Accessing the raw bus

As for the USB and I²C buses, the SPI bus supports the raw access in order to directly send and receive messages from the SPI slaves, so it's time to show you an example on how we can do it on our BeagleBone Black.

We are going to manage a really simple device: the thermocouple to digital converter, as shown in the following image:

Note

The device can be purchased at (or by surfing the Internet) http://www.cosino.io/product/thermocouple-max31855.

By looking at the chip's datasheet at the URL http://datasheets.maximintegrated.com/en/ds/MAX31855.pdf, we can see that its functioning is very simple: it has one 32-bit register, where we can read the temperature information. The register has the following format:

14-bit thermocouple data				Res	Fault	12-bit internal temperature				Res	SCV	SCG	OC
D31	D30	...	D18	D17	D16	D15	D14	...	D4	D3	D2	D1	D0
Sign	MSB	X	LSB	-	X	X	MSB	X	LSB	-	X	X	X

So, in order to read the temperature data, we have to read the preceding...