The serial peripheral interface (SPI) is a full duplex synchronous serial follower interface, which uses four lines: SDO, SDI, SCLK and nSCS. Data is transferred by the lines SDI and SDO at the rate given by SCLK. The falling edge of nSCS indicates the beginning of an access. Data is sampled in on line SDI at the falling edge of SCLK and shifted out on line SDO at the rising edge of SCLK. Each access must be terminated by a rising edge of nSCS.

A modulo 8/16 counter ensures that data is taken only when a multiple of 8 bit has been transferred after the first 16 bits. Otherwise a TER bit is asserted. In this way the interface provides daisy chain capability with 16 bit as well as with 8 bit SPI devices.