Week 3: Examining the I2C Option

Now that I know I'm going to be communicating with I2C, I feel that a quick refresher is in order...more for my benefit if anything.

To briefly sum everything up, I2C (Inter Integrated Circuit) is a bus protocol that facilitates communication between embedded electronic components. The hardware side specifies a two-wire system, consisting of an SDA line (Serial Data) and an SCL line (Serial Clock), both pulled up to whatever logic level is used. All devices on the bus are connected to these two lines, and all communication occurs when a designated 'master' device initiates comms. As the protocol (and our implementation) has it, the master is the only device that can initiate communications, the slaves are entirely reactive.

Fig 1. Example I2C configuration from Wikipedia.

In our system the NXT is the obvious choice to be the bus master. The way I envision the system working has the Arduino independently operating separate from the NXT as an I2C slave, to be polled for info when it is needed. The systems will obviously need to be connected, and thus I researched the pinouts for the NXT's sensor jacks. The lines very neatly expose the SDA/SCL lines in addition to Vdd and an AREF signal (GND). Given that the Arduino operates on a 5V logic level, the lines need to be pulled up with specific resistor values. Going off the suggestion of a site I found (link below), I am proceeding with 82K resistors. After all is said and done, we are left with the connection detailed below.

Fig 2. Basic I2C connection between NXT and Arduino.

One possible issue which may be apparent in the above schematic is that the NXT provides 4.3 volts in its sensor lines. This may be an issue if the Arduino is picky about its logic level voltages, however I am of the opinion that the I2C facilities (and the Wire library) of the Arduino are capable of operating with such voltages.

It bears mentioning that I do not have the full suite of hardware yet, and am thus still in the planning stages. Until I receive all the necessary equipment, the next few posts may be somewhat sparse. In the meantime though I have a pretty monumental decision to make regarding the dev environment for the NXT. The main contenders are LabView for Mindstorms which operates using the graphical language NXT-G, or RobotC which utilizes traditional C. Having taken ECE 1882 I know how painful it is to use LabView on the NXT, and that I despise it with all my soul. Thus my completely unbiased and scientifically formulated conclusion is that RobotC be used at all costs.


Resource website with NXT-Arduino info, a good deal of my work is derived from here:
http://www.dexterindustries.com/howto/connect-the-arduino-and-the-lego-mindstorms-together/