SPI vs. I2C
Both SPI and I2C provide good support for communication with slow peripheral devices that are accessed intermittently. EEPROMs and real-time clocks are examples of such devices. But SPI is better suited than I2C for applications that are naturally thought of as data streams (as opposed to reading and writing addressed locations in a slave device). An example of a “stream” application is data communication between microprocessors or digital signal processors. Another is data transfer from analog-to-digital converters.
SPI can also achieve significantly higher data rates than I2C. SPI-compatible interfaces often range into the tens of megahertz. SPI really gains efficiency in applications that take advantage of its duplex capability, such as the communication between a “codec” (coder-decoder) and a digital signal processor, which consists of simultaneously sending samples in and out.
SPI devices communicate using a master-slave relationship. Due to its lack of built-in device addressing, SPI requires more effort and more hardware resources than I2C when more than one slave is involved. But SPI tends to be simpler and more efficient than I2C in point-to-point (single master, single slave) applications for the very same reason; the lack of device addressing means less overhead.
Inside the box
SPI is a serial bus standard established by Motorola and supported in silicon products from various manufacturers. SPI interfaces are available on popular communication processors such as the MPC8260 and microcontrollers such as the M68HC11. It is a synchronous serial data link that operates in full duplex (signals carrying data go in both directions simultaneously).
Devices communicate using a master/slave relationship, in which the master initiates the data frame. When the master generates a clock and selects a slave device, data may be transferred in either or both directions simultaneously. In fact, as far as SPI is concerned, data are always transferred in both directions. It is up to the master and slave devices to know whether a received byte is meaningful or not. So a device must discard the received byte in a “transmit only” frame or generate a dummy byte for a “receive only” frame.
Links: http://embedded.com/columns/beginerscorner/9900483
