Thanks for the link. I did some searching for digital multiplexers since the QTR sensors on the 3pi are controlled and read using digital I/Os and found some that are even cheaper, but I'm not convinced the added complexity and cost are justified when the reward is freeing up two I/O lines (since there are five sensors, all three multiplexer inputs would be required.
It seems like the best way to get more I/O lines is to use an AVR in a package that natively provides more I/O lines (e.g. the mega644). The increased cost would be comparable to that of using a mux or port expander, but the performance would be much higher and the circuit complexity would be much lower.
If your design to expand the 3pi robot requires more than three digital I/Os and two analog inputs (or two digital I/Os and three analog inputs), the solution we envision is to turn the base into a serially controlled slave device driven by a secondary microcontroller such as an Arduino.
We intend the 3pi to be the first of a line of programmable robots that provide a quality mechanical platform for your software and custom electronics. We're already tossing around an idea for a larger robot based on the mega644 (64k flash, 4k SRAM, 2k EEPROM, 32 digital I/Os, 8 analog inputs). As far as programming goes, the mega644 is very similar to the mega168, and there would be plenty of I/O lines and prototyping space available for custom electronics.
- Ben