Servo - January 2013

only requires two GPIO pins (not including power and ground): one for the SCL signal and one for the SDA signal. From this two-wire bus, we can connect as many sensors as we want, up to the addressing limits mentioned previously.

This new VEX Cortex feature seems to leave all the VEX 0.5 microcontroller users out in the cold, unfortunately, since it does not provide a dedicated I2C port. Still, we can do something about this state of affairs in that we can simulate this interface on the original VEX v0.5 microcontroller using a technique known as bit banging (this will be demonstrated in Part 2). When using bit banging on the VEX 0.5 microcontroller, the data rates will be considerably slower than those referred to above, but we should still be able to read most of the devices with no problems.

In fact, the EasyC firmware provided for the VEX 0.5 microcontroller can easily be adapted to run on the VEX Cortex and the VEX Pro (ARM9) microcontrollers using their I2C bus. This means some minor changes to the application are required to use direct EasyC I2C instructions supported by those controllers instead of the bit banged instructions. In fact, only the main routine controlling the compass would need to be converted.

We mentioned that an electronic compass such as the HMC6352 module can be interfaced to the VEX microcontroller (what better way to get your robot to travel in a specific direction than by using an electronic compass). Using it, we can tell our robot to simply move north 20 inches from its current position, then move east 20 inches, then move south 20 inches, then move west 20 inches, and then move north another 20 inches back to its original position. It will have traveled in a square trajectory path.

What does all this information mean for VEX users? The I2C port on the Cortex microcontroller opens a whole array of new sensors from both IFI and third-party vendors. These are sensors that require little or no calibration. They can also be auto-configured for your VEX robotics application using the device ID and device configuration information stored in the sensor’s local non-volatile memory (usually EEPROM). Just think of how many sensors you can use on your robot using only one I2C port.

THE grEAT COMPASS EXPEriMENT

In this experiment, we will show you how to make your

FigurE 5. Setup with the I2C interface connected to a typical microcontroller.

own electro-magnetic compass using some VEX components and an I2C based electronic compass to interface to the HMC6352 electronic compass. The connection to the compass module using a VEX v0.5 microcontroller is shown in Figure 5.

The bill of materials used in this experiment is shown in Table 1. Be sure to connect the ground wire and power to

QT Y

1

TABLE 1. Bill of materials for the VEX electromechanical compass.

DESCRIPTION SOURCE VEX microcontroller Innovation First, Inc. (IFI) www.vexforum.com

7. 2 volt battery IFI Wire-wrap cable RadioShack www.radioshack.com

IFI SparkFun

www.sparkfun.com

IFI

1

IFI

1

IFI

1

VEX bumper switch HMC6352 electronic compass 12-tooth gears: 0.5" ( 12. 7 mm) pitch dia. 36-tooth gears: 1.5" ( 38.1 mm) pitch dia. 60-tooth gears: 2. 5" ( 63. 5 mm) pitch dia. 84-tooth gears: 3. 5" (88.9 mm) pitch dia.