Understanding the MAX6675

[Chris S]

I dont need help getting code to work, but I am having a hard time visualizing how this works. I almost thought of drawing it out with blocks to get an idea but I have no one else to check my knowledge. So I'll ask here:

I have in hand, one of those MAX6675 modules. My code works! Yay, and it reports room temp and then close to boiling water (91C, but it was in a mug). My question is this:

1) When doing an SPI transfer, is the MSB transfer first? So then if I put that in a byte, or upper byte of a word, the MSB's, would match, correct?

Dim L_byte as Byte
Dim Temperature as Word

  SPI_CS=0
  SPITransfer 0,Temperature
  Temperature=FnLSL(Temperature, 8) ;shift left 8 bits
  SPITransfer 0, L_byte
  Temperature = Temperature or L_byte
  Temperature = FnLSR(Temperature, 5)
  SPI_CS=1

So this was a snippet of code I found around here. Now the MAX6675 has 3 lower bits that are ignored. If we are shifting 8 bits to the left, and then 5 bits to the right, wouldnt the upper 3 bits of the var "Temperature" be unknown? This is where Im getting stuck. It doesnt seem like we are right justifying the bits for correct temperature (But The code does work and report back correctly!) Maybe I'll draw it out..

[Anobium]

It seems to me that you are shifting a value in Temperature from the low byte to the high bytes, then, loading the low byte then shifting the whole word to the left. Is this what you wanted to do? At the top three bits of the high byte of the word will get lost,

[Chris S]

This is the thread I found where I got the snippet from
https://sourceforge.net/p/gcbasic/discussion/projects%26guides/thread/f0a238743b/

Really, I need to get rid of the 3 lower bits (0,1,2) but I still want to emphasize that the code above does work, I just am trying to understand how.

It seems that the arduino library does the following, which also works:

  SPI_CS=0
  SPITransfer 0,Temperature
  Temperature=FnLSL(Temperature, 8) ;shift left 8 bits
  SPITransfer 0, L_byte
  Temperature = Temperature or L_byte
  SPI_CS=1

  Temperature = FnLSR(Temperature, 3)
  Temperature=Temperature/4

Adapted from:https://github.com/adafruit/MAX6675-library/blob/master/max6675.cpp

[Anobium]

I am home now.. back online.

Your code analysed.

Get the byte value with the result being loaded in Temperature variable
SPITransfer 0,Temperature

Put the low byte of the word variable into the high byte of the word variable
Temperature=FnLSL(Temperature, 8) ;shift left 8 bits

Get the byte value with the result being loaded in L_byte variable
SPITransfer 0, L_byte

'OR' the L_byte variable with Temperature variable to fully populate the word variable. Now the word variable is as per mmotte post - see https://sourceforge.net/p/gcbasic/discussion/579126/thread/7038dd6b3d/?limit=25#8068
Temperature = Temperature or L_byte

Remove the variable top 3 bits by shifting left
Temperature = FnLSR(Temperature, 3)

Shift the variable right by 2 bits by divide by 4
Temperature=Temperature/4

---

So, to be honest I do not know how this works in the reality. Assume the process has load the max value into the word variable, so, bits 5 thru 14 are all set - this equates to 32736 ignoring all the other bits for now. So, to get to 1023 the real value this would required a shift right 5 times - variable >> 5 = 32736 >> 5 =1023.

Using the shifts in the method would be 32736 << 3 >> 2 = 16320

Really, puzzled how this works....

[Chris S]

So I actually amended the code a bit, to show step by step via UART what was getting received. It makes sense when you look at it that way. Here is my full code:

Note: the 1 second delay is for the chip itself. It actually needs 300mS or so to sense the temperature. Realistically, in practice (like control loops), 1 second is more than enough.

#chip PIC18F13K22, 8
#define USART_BAUD_RATE 9600
#define SPI_HardwareSPI
#include "gcbversionnumber.cnt"


#define SPI_CS PORTC.2
#define SPI_DI PORTB.4
#define SPI_SCK PORTB.6
dir SPI_CS    out
dir SPI_DI    In
dir SPI_SCK   Out
SPIMode MasterSlow

Dim L_byte as Byte
Dim Temperature as Word
dim timercount as Word
  dim versionString as string * 40
  versionString = "Build"+GCBBuildStr
  versionString = versionString + "@"+GCBBuildTimeStr

timercount=0

Dim SPIstring as string

set SPI_CS ON

HSerSend 13
HSerSend 10
HSerPrint versionString
HSerSend 13
HSerSend 10

do

  SPI_CS=0
  SPITransfer 0,Temperature
  Temperature=FnLSL(Temperature, 8) ;shift left 8 bits
  SPITransfer 0, L_byte
  Temperature = Temperature or L_byte
  SPI_CS=1

//Temperature = FnLSR(Temperature, 3)
//Temperature=Temperature/4
  SPIstring=Wordtobin(Temperature)
  HSerPrint "raw: "
  HSerPrint SPIstring
  HSerSend 13
  HSerSend 10
Temperature = FnLSR(Temperature, 5)
 SPIstring=Wordtobin(Temperature)
  HSerPrint "shf: "
  HSerPrint SPIstring
  HSerSend 13
  HSerSend 10

//Temperature=Temperature/4
 SPIstring=Wordtobin(Temperature)
  HSerPrint "tem: "
  HSerPrint SPIstring
  HSerSend 13
  HSerSend 10



  HSerPrint "Current Temperature: "
  HSerPrint Temperature
  HSerSend 13
  HSerSend 10
'  HSerPrint "Current Timer Count: "
'  HSerPrint timercount
'  HSerSend 13
'  HSerSend 10

wait 1 s
timercount++

if timercount >=1023 then
  timercount=0
end if

loop



end

[Chris S]

As you can see, I amended the code and commented stuff out. Using shift 3, and then dividing by 4 (or shifting by 2) gives the same result. I ran the experiment twice and got the same result.

Edit: There might be some duplicate code here:

Temperature = FnLSR(Temperature, 5)
 SPIstring=Wordtobin(Temperature)
  HSerPrint "shf: "
  HSerPrint SPIstring
  HSerSend 13
  HSerSend 10

//Temperature=Temperature/4
 SPIstring=Wordtobin(Temperature)
  HSerPrint "tem: "
  HSerPrint SPIstring
  HSerSend 13
  HSerSend 10

Not that it will cause issues or anything. But its left over from the divide by 4.

[Anobium]

It seems to me that you are shifting a value in Temperature from the low byte to the high bytes, then, loading the low byte then shifting the whole word to the left. Is this what you wanted to do? At the top three bits of the high byte of the word will get lost,

[mmotte]

The range on the max6675 is 0 to 1024 deg C with .25 deg resolution.
so the Msb 10 bits are the whole degs
bits 0 tristate
bit 1 is ID
bit 2 is open thermocouple
bit 3 and 4 are the 0.25 deg C increments
bits 5 thru 14 are the whole degC
bit 15 is 0

Shifting right 5 throws away the .25 deg resolution and the lower flags

[Anobium]

I understand now. Looks good.

[Anobium]

I did make this test program to ensure it worked. :-)

I set the Temperature and L_byte to the values that should represent 1023.... and, the result is correct. :-)

Dim L_byte as Byte
Dim Temperature as Word

  Temperature = 127
    HserPrintCRLF 2
    HserPrint "1: "
    HserPrint Temperature
    HserPrintCRLF

  Temperature=FnLSL(Temperature, 8) ;shift left 8 bits
    HserPrint "2: "
    HserPrint Temperature
    HserPrintCRLF

  L_byte = 224
  Temperature = Temperature or L_byte
    HserPrint "3: "
    HserPrint Temperature
    HserPrintCRLF


  Temperature = FnLSR(Temperature, 5)
    HserPrint "4: "
    HserPrint Temperature
    HserPrintCRLF