Having trouble with SERIN command and MAX6675

[richard]

for a start your temp var is a declared as byte , how can 12 bit reading fit in that ?

[flotulopex]

I corrected my code just after I sent it - couldn't edit the post anymore, should have mentionned it, sorry

[flotulopex]

Anyway, I understand the calculation to get the temperature value BUT why do I have around 5°C diffrence between the thermocouple and my home thermometer?

Must a thermocouple be kind of "adjusted" (correct the result in the code) or something like that?

[flotulopex]

My actual code is:

' ====== FUSES ====================================================================================
' PIC 16F690
@ __Config _FCMEN_OFF &_IESO_OFF &_CPD_OFF &_WDT_OFF &_INTRC_OSC_NOCLKOUT &_BOR_OFF &_CP_OFF &_PWRTE_OFF &_MCLRE_OFF 

' ====== DEFINES ==================================================================================
DEFINE OSC 4

DEFINE SHIFT_PAUSE 250  'for MAX6675 conversion time

DEFINE LCD_DREG PORTC   'LCD data port 
DEFINE LCD_DBIT 0       'LCD data starting bit 0 or 4 on µC 
DEFINE LCD_RSREG PORTC  'LCD Register Select port 
DEFINE LCD_RSBIT 4      'LCD Register Select bit 
DEFINE LCD_EREG PORTC   'LCD Enable port 
DEFINE LCD_EBIT 5       'LCD Enable bit 
DEFINE LCD_BITS 4       'LCD bus size 4 or 8 
DEFINE LCD_LINES 2      'Number lines on LCD 

' ====== REGISTERS ================================================================================
'             76543210
OPTION_REG = %10000000 ' PORT A&B Pull-Ups disabled (look WPUA & WPUB)
'OSCCON     = %01100000 ' Internal RC set to 4Mhz - not to be used with XTal
ANSEL      = %00000000 ' Analog inputs Channels 0 to 7
ANSELH     = %00000000 ' Analog inputs Channels 8 to 11
ADCON0     = %00000000 ' A/D Module is OFF
CM1CON0    = %00000000 ' Comparator1 Module is OFF
CM2CON0    = %00000000 ' Comparator2 Module is OFF
INTCON     = %00000000 ' INTerrupts CONtrol / RA2 is ON
PORTA      = %00000000 ' Ports High/Low (0 to 5)
TRISA      = %00000000 ' Set Input/Output (0 to 5)
PORTB      = %00000000 ' Ports High/Low (4 to 7)
TRISB      = %00000000 ' Set Input/Output (4 to 7)
PORTC      = %00000000 ' Ports High/Low (0 to 7)
TRISC      = %00000000 ' Set Input/Output (0 to 7)

' ====== VARIABLES ================================================================================
MAX6675_SDO var PORTB.4
MAX6675_CS  var PORTB.5
MAX6675_SCK var PORTB.6

Temperature var word
Temperature = 0

' ====== PROGRAM ==================================================================================

MAX6675_READ:
    
    MAX6675_CS = 0
    shiftin MAX6675_SDO,MAX6675_SCK,0,[Temperature\16]
    MAX6675_CS = 1

    Temperature = Temperature >> 3
    
    lcdout $FE,2,DEC Temperature/4,",",DEC (Temperature//4)*25," C"
    
    PAUSE 1000

Goto MAX6675_READ:

END

[richard]

if you have cheap ebay sourced k type probes a reading of 5 -+ is typical , they seem more accurate [precise anyway] at higher temps >200c
but there is still a fair spread of variation in the readings

how to interpret reading

[flotulopex]

Thanks Richard,

With your calculation example you gave me above, I could understand how to read my result.

Do you know if thermocouples are "qualified" according to their precision?

PS: yup, mine has made the travel from China

[richard]

Do you know if thermocouples are "qualified" according to their precision?

not cheapies on ebay .

a more likely source of some probes with actual specs would be rs components ,farnel/element14 ,digikey or similar
but they will cost more.

I have never had the need for real absolute accuracy and just add or subtract a suitable offset to compensate for error,
so far it works acceptably for my 3d printer [precision being more important]

[mackrackit]

And I will add... Take extra care with your cold junctions. The 6675 does a good job compensating if the junction is close and in the same environment as it is. As do not have the cold junction on the outside of an enclosure and the 6675 on the inside. And noise that can be picked up from breadboard connections can be trouble.