Serial Peripheral Interface (SPI)

Serial Peripheral Interface (SPI) is a high-speed, synchronous serial communication protocol used primarily for short-distance communication between microcontrollers and peripheral devices (like sensors, SD cards, or displays).

MOSI: The data line for sending information from Master to Slave.
MISO: The data line for sending information from Slave back to Master.
SCLK: The clock signal generated by the controller to synchronize data.
CS/SS: Used by Master to “wake up” a specific Slave before talking to it.

SPI Mode: The four SPI modes determine the clock edge on which data is sampled.

CPOL (Clock Polarity) – idle level of SCK

• CPOL = 0 → idle low
• CPOL = 1 → idle high

CPHA (Clock Phase) – sampling edge

• CPHA = 0 → first edge
• CPHA = 1 → second edge

Green arrows indicate sampling edges (when data is read).
MOSI data is expected to be stable at those arrows.

SPI Mode 0 (CPOL = 0, CPHA = 0)

• Clock idles low
• Data is sampled on the rising edge
• Data changes on the falling edge

SPI Mode 1 (CPOL = 0, CPHA = 1)

• Clock idles low
• Data is sampled on the falling edge
• Data changes on the rising edge

SPI Mode 2 (CPOL = 1, CPHA = 0)

• Clock idles high
• Data is sampled on the falling edge
• Data changes on the rising edge

SPI Mode 3 (CPOL = 1, CPHA = 1)

• Clock idles high
• Data is sampled on the rising edge
• Data changes on the falling edge

SPI Communication Example

Reference
[1] SPI通讯协议
[2] Basics of SPI: Serial Communications