The SPI Bus - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 184

HardwareConiguration

Four data lines, at a minimum, are present in any SPI system. Additional SS
lines are added for each slave device appended to the network. Before you learn
how to actually send and receive data to and from an SPI device, you need to
understand what these I/O lines do and how they should be wired. Table 9-2
describes these lines.

Table 9-2: SPI Communication Lines

SPI COMMUNICATION LINE DESCRIPTION

MOSI Used for sending serial data from the master device

to a slave device.

MISO Used for sending serial data from a slave device to

the master device.

SCLK The signal by which the serial data is synchronized

with the receiving device, so it knows when to read
the input.

SS A line indicating slave device selection. Pulling it low

means you are speaking with that slave device.

Unlike with the I 2 C bus, pull-up resistors are not required, and communica-
tion is fully bidirectional. To wire an SPI device to the Arduino, all you have to
do is connect the MOSI, MISO, SCLK, and SS pins and you'll be ready to use to
the SPI communication library.

NAMING CONVENTIONS

BecauseSPIisnotauniversalstandard,somedevicesandmanufacturers
mayusedifferentnamesfortheSPIcommunicationlines.Slaveselectis
sometimesreferredtoaschipselect(CS),serialclockissometimesjust
calledclock(CLK),MOSIandMISOpinsonslavedevicesaresometimes
abbreviatedtoserialdatain(SDI),andserialdataout(SDO).

CommunicationScheme

The SPI communication scheme is synced with the clock signal and depends on
the state of the SS line. Because all devices on the bus share the MOSI, MISO, and
SCLK lines, all commands sent from the master arrive at each slave. The SS pin
tells the slave whether it should ignore this data or respond to it. Importantly,
this means that you must make sure to only have one SS pin set low (the active
mode) at a time in any program that you write.