Basics of SPI protocol:

Serial Peripheral Interface:

Common serial interface on many microcontrollers

 Simple 8-bit exchange between two devices

 Master initiates transfer and generates clock signal

 Slave device selected by master

 One-byte at a time transfer

 Data protocols are defined by application

 Must be in agreement across devices

SPI Block Diagram:

 8-bits transferred in each direction every time

 Master generates clock

 MOSI: “Master Out Slave In”; MISO: “Master In Slave Out”

 Connect MOSI to MOSI and MISO to MISO

 Very clean terminology, unlike “TX” and “RX” which are easy to confuse

 Slave Select (SS) used to select one of many slaves

 Terminology varies:

 Instead of SS, “Chip Select” (CS)

 Instead of MOSI and MISO, SIMOD and SOMI

Using SPI as a bus:

Configuration details to watch out for:

 CPHA (Clock PHase) aka ~CKPH (MSP430 terminology)

 = 0 or =1, determines when data goes on bus relative to clock

 CPOL (Clock POLarity) aka CKPL (MSP430)

 =0

 clock idles low between transfers

 =1

 clock idles high between transfers

This leads to 4 SPI clock modes

SPI timing diagram:

SPI properties:

 Pros

 Simplest way to connect 1 peripheral to a micro

 Fast (10s of Mbits/s, not on MSP) because all lines 

actively driven, unlike I2C

 Clock does not need to be precise

 Nice for connecting 1 slave

 Cons

 No built-in acknowledgement of data

 Not very good for multiple slaves

 Requires 4 wires

 3 wire variants exist…some get rid of full duplex and share a 

data line, some get rid of slave select.