The Serial Peripheral Interface Bus or SPI bus is a very loose standard for controlling almost any digital electronics that accepts a clocked serial stream of bits. A nearly identical standard called "Microwire" is a restricted subset of SPI, trademarked by National Semiconductor.

A SPI is cheap, in that it does not take up much space on an integrated circuit, and effectively multiplies the pins, the expensive part of the IC. It can also be implemented in software with a few standard IO pins of a microcontroller.

Many real digital systems have peripherals that need to exist, but need not be fast. The advantage of a serial bus is that it minimizes the number of conductors, pins, and the size of the package of an integrated circuit. This reduces the cost of making, assembling and testing the electronics.

A serial peripheral bus is the most flexible choice when many different types of serial peripherals must be present, and there is a single controller. It operates in duplex (sending and receiving at the same time), making it an excellent choice for some data transmission systems.

In operation, there is a clock, a "data in," a "data out," and a "chip select" for each integrated circuit that is to be controlled. Almost any serial digital device can be controlled with this combination of signals.

Most often, data goes into an SPI peripheral when the clock goes low, and comes out when the clock goes high. Usually, a peripheral is selected when chip select is low. Most devices have outputs that become high impedance (switched-off) when the device is not selected. This arrangement permits several devices to talk to a single input. Clock speeds range from several thousand clocks per second (usually for software-based implementations), to several million per second.

In practice, many devices have exceptions. Some read data as the clock goes up, others read as it goes down. Some read on the leading edge of the clock, some on the trailing edge. Writing is almost always on the opposite direction of clock movement. Some devices have two clocks, one to "capture" or "display" data, and another to clock it into the device. In practice, many capture clocks can be run as inversions or inputs of the chip select. Chip selects can be either selected high, or selected low. Many devices are designed to be daisy-chained into long chains of identical devices.

SPI looks at first like a non-standard. However, many programmers that develop embedded systems have a software module somewhere in their past that drives such a bus from a few general-purpose I/O pins, often with the ability to run different clock polarities or clock edges for different devices. The classic way to implement an SPI interface from a personal computer to custom electronics is via a custom cable to the parallel printer port. The parallel port uses standard TTL logic voltages- 5V is high.

In some organizations, these pieces of code have become quite stylized and generalized. Often the serial bus passes through a programmable logic array, making the hardware programmable, as well as the software.

Motorola actually developed a standard microcontroller peripheral for these sorts of busses, which gave them a name.

See also I²C, computer bus, Controller Area Network

External links

• Introduction to Serial Peripheral Interface (http://embedded.com/showArticle.jhtml?articleID=9900483)/SPI vs I²C