Simulator
This function needs real hardware to work with. It’s not supported in the simulator.
Set the Serial Peripheral Interface (SPI) format.
pins.spiFormat(8, 3);The data sent over a SPI connection has a number of bits to represent each value. Also, position of the clock signal (SCK), high or low, and the exact time when a data value is read is determined by the SPI mode. Both the bits and mode values together are called the SPI format.
The default number of bits is 8 and the default mode value is 3.
8 since only 8 bits is currently supported for SPI data values. 
0: the data line is active when SCK goes tohighand the data values are read when SCK goes tohigh
1: the data line is active when SCK goes tohighand the data values are read when SCK goes tolow
2: the data line is active when SCK goes tolowand the data values are read when SCK goes tohigh
3: the data line is active when SCK goes tolowand the data values are read when SCK goes tolow
Set the pins and format for the SPI connection.
pins.spiPins(DigitalPin.C15, DigitalPin.C14, DigitalPin.C13);
pins.spiFormat(8, 3);