SPI - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 126

the temperatures that thermocouples can support, copper becomes liquid at
1084° C (1983° F) and gold becomes liquid at 1063° C (1946° F). They can there-
fore be placed in almost every oven, i re or barbecue. If ever you want to create
a smokehouse to make smoked salmon, a thermocouple is an excellent way to
keep track of the temperature directly inside the i re and on the racks.
Thermocouples do not report a temperature; rather, they report a temperature
difference between their hot junction (the tip) and the cold junction (the other
end of the thermocouple that is connected to the printed circuit board). To use
a thermocouple effectively, it is important to know the temperature on the cold
junction, and integrated drivers do this automatically.
The MAX31855 is a thermocouple driver, capable of working with a variety
of thermocouples. It has good accuracy, fast conversion, and excellent range.
(This device, coupled with a type K thermocouple, can register up to +1350° C
(+2462° F). Different thermocouples exist, using different metals and handling
different temperature ranges. A thermocouple driver must be connected to
the correct thermocouple to function. To communicate this data with another
device, the MAX31855 uses the SPI bus and is a read-only device. It outputs the
thermocouple temperature, reference junction temperature, and fault indicators.
The MAX31855 can warn when a thermocouple short occurs, or when the con-
nection is broken, making it excellent for industrial applications.
The MAX31855 is only available in a surface-mounted format (SO-8), but
Adafruit has created a small, reliable breakout board for this component. The
MAX31855 itself can support only 3.3 V power, but Adafruit have added volt-
age shifting onto its breakout board, allowing this component to be used by
both AVR (which typically operate at 5 V) and the Cortex-M (running at 3.3 V)
based Arduinos.
Hardware
For this example, you use an Arduino Due. The Due is a powerful device, pow-
ered by 3.3 V and with advanced SPI functionality. You also use an Adafruit
MAX31855 breakout board and thermocouple. This board has two connectors:
One is placed on the breadboard and the thermocouple connects to one. It
requires some soldering; the connectors are packaged with the card but not
connected, but it is easy to do and requires only a few minutes.
The Arduino Due has three slave select pins available; for this example, you
use the digital pin 10. The layout is shown in the Figure 7-3.
The layout is extremely simple; the breakout board is connected to the Arduino
Due's 3.3 V power and also to the ground. The driver's SS pin is connected to
digital pin 10; this is the slave select pin and will be pulled low when the Arduino
Due requests information from the MAX31855. The SPI clock on pin 21 is con-
nected to the breakout board's clock connector. To read information from the