Wire - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 138

interrupt occurs), and possibly others with different specialized functions. To
read or write this data, you need to know several details:

The slave address

■

The register number

■

If it is a read or write operation

■

The length of the data to be received

■

It is important to know exactly how much data is to be sent and received.
Each I 2 C device is different and will function in a different way. Devices that
have only one writable register might accept a single byte of data directly and
will place that byte into the register. Other devices with several writable regis-
ters might require you to send the register number, followed by the contents,
or maybe send the contents of all the registers in multiple writes. I 2 C describes
a way to send data and receive data, but for your own implementation, it is up
to you what you need.
All Arduinos have a pair of I 2 C pins. The Arduino Due has two separate
I 2 C buses, SDA and SCL, as well as SDA1 and SCL1. The pins reserved for I 2 C
operations are listed in Table 8-1.

Table 8-1: I 2 C Pins on Diff erent Arduino Boards

BOARD SDA SCL

Uno A4 A5

Ethernet A4 A5

Mega2560 20 21

Leonardo 2 3

Due 20 21

Communicating

To communicate on the I 2 C bus, the Wire library must i rst be initialized. As
with all Arduino libraries, you must import the Wire library. This is done by
either adding the library from the Arduino IDE (Sketch ➪ Import Library ➪

Wire) or by manually typing in the sketch.

#include <Wire.h>

To declare the Arduino as an I 2 C device, call Wire.begin() . If the Arduino
is used as a slave, you must specify an address.

Wire.begin(address); // configures the Arduino as an I2C slave