SPI BUS Electronic Design & Firmware by Embedded Design Engineer John Heritage


SPI BUS (Serial Peripheral Interface)

The Serial Peripheral Interface (SPI) is a synchronous serial bus developed by Motorola and present on many of their microcontrollers.

I currently design and program daily using the SPI protocol when communication between peripheral ICs and the microprocessor needs to be at a relatively high speed, many IC’s use support the SPI data bus.

The SPI bus consists of four signals: master out slave in (MOSI), master in slave out (MISO), serial clock (SCK), and active-low slave select (/SS). As a multi-master/slave protocol, communications between the master and selected slave use the unidirectional MISO and MOSI lines, to achieve data rates over 1Mbps in full duplex mode. The data is clocked simultaneously into the slave and master based on SCK pulses the master supplies. The SPI protocol allows for four different clocking types, based on the polarity and phase of the SCK signal. It is important to ensure that these are compatible between master and slave.

In addition to the 1Mbps data rate, another advantage to SPI is if only one slave device is used, the /SS line can be pulled low and the /SS signal does not have to be generated by the master. (This capability is, however, dependent on the phase selection of the SCK.)

A disadvantage to SPI is the requirement to have separate /SS lines for each slave. Provided that extra I/O pins are available, or extra board space for a demultiplexer IC, this is not a problem. But for small, low-pin-count microcontrollers, a multi-slave SPI interface might not be a viable solution.