The Electric Imp has two sets of pins for SPI. Those sets are pins 1,8 & 9, and pins 2,5 & 7. For this tutorial we will be using SPI189 as it is referred to in hardware configuration. When you configure SPI189 you get the following pin configuration in the Electric Imp.

• PIN 1 - CLK or Clock
• PIN 8 - MOSI (Microcontroller Out/Sensor In)
• PIN 9 - MISO (Microcontroller In/Sensor Out) or Data Out

This line of code configures SPI189 in the device:

hardware.spi189.configure(MSB_FIRST | CLOCK_IDLE_LOW , 1000);

Since we will only be reading data from the MAX31855 board (sensor), we only need to connect the MISO pin, which is pin 9, and not MOSI. We also need Pin 1 to provide a clock signal, and one more pin to act as a Chip Select (CS) pin. In order to tell the MAX31855 that we want to read SPI data, we pull the CS pin LOW, read the data, and then set the pin back to HIGH. We can configure any of the other pins as a CS pin, so we will use Pin 2, since it is directly across from the CS pin.

This code reads SPI data from the chip that is connected to Pin 2:

hardware.pin2.write(0); //pull CS LOW to start the transmission of temp data
local temp32=hardware.spi189.readblob(4);//SPI read is totally completed here
hardware.pin2.write(1); // pull CS HIGH

Note: You can read as many chips as you like from a single SPI bus, as long as you have a Chip Select Pin for each. Using just the April board, you could read up to four MAX31855 chips. This is very handy for a project with multiple thermocouples.

To complete the wiring, connect these pins as shown above.

April - MAX31855
PIN1 - CLK
PIN2 - CS
PIN9 - DO
3V3 - 3Vo
GND - GND