NTC thermistor temperature sensing

**Acetronics2** · - 14th January 2011, 08:24

Hi,

IF you want to show 1/100 ° ...

better finely tune your calculations @ first :

255/ 3.3 = 77.272727 ... not 78 !!!

PBP can handle ADRes * 2550 / 33 ... sooooooo ...

add to that , here, a linear regression is not the best for a thermistor, and we conclude you will show " something " , but nothing that could be called a temperature.

and I don't talk about the least digit meaning ...

just my two cents ...

Alain

**pxidr84** · - 14th January 2011, 18:44

Well, sorry for these newbie questions...

The precision for my thermoter is not critical, is just a temperature sensor for security purposes. But I want to keep the code for current and voltage sensing purposes (linear curves), so in these cases I need more precision...

I've read many exemples given... I've modified my code in consequence, I can read the integer part (like 23.00 °C), but not the float part...

Here's my code :

Code:

define OSC 20
DEFINE ADC_BITS 8      ' Set number of bits in result
DEFINE ADC_CLOCK 3     ' Set clock source (rc = 3)
DEFINE ADC_SAMPLEUS 10 ' Set sampling time in microseconds 
DEFINE LCD_DREG PORTC 'LCD data port
DEFINE LCD_RSREG PORTC 'LCD data port
DEFINE LCD_EREG PORTC
DEFINE LCD_EBIT 5 
DEFINE LCD_BITS 4 'LCD bus size 4 or 8 
DEFINE LCD_LINES 2 'Number lines on LCD 
DEFINE LCD_COMMANDUS 10000'Command delay time in us 
DEFINE LCD_DATAUS 1000 'Data delay time in us 

AD_AN1_VALUE VAR WORD
B0 VAR WORD
B1 VAR WORD
B2 VAR WORD
B3 VAR WORD

TRISA = %11111111     

ADCON1 = %10000010   

PAUSE 4000

lp:

ADCIN 1, AD_AN1_VALUE

B0 = AD_AN1_VALUE*129   
B1 = B0-8143                                      
B2 = B1/319
PAUSE 100
                      
LCDOUT $fe,1
LCDOUT #AD_AN1_VALUE
Lcdout $fe,$c0,"Temp.: ",DEC (B2 DIG 0),DEC (B2 DIG 1),".",DEC (B2 DIG 2),DEC (B2 DIG 3),"°C"

goto lp

I like to have XX.Y °C (23.2 °C for example).
I know that is a very common problem, but I've spend many hours to try solve this, so...

Example of my calculation :
AD_AN1_VALUE=137
B0=137*129=17673
B1=17673-8143=9530
B2=9530/319=29,874 °C

But here I only get 29,00 °C...

**mackrackit** · - 14th January 2011, 20:10

No problem with the newbie questions, just figured a search would get you there.

Powers of ten...
If you are using an 18F chip it is easy with LONG type variables.

Make the 9530, 953000, mult by 100.
then
953000 / 319 = 2987
then
do the DIG.

What chip are you using?

**Plcguy** · - 14th January 2011, 20:35

PBP can only handle integer, it clips off the .874 dec portion.

where analog in is 0-1024 I would do something like this, tho your scaling would vary :

Analog_In_1:
ADCIN 1, AIN1
Volts_1=AIN1*50/51/2
Start_Seconds=AIN1*50/51/2
HI_1 = Volts_1/100
LOW_1 = Volts_1//100
Lcdout $fe,$c0,"Temp.: ",#HI_1,".",#LOW_1,"°C"

Return

So work in values*100
Divide by 100 for the integer
get the modulus of the Divide by 100 for the floating portion
if you support longs even better!
note // is modulus
I thought B0 was reserved for Byte zero, but I may be thinking of something else.

**Acetronics2** · - 14th January 2011, 21:02

not much better ...

Code:

 
.
.
.
 
AD_AN1_VALUE VAR WORD
B0 VAR WORD
B1 VAR WORD
B2 VAR WORD
B3 VAR WORD
B3 = 100
TRISA  = %11111111     
.
.
.
 
PAUSE 4000
lp:
ADCIN 1, AD_AN1_VALUE
B0 = AD_AN1_VALUE*129   
B1 = B0 - 8143
B1 = B1 * B3                                      
B2 = Div32 319
PAUSE 100
B0 = B2 / 100   ' we get integer part of temp
B1 = B2 // 100  ' we get the decimal part x 100
 
LCDOUT $fe,1
LCDOUT #AD_AN1_VALUE
Lcdout $fe,$c0,"Temp.: ",DEC (B0 DIG 1),DEC (B0 DIG 0),"." ,DEC (B1 DIG 1),DEC (B1 DIG 0),"°C"
goto lp

But with whisles and Bells ...

Alain

**Plcguy** · - 14th January 2011, 21:22

I'm not seeing what "DEC" and "dig" offers over my ver.....
Please enlighten me!

**ScaleRobotics** · - 14th January 2011, 21:46

Oops, sorry Picguy (I thought you were the original poster....

**Plcguy** · - 14th January 2011, 22:22

Originally Posted by scalerobotics

Oops, sorry Picguy (I thought you were the original poster....

I appreciate you saying that, thank you. But my questions stand, AND I have read the manual. My version SEEMS more succinct than the dig / dec version. However, I assume there are advantages to that style, I'm just not seeing them.

Please point them out to me.

**ScaleRobotics** · - 15th January 2011, 00:18

Originally Posted by Plcguy

My version SEEMS more succinct than the dig / dec version. However, I assume there are advantages to that style, I'm just not seeing them.

As for me, I missed your earlier post first, hence my foot in my mouth. But now that I look closer at it, if your //100 remainder is 9, then you will print out xx.9. In Alain's example dig 1 will be 0, dig 0 will be 9, so his result will be xx.09.

Walter

**Acetronics2** · - 15th January 2011, 10:12

Originally Posted by Plcguy

I'm not seeing what "DEC" and "dig" offers over my ver.....
Please enlighten me!

Hi, PlcGuy

The Goal here is to offer solutions showing the differents uses of PBP and try to make people understand what they do ... not to show as-is the - so called - " best " solution.

Pxidr84 had chosen this way of displaying its numbers ... no need to change it, as long as he understands what he does. - I perfectly know it is not the shorter or easier or smarter or .... way of doing it. And we do not talk about Walter's # 12 post ...

The question was about scaling a value ... whithout knowing the Pic type, I showed the DIV 32 use ... which is the simpler for him.

Darrel posted about "BIG Numbers" operations ... and it is the way to get more decimals here ... but not so simple - for a newbee !!!

Now, We Know it is a Pic 18F ... the use of LONGs becomes obvious. It still is easy to use.

I see in your example you just display the ADC input voltage, and not the required temp value, what could raise more questions than offer ideas, for a PBP "newbee" as our friend, as you didn't tell him it doesnt give the awaited result ...
but was just an example.

Not so easy to give a useful and usable answer to a post ... don't you think ???

Alain

PS: for B0, B1 .... absolutely no problem till you do not use the BS1 or 2 to include " compatibility files " ...

**pxidr84** · - 15th January 2011, 14:11

Okay, I've rectified the code for use the 10-bit ADC. Now I get more accurate temperature measurement.
I've also modified the temperature formula (the bin->voltage conversion is now "integrated" within), so I don't need anymore the bin->voltage calculation effetued by the PIC.

Code:

define OSC 20
DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 3  
DEFINE ADC_SAMPLEUS 10
DEFINE LCD_DREG PORTC
DEFINE LCD_RSREG PORTC
DEFINE LCD_EREG PORTC
DEFINE LCD_EBIT 5 

tb0 VAR WORD
tb1 VAR WORD
tb2 VAR WORD
tb3 VAR WORD
tb3=100

TRISA=%11111111     
ADCON1=%10000010   

PAUSE 4000

lp:

ADCIN 1,tb0
tb1=tb0-16170
tb1=tb1*tb3                                      
tb2=Div32 634
tb0=tb2/100   
tb1=tb2//100    
           
PAUSE 100
 
Lcdout $fe,$c0,"IGBT t.: ",DEC (tb0 DIG 1),DEC (tb0 DIG 0),".",DEC (tb1 DIG 1),"°C"

goto lp

**Acetronics2** · - 15th January 2011, 15:25

Hi,

I'm somewhat affraid ...

ADCIN 1,tb0
tb1=tb0-16170
tb1=tb1*tb3
tb2=Div32 634
tb0=tb2/100
tb1=tb2//100

0 < Tb0 < 1024 ... ok ?

1024 - 16170 = ??? - 15146 !!!
so, tb1 is NEGATIVE ...

and PBP do not deal with negative numbers .... but " read " it as 65536 -15146 = ???

a hole in the somewhere ???

Alain

**pxidr84** · - 14th January 2011, 22:12

Originally Posted by Acetronics

not much better ...

Code:

 
.
.
.
 
AD_AN1_VALUE VAR WORD
B0 VAR WORD
B1 VAR WORD
B2 VAR WORD
B3 VAR WORD
B3 = 100
TRISA  = %11111111     
.
.
.
 
PAUSE 4000
lp:
ADCIN 1, AD_AN1_VALUE
B0 = AD_AN1_VALUE*129   
B1 = B0 - 8143
B1 = B1 * B3                                      
B2 = Div32 319
PAUSE 100
B0 = B2 / 100   ' we get integer part of temp
B1 = B2 // 100  ' we get the decimal part x 100
 
LCDOUT $fe,1
LCDOUT #AD_AN1_VALUE
Lcdout $fe,$c0,"Temp.: ",DEC (B0 DIG 1),DEC (B0 DIG 0),"." ,DEC (B1 DIG 1),DEC (B1 DIG 0),"°C"
goto lp

But with whisles and Bells ...

Alain

Thank you so much, it works flawlessly now. Very helpful.

For mackrackit, I use the 18F26K20 (3.3V, 64MHz, 28-DIP), but I will switch to 18F4431 (because mine haven't enough I/O ports, and not enough hardware PWM ports).

**Acetronics2** · - 15th January 2011, 09:47

Originally Posted by pxidr84

Thank you so much, it works flawlessly now. Very helpful.

For mackrackit, I use the 18F26K20 (3.3V, 64MHz, 28-DIP), but I will switch to 18F4431 (because mine haven't enough I/O ports, and not enough hardware PWM ports).

Hi,

IF you use a 18F series, it would be much better to make temp calculations with LONG types ... and then use 10 bits ADC result.

you could easily get a .1°C resolution ( here it is .4°C ... I'm sure you had noticed ! )

...

Alain

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