Is this a K Type sensor?

[jessey]

The MAX6675 doesn't need a ADC to interface with a PIC.
You just shiftin the serial data and it reads pretty much directly in degrees C on an LCD without too much else.

This is pretty much all the code involved...

'Alias pins - MAX6675 temp sensor
MXSCLK  var     PORTB.5   'Clock
MXCS     var     PORTB.4    'Chip Select
MXSO     Var     PORTB.3   'Serial out


 'Allocate MAX6875 Variables
MXTemp   var     word    'raw data from 6675/type K sensor
TempC   var     word    'converted to degrees C


'-----------Read and display temperature from MAX6675-----------
MXCS = 0  'Chip select low
shiftin MXSO, MXSCLK, 0, [MXTemp\16]   'read the data to MXTemp
MXCS = 1    'Chip select high
TempC = MXtemp >> 5   'right shift the data 5 places to get degrees C (Read the Data sheet)
LCDOUT $fe,2,"Temp = ", DEC TempC, "degrees C "  ' Spit it out on an LCD

Works for me...

Steve

Hi Steve,

I finally received 2 of each MAX6675's and AD595's and thought I'd use one of the 6675's first to try out your code. I found this article: http://emesystems.com/OL2therm.htm that shows a schematics for the 6675 and I have a couple of questions for you.

This article shows 200 ohm resistors on pins 5, 6 and 7 to the microprocessor for input protection which sounds like a good idea but are not shown in the data sheet. Do you think they could cause any problems in reading the higher temperatures?

Also in the same article the author dismisses grounding pin 2 as shown in the datasheet and I was wondering what you think of that?

Now that I have my pickit2 programmer working good on my laptop, I'll take a breadboard to my friends place next week sometime to try your code. Once again thanks for posting your code.

Thanks
jessey

[Byte_Butcher]

Jessey,
I didn't use any series resistors on my MAX6675, but it's probably a good idea to use them. It shouldn't cause any accuracy problems... it's all serial data, not analog signals.

I grounded pin 2 on mine. If it's grounded, then the MAX6675 can tell when there's an open probe. (type K probes almost always fail "open") The data sheet explains it.

Have fun!


Steve

[jessey]

Hi Steve,

It took some time but I finally got around to trying to get the sensor up and running but I'm not having any success. I'm pretty sure I have everything configured ok with your code. Could you please have a look and see if there's something that you can see that I've done wrong here? Any help from anyone would be very much appreciated. I included the schematics I used in an attachment here.

Thanks
jessey

'**************************************************************************
'* Name    : Furnace Temperature Probe                                    *
'* Author  : Jessey Montgomery                                            *
'* Includes: Byte_Butchers code to read temperature using a MAX6675       *  
'* Date    : April 24/th 2009                                             *
'* Version : Using PicBasic Pro Ver 2.50, MPASM Ver 5.20, Using MicroCode *
'*         : Studio 3.0.0.5, PICkit 2 programmer - Application Version    *
'*         : 2.50.02 - Device File Version 1.5100 - OS Firmware Version   *
'*         : 2.30.01. Using a Pic16F688-I/P 04470AY.                      *
'**************************************************************************

'(1) Comments & Objectives
'   =====================
' this program is used to display the temperature from a K type sensor using a
' MAX6675 interface...

'------------------------------------------------------------------------------'

'Connections for the PICKIT 2 Programmer for a 16F688
'Pickit 2 pins                   16f688 pins
'=============                   ===========
'     1 ...... Connect to ...... pin 4       RA3/MCLR/VPP
'     2 ...... Connect to ...... pin 1       VDD (positive)
'     3 ...... Connect to ...... pin 14      VSS (ground)
'     4 ...... Connect to ...... pin 13      RA0/AN0/C1IN+/ICSPDAT/ULPWU
'     5 ...... Connect to ...... pin 12      RA1/AN1/C1IN-/VREF/ICSPCLK
'     6......................................no connection
'*Note
'Put a 0.1mf capicator on pin 1 of 16f688 to ground

'------------------------------------------------------------------------------'

'                               --------------
'                               |   16f688   |
'                   VDD+ (--->) |1         14| (<---) VSS-
'                               |            |
'   RA5/TICKI/OSC1/CLKIN (<-->) |2         13| (<-->) RA0/AN0/C1IN+/ICSPDAT/ULPWU 
'                               |            |
'RA4/AN3/TIG/OSC2/CLKOUT (<-->) |3         12| (<-->) RA1/AN1/C1IN-/Vref/ICSPCLK 
'                               |            |
'            RA3/MCLR/Vpp(--->) |4         11| (<-->) RA2/AN2/TOCKI/INT/C1OUT
'                               |            |
'              RC5/RX/DX (<-->) |5         10| (<-->) RC0/AN4/C2IN+
'                               |            |
'        RC4/C2OUT/TX/CX (<-->) |6          9| (<-->) RC1/AN5/C2IN-
'                               |            |
'                RC3/AN7 (<-->) |7          8| (<-->) RC2/AN6
'                               |            |
'                               --------------

'------------------------------------------------------------------------------'
' --------------------------------------------------------------------------------
 

'                            Configuration Of Pins
'                            =====================
'                                ------------
'                                |  16f688  |
'                           VDD+ |1       14| VSS-
'                                |          |
'                       Not Used |2       13| Clock output for 6675 to pin 5     
'                                |          |
'                       Not Used |3       12| Chip Select output for 6675 pin 6              
'                                |          |
'                       Not Used |4       11| Serial input from 6675 to pin 7      
'                                |          |
'  Blue/White.........Enable bit |5       10| Data bit 4...............Blue
'                                |          |
'  Orange/Black..Register select |6        9| Data bit 5..............White
'                                |          |
'  Orange.............Data bit 7 |7        8| Data bit 6..............Green
'                                |          |
'                                ------------

'------------------------------------------------------------------------------'

'                               --------------
'                               |  MAX6675   |
'                        -5 VSS |1          8| No Connection
'                               |            |
'          T- Alumel Lead (Red) |2          7| Serial Output to 688 pin 11  
'                               |            |
'       T+ Chromel Lead (yellow)|3          6| Chip Select input to 688 pin 12        
'                               |            |
'                        +5 VDD |4          5| Serial Clock in from 688 pin 13 
'                               --------------


'(2) PIC Config Fuse Definitions for 16f688
'   ======================================
 @ __config _INTRC_OSC_NOCLKOUT & _WDT_ON & _MCLRE_OFF & _CP_OFF & _BOD_ON   
 @ ERRORLEVEL -306; ' turns off cross page boundary message alerts after compile

'(3) PIC Hardware Definitions (ie Pin-Out & Port Assignments)
'   ========================================================
 DEFINE LCD_RSREG PORTC' LCD Register Select Port  
 DEFINE LCD_RSBIT 4    ' LCD Register Select 
 DEFINE LCD_EREG PORTC ' LCD Enable Port  
 DEFINE LCD_EBIT 5     ' LCD Enable      
 DEFINE LCD_DREG PORTC ' LCD Data Port 
 DEFINE LCD_DBIT 0     ' LCD data starting bit 0 or 4 
 
'------------------------------------------------------------------------------'
'------------- Clear Each Port Before Setting The TRIS Registers --------------'
'------------------------------------------------------------------------------'
                                
 Clear   ' Set all ram registers to zero
 PORTA = 0
 PORTC = 0                           

'--------------------------- VSS VDD MCLR Ect. Pins ---------------------------'
 
'VDD +       '(pin 1)
'VSS -       '(pin 14)
 
 '------------------- SET THE TRIS  ------------------'               
 
 '-------------------- PORTA PINS --------------------' 

 TRISA.0 = 0 '(pin 13) ...................................... MXSCLK VAR PORTA.0   
 TRISA.1 = 0 '(pin 12) .........................................MXCS VAR PORTA.1   
 TRISA.2 = 1 '(pin 11) .........................................MXSO VAR PORTA.2               
 TRISA.3 = 1 '(pin 4)  ............... NOT USED VAR PORTA.3      
 TRISA.4 = 1 '(pin 3)  ............... NOT USED VAR PORTA.4      
 TRISA.5 = 1 '(pin 2)  ............... NOT USED VAR PORTA.5   

 '-------------------- PORTC PINS --------------------'
 TRISC.0 = 0 '(pin 10) Blue........... Lcd Data bit 4   
 TRISC.1 = 0 '(pin 9)  White.......... Lcd Data bit 5 
 TRISC.2 = 0 '(pin 8)  Green.......... Lcd Data bit 6              
 TRISC.3 = 0 '(pin 7)  Orange......... Lcd Data bit 7  
 TRISC.4 = 0 '(pin 6)  Orange/Black... Lcd Register select     
 TRISC.5 = 0 '(pin 5)  Blue/White..... Lcd Enable bit 

 ANSEL = %00000000  ' set all adc pins to digital
 CMCON0 = 7         ' turn off comparators 

'Alias pins - MAX6675 temp sensor
 MXSCLK   var     PORTA.0   'pin 13 of 16f688 (Clock) to pin 5 of MAX6675
 MXCS     var     PORTA.1   'pin 12 of 16f688 (Chip Select) to pin 6 of MAX6675
 MXSO     Var     PORTA.2   'pin 11 of 16f688 (Serial) to pin 7 of MAX6675   

 'Allocate MAX6875 Variables
 MXTemp  var     word    'raw data from 6675/type K sensor
 TempC   var     word    'converted to degrees C 
 a VAR BIT ' a always equals zero, used in IF-THEN's instead of using GOTO's
 a = 0

 INCLUDE "modedefs.bas"     

'5. Actual Program Start (usually a "Goto MainLoop" jump)
'   =====================================================
 Pause 1000 ' Pause for LCD to initialize

    LCDOut $fe, 1, "Temp 6675 Laptop"
PAUSE 2500

 IF a = 0 THEN MainLoop

'------------------------------------------------------------------------------'
MainLoop:

'-----------Read and display temperature from MAX6675-----------
MXCS = 0  'Chip select low
shiftin MXSO, MXSCLK, 0, [MXTemp\16]        'read the data to MXTemp 
MXCS = 1                                    'Chip select high
TempC = MXtemp >> 5'right shift data 5 places to get degrees C (in data sheet)
LCDOut $fe, 1,"Temp = ", DEC TempC, " C  "  'Spit it out on an LCD 

PAUSE 100                                   'reduce lcd flicker

IF a = 0 THEN MainLoop ' a always equals zero, use instead of GOTO... 

End

[DocDingwall]

I'm trying to do a similar thing just now with the MAX6675 and the breakout board from Sparkfun (at least I think it's the MAX6675, I'm at work now). It's not clear to me if the wires from the female thermocouple socket to the MAX6675 breakout also need to be K type or is copper OK? At what point is it OK to switch to copper? Could the thermocouple lead be soldered right to the breakout board for prototyping?

Thanks for your help. Sorry if I should be starting a new thread.

DD.

[mackrackit]

Anyplace the wire material changes you need a reference temperature and calcs. A couple exceptions here and there though.

[ptig185]

I'm writing some code for this device also. I'm wondering why the shift of 5 places to the right when in the data sheet it looks like only 3 shifts right should put the Temperatures lsb in the correct location.

At least that's how my initial coding is. I'll try it tonight.
I also use MSBPOST=2 :

Get6675_Raw:
LOW NCS:PAUSEUS 1
Shiftin SO, SCK, MSBPOST, [raw1\16] ' Read 16 bit temperature register
raw1 = raw1>>3
HIGH NCS
RETURN

Brian

[ptig185]

OK so I tried the code. Now I see you guys are right with the 5 bit shift.
The device is capable of 12bits or 0.25 deg resolution if shifted only 3 times.

Brian