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The Serial Peripheral Interface bus (SPI) is a synchronous serial communication interface specification used for short distance communication, primarily in embedded systems. The interface was developed by Motorola in the late eighties and has become a de facto standard. Typical applications include Secure Digital cards and liquid crystal displays.

SPI devices communicate in full duplex mode using a master-slave architecture with a single master. The master device originates the frame for reading and writing. Multiple slave devices are supported through selection with individual slave select (SS) lines.

Sometimes SPI is called a four-wire serial bus, contrasting with three-, two-, and one-wire serial buses. The SPI may be accurately described as a synchronous serial interface, but it is different from the Synchronous Serial Interface (SSI) protocol, which is also a four-wire synchronous serial communication protocol. But SSI Protocol employs differential signaling and provides only a single simplex communication channel.

The SPI bus specifies five logic signals:

While the above pin names are the most popular, in the past alternative pin naming conventions were sometimes used, and so SPI port pin names for older IC products may differ from those depicted in these illustrations:

Serial Clock:

Master Output ${\displaystyle \rightarrow }$ Slave Input:

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