SPI - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 128

There are several versions of the MAX31855 chip: one per cable type. The chip
on Adafruit's breakout board can use only K-type thermocouples. Connect the
wire to the breakout board, being careful to use the correct polarity (red and
yellow wires).

Sketch

Now that the hardware is connected, it is time to deal with the software. This
sketch communicates with the MAX31855 through the SPI bus. The datasheets
explain how the data transmits. The MAX31855 sends 32-bits of data (unless
stopped by the slave select pin), corresponding to several pieces of information.
The transmission is shown in Table 7-3.

Table 7-3: MAX31855 Data Output

BIT NAME FUNCTION

D[31:18] 14-bit thermocouple tempera-

ture data

Contains signed 14-bit thermocouple
temperature

D17 Reserved Always reads as 0

D16 Fault Reads as 1 if a fault is detected, otherwise 0

D[15:4] 12-bit internal temperature

data

Contains signed 12-bit cold junction
temperature

D3 Reserved Always reads as 0

D2 SCV Fault Reads 1 if the thermocouple is shorted to

VCC

D1 SCG Fault Reads 1 if the thermocouple is shorted to

ground

D0 OC Fault Reads 1 if the thermocouple is not

connected

The data is delivered in a 32-bit package, but there is something interesting
about the layout. It can be seen as two 16-bit values: bits D31 to D16 and D15
to D0. The i rst 16-bits contains everything that is essential: the temperature
detected on the thermocouple and a fault bit. If there is a fault, or if the user
wants to know the cold-junction temperature, then the second 16-bit value can
be read, but otherwise, it is not required.
Time to write the sketch as follows in Listing 7-1:

Listing 7-1: Digital Thermometer Sketch (fi lename: Chapter7.ino)

1 #include <SPI.h>
2
3 const int slaveSelectPin = 10;