16f877 with LM335 problem

[Ioannis]

Of course it it workable if you accept the 2,59 counts per degree. Noise will be an issue also and especially in this case I would insist on using oversampling to 12 bits to iron out the noise as much as possible.

Ioannis

[richard]

having in mind the accuracy I need is not that critical (+/- 2°C is okay),

with a bit of averaging or oversampling 0.5 resolution is achievable, the software solution is at least cheap and easy to try out

[flotulopex]

Hello,

I have built the circuit and I'm now testing.

I'll send some pictures from the the values I read from the LM35 in a moment - very strange values, not stable at all... :-(

[flotulopex]

So this is where I am right now.

First, here's my hardware. What you won't see on the schema is the serial comm lead connected to PIC's PORTB.7 .


Here's the code:

' PIC 16F690 Fuses
@ __config _FCMEN_OFF &_IESO_OFF &_CPD_OFF &_WDT_OFF &_HS_OSC &_BOR_OFF &_CP_OFF &_PWRTE_OFF &_MCLRE_OFF

'_______________________________________________________________________________
' Registers   76543210
OPTION_REG = %10000000 'OPTION REGISTER
ANSEL      = %00000100 'Select analog inputs Channels 0 to 7
ANSELH     = %00000010 'Select analog inputs Channels 8 to 11
ADCON0     = %00000001 'A/D control register
ADCON1     = %00000000 'A/D control register
CM1CON0    = %00000000 'Comparator1 Module
CM2CON0    = %00000000 'Comparator2 Module
INTCON     = %00000000 'INTerrupts CONtrol
TRISA      = %00000100 'Set Input/Output (0 to 5)
PORTA      = %00000000 'Ports High/Low (0 to 5)
TRISB      = %00000000 'Set Input/Output (4 to 7)
PORTB      = %00000000 'Ports High/Low (4 to 7)
TRISC      = %10000000 'Set Input/Output (0 to 7)
PORTC      = %00000000 'Ports High/Low (0 to 7)

'_______________________________________________________________________________
' Defines
DEFINE OSC 4
DEFINE ADC_BITS 10

'_______________________________________________________________________________
' Variables
LM35_Vout       var word ' AN2 PORTA.2
LM35_VGND       var word ' AN9 PORTC.7
Temperature     var word
DataVar         VAR WORD(12)
DataTemp        var WORD
Counter_A       var WORD

'_______________________________________________________________________________
' Program

MAIN:
    ' LM35_Vout
    FOR counter_A = 0 to 11
        adcin 2, DataVar(counter_A)
    next counter_A
    Counter_a = 0
    GOSUB SORT
    LM35_Vout = DataTemp
    
    
    ' LM35_VGND
    FOR counter_A = 0 to 11
        adcin 9, DataVar(counter_A)
    next counter_A
    Counter_a = 0
    GOSUB SORT
    LM35_VGND = DataTemp

    Temperature = (LM35_Vout - LM35_VGND) * 100

    GOSUB DISPLAY

    pause 1000
    
Goto main:


'_______________________________________________________________________________
' Subroutine

DISPLAY:
    Serout2 PORTB.7,84,["Temp.: ", dec4 temperature,"  LM35_Vout: ", dec4 LM35_Vout,"  LM35_VGND: ", dec4 LM35_VGND,13]
RETURN

'_______________________________________________________________________________
' Subroutine

SORT:
' Melanie NEWMAN's array sorting routine
'    Aquire 12 values, delete 4 lower and 4 top values, average 4 remaining values

    If DataVar(counter_A + 1) < DataVar(counter_A) then
        DataTemp = DataVar(counter_A)
        DataVar(counter_A) = DataVar(counter_A + 1)
        DataVar(counter_A + 1) = DataTemp
        If Counter_A > 0 then Counter_A = Counter_A - 2
    endif
    Counter_A = Counter_A + 1
    If Counter_A < 11 then GOTO SORT
    
    ' Average four middle values
    DataTemp = 0
    For Counter_A = 4 to 7
        DataTemp = DataTemp + DataVar(counter_A)
    next Counter_A
    
    DataTemp = DataTemp / 4

Return

And here is the reading I get right now, knowing the temperature sensor is on my desk close to three other thermometers showing 21°C. Promised, I won't forget to format the "Temp." display in the final version (i.e. "9456" => "94,56°C")





What's wrong with it?

I have set and defined a 10 bits ADC result. How can I get values over 1023?

I might be wrong with the ADCON0.7 setting but I'm not used to modify it so I'm not sure about that.

Any other idea?

[Ioannis]

That's because you have left justified the ADC result in ADRESL and ADRESH.

Set the adcon0 to %10000001.

Also you have this calculation:

 Temperature = (LM35_Vout - LM35_VGND) * 100

What about LM35_Vout being less than LM35_VGND?

I expected you would do this and noted in earlier post (#33)

The simplest way is to have an if-then check first like this:

 if LM35_Vout >LM35_VGND then
       Temperature = (LM35_Vout - LM35_VGND) * 100
 else
 if LM35_Vout <= LM35_VGND then
       Temperature = (LM35_VGND - LM35_Vout) * 100
 endif

Ioannis

[flotulopex]

...What about LM35_Vout being less than LM35_VGND?

I expected you would do this and noted in earlier post (#33)

Sorry, I forgot this one . I was focused on the "mysterious" values I was reading.

I might be wrong with the ADCON0.7 setting but I'm not used to modify it so I'm not sure about that.

I changed to ADCON0.7 = 1 and the values I read look much better. Still the Temperature value has to be divide by 2 to give the correct reading.





Even if it's not the main subject here, can you shortly explain what the difference between ADCON0.7 = 0 and ADCON0.7 = 1 is?


The temporary code is now:

' PIC 16F690 Fuses
@ __config _FCMEN_OFF &_IESO_OFF &_CPD_OFF &_WDT_OFF &_HS_OSC &_BOR_OFF &_CP_OFF &_PWRTE_OFF &_MCLRE_OFF

'_______________________________________________________________________________
' Registers   76543210
OPTION_REG = %10000000 'OPTION REGISTER
ANSEL      = %00000100 'Select analog inputs Channels 0 to 7
ANSELH     = %00000010 'Select analog inputs Channels 8 to 11
ADCON0     = %10000001 'A/D control register
ADCON1     = %00000000 'A/D control register
CM1CON0    = %00000000 'Comparator1 Module
CM2CON0    = %00000000 'Comparator2 Module
INTCON     = %00000000 'INTerrupts CONtrol
TRISA      = %00000110 'Set Input/Output (0 to 5)
PORTA      = %00000000 'Ports High/Low (0 to 5)
TRISB      = %00000000 'Set Input/Output (4 to 7)
PORTB      = %00000000 'Ports High/Low (4 to 7)
TRISC      = %10000000 'Set Input/Output (0 to 7)
PORTC      = %00000000 'Ports High/Low (0 to 7)

'_______________________________________________________________________________
' Defines
DEFINE OSC 4
DEFINE ADC_BITS 10

'_______________________________________________________________________________
' Variables
LM35_Vout       var word ' AN2 PORTA.2
LM35_VGND       var word ' AN9 PORTC.7
Temperature     var word
DataVar         VAR WORD(12)
DataTemp        var WORD
Counter_A       var WORD

'_______________________________________________________________________________
' Program

MAIN:
    ' LM35_Vout
    FOR counter_A = 0 to 11
        adcin 2, DataVar(counter_A)
    next counter_A
    Counter_a = 0
    GOSUB SORT
    LM35_Vout = DataTemp
    
    
    ' LM35_VGND
    FOR counter_A = 0 to 11
        adcin 9, DataVar(counter_A)
    next counter_A
    Counter_a = 0
    GOSUB SORT
    LM35_VGND = DataTemp

    if LM35_Vout >LM35_VGND then
        Temperature = ((LM35_Vout - LM35_VGND) * 100) / 2
    endif
    if LM35_Vout <= LM35_VGND then
        Temperature = ((LM35_VGND - LM35_Vout) * 100) / 2
    endif

    GOSUB DISPLAY

    pause 1000
    
Goto main:


'_______________________________________________________________________________
' Subroutine

DISPLAY:
    Serout2 PORTB.7,84,["Temp.: ", dec2 (temperature/100),",",dec1 (temperature/10),"°C  LM35_Vout: ", dec4 LM35_Vout,"  LM35_VGND: ", dec4 LM35_VGND,13]
RETURN

'_______________________________________________________________________________
' Subroutine

SORT:
' Melanie NEWMAN's
' This subroutine sorts, in this example, 12 elements,
'   MyData array in sequence placing the smallest value into MyData[0] and
'   the largest value into MyData[11]...

    If DataVar(counter_A + 1) < DataVar(counter_A) then
        DataTemp = DataVar(counter_A)
        DataVar(counter_A) = DataVar(counter_A + 1)
        DataVar(counter_A + 1) = DataTemp
        If Counter_A > 0 then Counter_A = Counter_A - 2
    endif
    Counter_A = Counter_A + 1
    If Counter_A < 11 then GOTO SORT
    
    ' Average four middle values
    DataTemp = 0
    For Counter_A = 4 to 7
        DataTemp = DataTemp + DataVar(counter_A)
    next Counter_A
    
    DataTemp = DataTemp / 4

Return

[Tabsoft]

Roger,

Check out section 9.1.6 "Result Formatting" of the Datasheet.
It shows the difference in the two formatting options.

In essence Right Shifted will place the 10-bit value in bits 9 through 0 (MSB is bit9).
Left Shifted will place the 10-bit value in bits 15 through 6 (MSB is bit15)

So if your ADC reading is 250 then here would be the values.
Right Shifted = 250 (%0000 0000 1111 1010)
Left Shifted = 1600 (%0011 1110 1000 0000)