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Requan
Joined: 11 May 2008
Posts: 74
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| Simple thing but problem |
 Posted: Fri Jan 16, 2015 11:36 am |
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Dear CCS Member,
I wrote program to read temperature from PT1000 via external ADC.
| Code: | const float ResistanceTable[]=
{
803.06,
842.71,
882.22,
921.6,
960.86,
1000,
1039.02,
1077.92,
1116.71,
1155.39,
1193.95,
1232.39,
1270.71,
1308.92,
1347.02,
1385,
1422.86,
1460.6,
1498.23,
1535.75,
1573.1
};
float get_res_x()
{
int16 upper, lower, config, ans;
i2c_start();
i2c_write (0xD0); // 1101 0000 -address byte write (SELECT ADDRESS A0 AND SET IN WRITE MODE) FOR MCP3421A0
i2c_write (0x88); // ADC Gain 1
i2c_stop();
i2c_start ();
i2c_write (0xD1); // 1101 0001 - CHANGE TO READ MODE (ADDRESS A0) FOR MCP3421A0
upper = i2c_read (1); // read slave data
lower = i2c_read (1); // read slave data
config = i2c_read (0); // read slave data
i2c_stop ();
ans =( upper * 256) + lower;
if(ans>32767 )
{
ans=0;
}
return ans;
}
float CalcPT1000(int32 AdcRes)
{
float PtR, HiR, LoR;
float LoTemp;
int8 i;
PtR = (2.048f / 32768.0f) * (float)AdcRes;
PtR *= 1000;
for(i=0; i<22; i++)
{
if(ResistanceTable[i]>PtR)
{
HiR = ResistanceTable[i];
LoR = ResistanceTable[i-1];
LoTemp = (i-6) * 10;
break;
}
}
PtR -= LoR;
PtR /= (HiR - LoR);
PtR *= 10;
PtR += LoTemp;
return (float)(PtR*10);
}
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It worked fine, but when i put data to int16 table index 1, example:
| Code: |
adc_val = get_res_x();
valuemodbus[1] = (int16)CalcPT1000(adc_val);
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For first measure data is in index1, for second is on index 2, so in valuemodbus[2].
I think i made forbidden think, cause to weird table behavior.
Could You look at code and write me any suggestion?
Martin |
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asmallri
Joined: 12 Aug 2004
Posts: 1636
Location: Perth, Australia
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| Posted: Fri Jan 16, 2015 2:22 pm |
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Your two line sample code at the end does not provide enough information to see what is going wrong and I could not understand your description of the problem.
For example, where is the declaration of the array valuemodbus[] ?
In C arrays start at offset 0 therefore the first array element of your array would be valuemodbus[0]
_________________
Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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gpsmikey
Joined: 16 Nov 2010
Posts: 588
Location: Kirkland, WA
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| Posted: Fri Jan 16, 2015 2:24 pm |
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I may be missing it (it has been one of those days today), but where is the array "valuemodbus[]" defined??
mikey
[edit] I see Andrew posted while I was posting :-)
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3 |
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Requan
Joined: 11 May 2008
Posts: 74
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| Posted: Sat Jan 17, 2015 4:37 am |
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sorry, i put piece of code i have problem and forgot to paste declaration, here it is:
| Code: | int16 valuemodbus[10];
int32 adc_val; |
i tried also as int32 but no help.
Now, i am wondering if compiler can do this conversion in proper way:
| Quote: | | (int16)CalcPT1000(adc_val); |
maybe i should to do it by union? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19562
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| Posted: Sat Jan 17, 2015 4:47 am |
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A union can't help with this, and 'yes', the compiler will convert a float to an integer for you.
I'd suspect the value you are getting from your function is not what you think it is. Step back, and get rid of all the maths, and display the number you are actually reading from the chip.
If this isn't what you expect, your maths will then not work... |
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