Temperature does not go below 0

[amodpathak]

IAM MAKING TEMPERATURE CONTROLLER USING ntc THERMISTOR FROM -10 DEGREE TO 50 DEGREE.BUT MY TEMPERATURE IS NOT GOING BELOW 0. PLS CHECK MY CODE AND GUIDE ME.THANKS




DG1 VAR BYTE
DG2 VAR BYTE
DG3 VAR BYTE
DIGIT VAR BYTE
DG VAR BYTE
COUNTT VAR BYTE

NUMB VAR WORD
N VAR WORD
adval var word
temp var word
TEMPP VAR WORD

ADCON1=%10000000
adcon0=%00000101
TRISA=1
TRISB=%00111000
TRISC=0

numb=0
gosub mread
'n=numb
GOSUB DIGITCALC
'-------------------------------------------------------------------------


MAIN1:


GOSUB DISPLAY

IF PORTB.3=1 THEN MREAD

IF PORTB.3=0 THEN MAIN




GOTO MAIN1









'-------------------------------------------------------------------------
MAIN:


ADCON0.2=1


GO:
IF ADCON0.2=1 THEN GO

adval.HIGHBYTE = ADRESH
adval.lowbyte = ADRESL
temp=adval
N=temp
gosub display
gosub digitcalc


If adval>NUMB then PORTB.7=0
IF ADVAL< NUMB-5 THEN PORTB.7=1
GOTO MAIN1




'_____________________________________


UP:
IF NUMB=50 THEN MAIN
NUMB=NUMB+1
N=NUMB
GOSUB DIGITCALC
IF PORTB.4=1 THEN MEMORY
GOTO MAIN1


memory:
WRITE 0,numb.HighByte

WRITE 1,numb.LowByte

GOTO MAIN1


'_____________________________________


DOWN:
IF NUMB=-10 THEN MAIN 'MIN LIMIT
NUMB=NUMB-1
N=NUMB
GOSUB DIGITCALC
IF PORTB.5=1 THEN MEMORY
GOTO MAIN1


'_____________________________________
MREAD:
READ 0,numb.HighByte

READ 1,numb.LowByte

n=numb
gosub digitcalc
IF PORTB.4=1 THEN UP
IF PORTB.5=1 THEN DOWN
goto main1











__________________________________________________ _
DISPLAY:


FOR COUNTT=0 TO 99


PORTC=DG1
PORTB.2=1
PAUSEUS 300
PORTB.2=0


PORTC=DG2
PORTB.1=1
PAUSEUS 300
PORTB.1=0


PORTC=DG3
PORTB.0=1
PAUSEUS 300
PORTB.0=0


NEXT COUNTT


RETURN


'_________________________________________________ _______________






'_________________________________________________ _________________________
DIGITCALC:


DIGIT=0
LP1:
IF N<100 THEN DS1
N=N-100
DIGIT=DIGIT+1
GOTO LP1
DS1:
GOSUB FND
DG1=DG



DIGIT=0
LP2:
IF N<10 THEN DS2
N=N-10
DIGIT=DIGIT+1
GOTO LP2


DS2:
GOSUB FND
DG2=DG



DIGIT=N
GOSUB FND
DG3=DG





RETURN








'_________________________________________________ _________________________




'_________________________________________________ _________________________
FND:








FND0:
IF DIGIT>0 THEN FND1
DG=$7E '%0111 1110
GOTO FNDEND


FND1:
IF DIGIT>1 THEN FND2
DG=$48 '%0100 1000
GOTO FNDEND


FND2:
IF DIGIT>2 THEN FND3
DG=$3D '%0011 1101
GOTO FNDEND


FND3:
IF DIGIT>3 THEN FND4
DG=$6D '%0110 1101
GOTO FNDEND


FND4:
IF DIGIT>4 THEN FND5
DG=$4B '%0100 0011
GOTO FNDEND


FND5:
IF DIGIT>5 THEN FND6
DG=$67 '%0110 0111
GOTO FNDEND


FND6:
IF DIGIT>6 THEN FND7
DG=$77 '%0111 0111
GOTO FNDEND


FND7:
IF DIGIT>7 THEN FND8
DG=$4C '%0100 1100
GOTO FNDEND


FND8:
IF DIGIT>8 THEN FND9
DG=$7F '%0111 1111
GOTO FNDEND


FND9:
DG=$6F '%0110 1111


FNDEND:
RETURN

'_________________________________________________ _________________________

[HenrikOlsson]

Hi,
With PBP variables declared as BIT, BYTE and WORD are unsigned, they are always treated as positive.
Of course you can handle that yourself with something like.


IF NUMB.15=1 THEN ' MSB set, value is negative
IF ABS NUMB=10 THEN MAIN ' MIN LIMIT
ENDIF


/Henrik.

[amodpathak]

i tried that code but again temp is not going below 0.No minus sign.7848

[Scampy]

I always add comments to my listings, which also helps others follow the code and my "logic" when I have issues and post the code up seeking advice. I find it hard to work through the OP's code, but if you are using A-D I'm presuming that 0volts would equate to 0 and 5v 255, or something like that. I'm thinking that you need to scale the range or something so that, for example 127 digital (2.5v approx in this example) would be 0 degrees, and that any value between 0 and 127 would be treated as a minus value ? Then using the if then statement similar to if value is less than or equal to 126 then place the - symbol in the 1st LED,

[Archangel]

Your Code Goes Here

[amodpathak]

Thanks Scampy for clearing my doubts .what you explained is very easy to understand..thanks & regards

[amodpathak]

An example needed.iam using 10 bit adc and sensor is 10k thermistor.

[EarlyBird2]

An example needed.iam using 10 bit adc and sensor is 10k thermistor.

A Thermistor changes resistance with temperature change. To read a 'remote' Thermistor a 'completion resistor' (CR), typically 1000 ohms, is used in series and a known voltage is applied to the circuit then the voltage drop across the CR is measured. If the thermistor is on the board then voltage drop across the Thermistor can be measured directly. The resistance changes with temperature change which is not linear and this is required

http://en.wikipedia.org/wiki/Thermistor#NTC


Steinhart–Hart equation

For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The Steinhart–Hart equation is a widely used third-order approximation:
1 \ T = a + b\,\ln(R) + c\,(\ln(R))^3
where a, b and c are called the Steinhart–Hart parameters, and must be specified for each device. T is the absolute temperature and R is the resistance.

As you can see natural logarithms are needed to use this conversion, which are not available with PBP.

[Scampy]

Why not use a DS18B20 - It would be a lot simpler going digital

[Charlie]

As you can see natural logarithms are needed to use this conversion, which are not available with PBP.
Or you could simply use a look-up table...

[amodpathak]

Not able to solve problem.tried many times.

[AvionicsMaster1]

Does temp read correctly above zero? 'shows sensor connected and only negative indication of code is incorrect

Can you make the LED blink? 'shows PIC and LED hooked up correctly and working

I'm a novice programmer but aren't you supposed to RETURN from a sub then GOTO a different subroutine? Most of your FNDx routines GOTO a sub for a RETURN. The first FND is empty. The blinking LED would tell you if the program is stuck somewhere.

In DIGITCALC won't the variable N always be a negative number? If so PicBasic will always report that as a zero.

Hope it helps and I don't get flamed too much?

[Scampy]

I'm a novice programmer but aren't you supposed to RETURN from a sub then GOTO a different subroutine? Most of your FNDx routines GOTO a sub for a RETURN. The first FND is empty. The blinking LED would tell you if the program is stuck somewhere.

In DIGITCALC won't the variable N always be a negative number? If so PicBasic will always report that as a zero.

Hope it helps and I don't get flamed too much?

His code is messy with no comments and lots of spaces so it does make it hard to follow, but there is a FND start and finish.



FND8:
IF DIGIT>8 THEN FND9
DG=$7F '%0111 1111
GOTO FNDEND

FND9:
DG=$6F '%0110 1111

FNDEND:
RETURN



Basically the program go's to the subroutine FND, which then contains a lot of IF DIGIT statements and if it is above a value then jump to the next FNDx label, but if it equals that value then set set value for DG and goto FNDEND, which is a simple RETURN and thus closes the subroutine.

[AvionicsMaster1]

Thanks for the schooling. I see it now. Though it would be interesting to know if it indicates anything other than zeroes.

I usually use the LED blinking to help me know when I've done something less than desirable. Just add a TOGGLE to the main loop and a short pause will let you know if the program's hung.

Best of luck.

[amodpathak]

Temperature goes above ZERO.Positive temperature is displayed correctly.When i keep thermistor in freezeo then temperature does't go below ZERO.

[Scampy]

Temperature goes above ZERO.Positive temperature is displayed correctly.When i keep thermistor in freezeo then temperature does't go below ZERO.

Try some basic measurements. remove the thermistor from the board and place it in your freezer with wires connected. Measure the resistance using a digital volt meter and note it down, together with the temperature. Repeat the measurement when the thermistor is placed in ice water, and again when under a lamp or something that you can measure the temperature with. This may help you calibrate the routine better.

One possible issue could be that your chosen thermistor is positive only, ie at zero degrees C it's resistance is zero ohms (or as near to as dam it).

[Charlie]

Without the datasheet for your device, it's impossible to resolve it for you, but here's the data for a typical device:
http://www.arroyoinstruments.com/thermistors.html It has a table of values vs temperature. So, at -10C it's about 55Kohm, and at +50C it's about 3.6K ohms. So now you need to turn that into a voltage either using a resistor divider, or a current source. This is simple math, easy to do with Excel, and you can create a list of voltages for each degree or half degree or whatever you like. Then use those voltage values to lookup the corresponding temperature.