SX28 & MAX6675 thermocouple

Andrea · 2011-02-24 07:39

Hello,
I'm trying to read MAX6675 with this technique:

Dio             PIN     RC.4 INPUT
Clk             PIN     RC.3 OUTPUT
CS              PIN     RC.2 OUTPUT

result          VAR     Word
thC             VAR     Word

' =============
  PROGRAM Start
' =============

Start:
  CS = 1

Main:
  PAUSE 1
  Clk = 0
  CS = 0
  SHIFTIN  Dio, Clk, MSBPOST, result_MSB
  SHIFTIN  Dio, Clk, MSBPOST, result_LSB
  CS = 1

  thC = result >> 3
  thC = thC */ 640
  thC = thC / 10

  PAUSE 250
  GOTO Main

that I found on this forum...But it is out of my league to extract the value in to format like: "24.00" Centigrade,
and another concern is how to convert word value to ASCII (for LCD).

Thank you!!!

Capt. Quirk · 2011-02-24 12:05

Here is 3 programs written by Bean to format a word value to ascii. Just use the temp Celsius to Fahrenheit conversion.

http://forums.parallax.com/showthread.php?87337-Displaying-WORD-variable-as-DEC&p=638170
http://forums.parallax.com/showthread.php?87889-Sending-a-varible-to-a-serial-LCD&p=603003
http://forums.parallax.com/showthread.php?91950-is-it-possible-to-change-SX-B-8-bit-Commands-into-16-bit&p=631538

Andrea · 2011-02-24 14:44

Bill, Thank you.
First I'm OK with Celsius.
Now I'm able to red and send the temperature data to the display...But now I have a problem with temperature accuracy...On the start up error is about 10 Celsius more then real (compared with Fluke), when warm up with lighter it is going up but very slow, and cooling process is even slower. I'm not able to get correct readings compared to the Fluke.
Hardware that is used:
The thermocouple:

High quality K type thermocouple sensor probe with a versatile and flexible cable.

Specifications:
* Type: K type
* Resistance between terminals: 7.6Ω
* Cable length: 3.3 foot / 1 meter
* Thread: 5.85mm (0.2")
* Sensing tip length: 7.5mm (0.3")
* Threaded nut material: steel
* Temperature measurement range: 0 - 800˚C

...connected to the PCB with 2 small bolts

And SX/B:

So              PIN      RA.2 INPUT 	
Sck             PIN     RA.0 OUTPUT 
CS              PIN     RA.1 OUTPUT

result          VAR     Word
thC             VAR     Word

' =====================================
  PROGRAM Start
' =====================================
Word_To_Digits  SUB 2
' -------------------------------------------------------------------------
' Program Code
' -------------------------------------------------------------------------
Start:
  CS = 1

Main:
 PAUSE 1
  Sck = 0
  CS = 0
  SHIFTIN  So, Sck, MSBPOST, result_MSB
  SHIFTIN  So, Sck, MSBPOST, result_LSB
  CS = 1

  thC = result >> 3
  ' thC = thC */ 640
  ' thC = thC / 10

  thC = thC ** $4000

  Word_To_Digits thC    ' Value to the display

  PAUSE 1000
  GOTO Main

Should I solder probes to the PCB? Or something else?

Thank you

SX28 & MAX6675 thermocouple

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