Step 5: Some Caveats:
** Caveat #1 : The reason this trick works is that, unlike most Arduinos, the Lilypad Arduino is clocked by the ATMEGA's built-in oscillator instead of by an additional crystal oscillator circuit. This lack of additional oscillator circuitry makes wiring up the circuitry a lot simpler, but as a result the chip runs slower (8MHz instead of 16MHz) and its timing is not as accurate. The lilypad bootloader knows how to compensate for this speed difference to make sure all of your delays and baud-rates and other time-sensitive functions work correctly, but the fact remains that the chip will perform slower. This is not a big deal unless your application needs very accurate timing or needs to perform tasks very quickly.
If you do need your Arduino to run at the proper 16MHz speed, use two 22pf cermamic capacitors and an Abracon ABL-16.000MHZ-B2 crystal oscillator, and connect them as shown in the figure above. If you do decide to use this 16MHz crystal oscillator setup, don't use the Lilypad bootloader -- just use the correct bootloader for the board you have (eg diecimila, duemilanove, uno etc).
** Caveat #2: The ATMEGA chip has weird names for its pins -- ie ATMEGA pin 1 is not necessarily Arduino pin 1. So you need the chart above to translate (courtesy of http://arduino.cc/en/Hacking/PinMapping168 )
**Caveat #3: Whatever power supply you use (cell phone charger, usb jack, or AAA batteries are quick and easy methods) it may not be a perfectly solid 5V. So if you have problems with the chip hanging or resetting unexpectedly, add a .01 uF capacitor between 5V and Ground, as close to the chip as reasonably possible. This filters out noise on the power supply. You can also add an additional 10uF electrolytic cap in the same place (noting the polarity markings on the capacitor) --this protects against disruptions in the power supply.
**Caveat #4: The most expensive and complicated part of an Arduino is the serial-to-usb circuitry. By omitting it, we save a lot of cost and effort, but If you still want to use your ultra-bare-bones arduino to communicate using the normal Serial.print() commands, you will need to purchase a 5V TTL USB-to-rs232 adapter cable, and connect it like so:
Cable TX wire ---> ATMEGA Pin 2 (RXD)
Cable RX wire ---> ATMEGA Pin 3 (TXD)
Cable Gnd wire ---> ATMEGA Pin 8 (Gnd)