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SPI(Serial Peripheral interface)

串行外围设备接口。

常用标准

SPI为De-facto standard，即没有特定的组织来管理定义其规范，这也导致其有很多不同的版本。

对于Basic SPI，Freescale发布了一个标准版本： SPI Block Guide V04.01

三个版本，Basic SPI、Dual SPI（DSPI）、Quad SPI（QSPI）。

DSPI和QSPI和Basic SPI的差异为数据线为双向，但其是半双工，即同一时间只能单向传输数据，不能像千兆网口一样，同时双向传输数据。

Basic SPI的管脚定义如下表：

Name	Function
nCS	Chip select, a.k.a. slave select (nSS). This is driven low (hence the n) by the master to select a slave. There is a separate chip select line going from the master to each slave.
SCLK	Clock. Driven by the master, this provides the clock signal to the slaves to clock data in and out with. The exact polarity and edges that are used depend on the values of CPOL and CPHA.
MOSI	Master out, slave in. A.k.a. main out, subnode in. The master drives the line and provides data to the slaves. Only the slave with nCS asserted (low) listens to the data.
MISO	Master in, slave out. A.k.a. main in, subnode out. The selected slave can drive this line to send data to the master.