This library allows you to communicate with SPI devices, with the Arduino as the master device.
Serial Peripheral Interface (SPI) is a synchronous serial data protocol used by microcontrollers for communicating with one or more peripheral devices quickly over short distances. It can also be used for communication between two microcontrollers.
With an SPI connection there is always one master device (usually a microcontroller) which controls the peripheral devices. Typically there are three lines common to all the devices,
To write code for a new SPI device you need to note a few things:
The SPI standard is loose and each device implements it a little differently. This means you have to pay special attention to the device's datasheet when writing your code. Generally speaking, there are three modes of transmission. These modes control whether data is shifted in and out on the rising or falling edge of the data clock signal (called the clock phase, and whether the clock is idle when high or low (called the clock polarity). The three modes combine polarity and phase. The SPI.setDataMode() function lets you set the mode to control clock polarity and phase according to this table:
|Mode||Clock Polarity (CPOL)||Clock Phase (CPHA)|
Once you have your SPI parameters set correctly you just need to figure which registers in your device control which functions, and you're good to go. This will be explained in the data sheet for your device.
For more on SPI, see Wikipedia's page on SPI.
On the Arduino Duemilanove and other ATmega168 / 328-based boards, the SPI bus uses pins 10 (SS), 11 (MOSI), 12 (MISO), and 13 (SCK). On the Arduino Mega, this is 50 (MISO), 51 (MOSI), 52 (SCK), and 53 (SS). Note that even if you're not using the SS pin, it must remain set as an output; otherwise, the SPI interface can be put into slave mode, rendering the library inoperative.
It is possible to use a pin other than pin 10 as the slave select (SS) pin. For example, the Arduino Ethernet shield uses pin 4 to control the SPI connection to the on-board SD card, and pin 10 to control the connection to the Ethernet controller.