Getting to know the Raspberry Pi I2C bus

Users rely on the very efficient GPIO interface of the Raspberry Pi for many types of hardware projects. However, some functions, such as analog input and output, are generally not supported by the GPIO and therefore are incorporated in Rasp Pi projects in a limited fashion – if at all.

Similarly, the Raspberry Pi's one lone PWM outlet can quickly put the brakes on a project. Software solutions like ServoBlaster [1] offer a convenient fix, although they are restricted in terms of usefulness. If you really need more digital I/O, though, you will quickly run up against hard limits.

The I2C bus is a simple and professional solution for projects of large scope, and the Raspberry Pi has two corresponding interfaces on-board. In a series of articles, I discuss a number of semiconductors, each of which has an interface for the I2C bus.

General Information

The I2C bus is a serial master-slave bus suitable for communication over short distances – within a circuit board or within a device. The early 1980s technology came out of Philips Semiconductors (now part of NXP Semiconductors) for building control electronics in entertainment systems.

Data transmission occurs synchronously over two bidirectional lines: the serial data line, SDA, and the serial clock line, SCL. Resistors pull both lines up to a positive potential. The master node specifies the speed and operating mode and initiates communication byte for byte. The transmission speed of the bus varies from 100Kbps bidirectional in standard mode up to 5Mbps unidirectional in ultrafast mode (Table 1).

The I2C bus works with an address range of 7 bits, for up to 128 addresses. However, 16 of these addresses are reserved for special tasks, leaving 112 free. The eighth bit tells the slave whether it should receive data from the master or transmit data to the master.

Usually, you can only choose lower bits of the addresses on the slaves because the upper bits are predefined. The best way to figure out which addresses are available is to check the information sheet that comes with the product. Table 2 shows some examples of various address spaces.

If you are working on a large project and need more than the 112 available addresses, you can attach a bus multiplexer to the primary I2C bus; I will devote a section to this specialty component in a future article.

The I2C bus is susceptible to disturbances because it was originally designed to bridge just a few centimeters. Various possibilities exist to help improve the electrical characteristics of the bus, ranging from adaptation of the pull-up resistors to tunneling through CAN bus drivers. I will devote more attention to the question of different I2C bus drivers in a future article.

Testing

You will find an I2C interface directly on the Rasp Pi GPIO. Pins P1-03 (P1 header, pin 3), SDA_1, and P1-05, SCL_1, already have the necessary 1.8kohm pull-up resistors built-in that pull the Rasp Pi to 3.3V on idle. You can access a second I2C interface via the P5 connector, but you will first have to solder this onto the back of the board. You can communicate with the second interface via the P5-3 (SDA_0) and P5-4 (SCL_0) pins, although they have no premounted pull-up resistors.

To run a test, you can attach a PCF8574 microcontroller [2] to the I2C bus (Figure 1) and then observe the most important of its basic functions. The circuit diagram available online [3] for this simple test setup includes four LEDs and four switches.

Figure 1: A PCF8574 microcontroller and four LEDs, each with its own switch.