**Further reading:**  

- [Introduction to I²C and SPI protocols](http://www.byteparadigm.com/kb/article/AA-00255)
- [Microchip SPI tutorial](http://ww1.microchip.com/downloads/en/DeviceDoc/spi.pdf) (pdf)
- [Overview wiki on SPI bus.][1]  Of interest is the [chapter about lack of formal standard][2] and the variety of protocol variations.


  [1]: http://en.wikipedia.org/wiki/SPI_bus
  [2]: http://en.wikipedia.org/wiki/SPI_bus#Standards