Servo - January 2013

master (a microcontroller), three slave nodes (an ADC, a DAC, and a microcontroller), and pull-up resistors (Rp) uses only two bidirectional open-drain lines — Serial Data (SDA) and Serial Clock (SCL) — pulled up with resistors, with one on each signal line, ranging from 2.2K ohms to 10K ohms. These pull-ups are required for only one of the I2C devices connected to the bus. Typical voltages used are +5V or + 3.3V, although systems with other voltages are permitted.

The I²C reference design has a seven-bit or a 10-bit (depending on the device used) address space. Common I²C bus speeds are the 100 Kbit/s standard mode and the 10 Kbit/s low-speed mode, but arbitrarily low clock frequencies are also allowed. Recent revisions of I²C can host more nodes and run at faster speeds (400 kbit/s fast mode; 1 Mbit/s fast mode plus or Fm+; and 3. 4 Mbit/s high-speed mode). These speeds are more widely used with embedded systems rather than on PCs. There are also other features, such as 16-bit addressing.

Note that the bit rates quoted are for transactions between the master and slave without clock stretching or other hardware overhead. Protocol overheads include a slave address and perhaps a register address within the slave device, as well as per-byte ACK/NACK bits. So, the actual transfer rate of user data is lower than those peak bit rates alone would imply. For example, if each interaction with a slave inefficiently allows only one byte of data to be transferred, the data rate will be less than half the peak bit rate. The maximum number of nodes is limited by the address space, and also by the total bus capacitance of 400 pF which restricts practical communication distances to a few yards.

There is an I2C bus master and the I2C peripherals which respond to commands sent by the master controller. Each device on the bus listens for its address, and wakes up when the address sent by the master matches the selected peripheral address. Using the I2C bus, we can network up to 128 devices using seven-bit addressing, and up to 16,384

devices using 14-bit addressing. Bus capacitance will limit the distance at which we can reliably communicate, and will probably also limit the actual number of devices to well below the theoretical limits.

The master initiates the I2C message transfers using its Start condition. Each device on the I2C bus listens for its address before responding. The I2C interface is known as a synchronous serial interface since one of the signals is a clock signal, unlike serial RS-232 which is asynchronous. It is similar to the SPI four-wire interface.

The I2C message strings are just sequences of I2C conditions sent from the master to the peripherals. These include the Start condition, Re-start condition, Acknowledge condition, Not Acknowledge condition, and the Stop condition. We will describe these sequences in greater detail in Part 2 when we cover the necessary I2C firmware.

HOW I2C CAN BE USED ON VEX

You may be asking yourself how I2C relates to VEX robotics. Well, see for yourself in Figure 4, which shows the new VEX Cortex microcontroller and a new 393 motor with the integrated quadrature encoder connected to the I2C port. This is the first I2C sensor product to be released by IFI. The new VEX Pro microcontroller based on an ARM9 core also has a similar I2C port. Why would you want to use the I2C interface?

The main reason for VEX users is the variety of new digital sensors and actuators that can be connected to the I2C bus. These can easily be interfaced. Other advantages are that digital sensors are much simpler to use. In addition to providing digital readings, they don’t require much calibration since they are usually factory calibrated.

Another advantage is that they can easily be networked to processors with a limited number of general-purpose The new VEX Cortex microcontroller allows users to connect advanced digital sensors to it using its built-in I2C hardware interface. Just think of all the sensors and other devices that we will be able to use for our VEX robot applications! These include electronic compasses, GPS, serial EEPROMs, digital temperature sensors, pressure, humidity, XYZ accelerometers, six to nine DOF gyros, DC motor controllers, servo controllers, stepper motor controllers, LCD displays, and math co-processors.