This application note describes the operation of a general- purpose, microcontroller-based Serial Port Interface (SPI) boot circuit. This is a low cost solution for users who need to modify some of their device’s parameters at power up. This circuit addresses a 3-wire SPI application for programming converters, or any device that has a SPI option, and sends commands to user-defined SPI registers.
The SPI port consists of three pins. These are the serial clock pin (SCLK), the serial data IO pin (SDIO) and the chip select bar pin (CSB). SCLK is the serial shift clock in and is used to synchronize serial interface reads and writes. SDIO is a dual- purpose pin, which is used as an input and output depending on the instruction being sent and the relative position in the timing frame. CSB is an active low control that gates the read and write cycles.
Read more Here