SPI（Serial Peripheral Interface）是摩托羅拉開發的一種簡便的處理器和外設之間的接口，是一種同步傳輸接口，該接口使用四種信號，
Master Out Slave In (MOSI),
Master In Slave Out (MISO),
Serial CLocK (SCLK orSCK)
chip select (CS)
SPI接口基本連接如下圖
SPI has four modes of operation, depending on clock polarity and clock phase. For low clock polarity, the clock (SCK) is low when idle and toggles high during a transfer. When configured for high clock polarity, the clock is high when idle and toggles low during a transfer.
The two clock phases are known as clock phase zero and clock phase one. For clock phase zero, MOSI and MISO outputs are valid on the rising edge of the clock (SCK) if the clock polarity is low (Figure 7-4). If the clock polarity is high, these outputs are valid on the falling edge of SCK, for clock phase zero (Figure 7-5). The "X" bit output on MISO is an undefined extra bit and is a consequence of the SPI interface. You don't need to worry about it, as the SPI interfaces ignore it.