Having trouble with SERIN command and MAX6675

[flotulopex]

Unless you are trying to get the fractional part of the reading 10 bits is all you need.

I don't get it, Dave...

[richard]

why do I have to shift by "2" instead of what has been said above (shift by 3) or even shift by 1 since bit 14 is the second bit to start with?

the data can be read in msb first by shiftin but it is then translated to a normal binary representation so bit 14 is read as bit 14.

shifting by 2 is incorrect , as dave indicated
Temp = RawTemp >> 5
will get the high 10 bits of the 12bit reading ,resulting in the integer component of the reading

or

Temp = RawTemp >> 3
int_part=temp/4
fract_part=(temp//4)*25

lcdout $FE, $C0,"Tmp ", #int_part,".",dec2 fract_part," C"

[flotulopex]

This is confusing to me.

My code gives me almost exactly what my home thermometer shows:


This code

Temp = RawTemp >> 3
int_part=temp/4
fract_part=(temp//4)*25
lcdout $FE, $C0,"Tmp ", #int_part,".",dec2 fract_part," C"

display this:

Where is the truth?

[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