Servo - June 2009

subtle and really obvious once I saw it. There is the lesson: When the data you observe doesn’t make any sense, examine your assumptions; at least one of them is wrong!

So, there you have it. To add more sensors, simply daisy-chain more boards on to the SCI bus. Make sure that each of them has a different address though! Figure 8 shows my entire test setup with all of the boards wired together. Not pretty, but it worked great.

ENHANCEMENTS: You can always do better…

There is certainly room for future enhancement here. Here are some of the modifications and changes that I want to make:

1) Add a command to change the board address so I don’t have to compile the address in.

2) Carefully measure the required data setup time so I can run the bus as fast as possible – find out what is possible.

3) Find the maximum wire length before data corruption occurs.

4) Investigate devices that have an open drain output so that I don’t have to worry about damaging a part if bus contention occurs (two parts driving the line in opposite directions).

5) Add more kinds of sensors!

That’s all folks! Have fun and let me know what you come up with if you like this bus architecture.

Remember, the complete source code for this article is available on the SERVO Magazine website under Mr. Roboto as SPIsonar.zip. It will have three files in it: SPIsonar.c for the PIC16F630 and CCS C compiler, and atm168.c and atm168.h for the ATMega168 processor side of the project. I used avr-gcc 4. 3.0 as my compiler with Eclipse and the AVR Eclipse plugin as my development environment. If you want to know how to use this environment, see my columns in the August and September issues.

Well, we’ve come to the end of another Mr. Roboto column and I hope you’ve felt that it was time well spent. As usual, I can be reached for questions, comments, and criticisms at roboto@servomagazine.com and I’ll be happy to work on it! Until next time, keep on building those robots! SV

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