Reply with quote

I believe with the exception of the correction of the min value, we suggested the same idea....(logicwise)

I'll keep that in mind (the min value correction) if I ever need to do something like this.....

nice...

G
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CCS PCM 5.078 & CCS PCH 5.093
Ttelmah



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Posts: 19513

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PostPosted: Tue Sep 28, 2010 2:25 pm     Reply with quote

Absolutely.
I started typing my reply before your's was posted, so "wasn't aware of it".
Logic identical.

Best Wishes
Gabriel



Joined: 03 Aug 2009
Posts: 1067
Location: Panama

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PostPosted: Tue Sep 28, 2010 2:59 pm     Reply with quote

yeah it happens...(simultaneous posting)

yours is the " true answer " though... i didnt take in account the min value correction.... because i didnt consider "limit tests" on my inputs for my solution....

cheers

G
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CCS PCM 5.078 & CCS PCH 5.093
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