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It is straightforward to implement an interpreter-like interface, where user commands are transparently compiled, downloaded, and executed.

The Cricket virtual machine is burned into the PIC microprocessor's internal ROM

The user's code resides in a serial EEPROM

Built-in infrared communications routines include a protocol for reading and writing to this external EEPROM, and for asking the virtual machine to begin execution of byte codes already loaded into the EEPROM.

Users write programs for the Cricket in Cricket Logo, a dialect of Logo specialized for the Cricket virtual machine. Essentially, there is a one-to-one mapping between statements in Cricket Logo and primitive functions built into the virtual machine. This makes the implementation of the compiler far simpler than typical compilers.

The compiler includes an interactive mode—a text window where user expressions are compiled, downloaded, and executed in one step when the user presses the return key. A portion of the Cricket's memory is set aside for these dynamic programs.

we have found that a debugger is not necessary because of the interactive and incremental style of project development that occurs when using the Cricket.

The infrared protocol includes the following capabilities: Check that a Cricket is present and ready for other commands. Write a byte to the Cricket's EEPROM. Read a byte from the Cricket's memory. Begin program execution from a particular memory address.

User-level primitive functions compile to one, two, or three bytes of object code for the Cricket virtual machine.

The Cricket virtual machine has two process threads: a foreground process and a background daemon. In most Cricket programs, the foreground thread handles all the work, but for some tasks, the background daemon is valuable. For example, the background daemon can be used to instigate a periodic activity, or take action when some event occurs.

There are hardware-specific primitives for interacting with on-board Cricket hardware. Motor commands set state (on or off), direction, and power levels for each of the two integrated motor drivers. Analog sensor primitives (sensora and sensorb) return a value (0 to 255) for each of the two voltage inputs. These inputs also may be interpreted as digital values using the switcha and switchb primitives. There is a pair of primitive functions for generating tones on the piezo beeper: beep and note, the latter taking pitch and duration arguments.

there is a background millisecond timer that is updated every four milliseconds

One foreground thread plus one background daemon Daemon fires when provided Boolean expression makes false-to-true transition

Eventually, we got to the point where we don’t know what Pleo will do next because he learns. If Caleb and I went to your house to see your Pleo, we couldn’t predict a lot of the things he would do, even though we know everything we put in him. Pleo has the ability to change and figure things out on his own.

Consumers will be able to download and customize Pleo later this year or early next year. We want to give the user the ability to change Pleo’s personality, animations and tricks. We also want to allow developers and hobbyists to take the SDK and motion system and behavior system and choreograph advanced features and animations for new AI functionality.

We didn’t include a camera (or voice recognition) in Pleo because of the price point for the product. Pleo is probably a good hack for a CMU camera, and we want people to develop these sorts of things.

The only way you can create life is to give it choice. Life is very complex, and it has to evolve, otherwise it is a robot. The only way to get complex systems to work is to let them chose for themselves.