Instructables

The RRRRRRRRRRBBA, a $3 Arduino

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)
 
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ZachyKras3 years ago
i'm pretty new to electronics, but i've been thinking about getting an arduino a lot lately. I've done some research and this is the answer to a lot of my questions. thank you so much! do you think you might be able to add a little schematic about where to include the crystal and capacitors when you move the chip to its project? thanks for making a great instructable for beginners like me!
As a longtime microcontroller user, it would recommend that anyone wanting an Arduino buy an Arduino. Then you can learn by using all the tutorials that actually match the hardware that you have purchased. The single chip will just frustrate you if you are not experienced.

- Ray
XCVG3 years ago
The part about needing the AVRISP to program the chip initially is not strictly true. The ArduinoISP sketch can be used to burn the bootloader using your Arduino! Of course, you need something else to put the target microcontroller in, which is why breadboards were invented. Alternatively, this can be used to solve the infamous programmer chicken-and-egg problem. I built a USBTinyISP clone using this method.
ipfone3 years ago
Whit a 16 k ressonator you don´t need the capacitors and you can run at normal speed.. so all your scripts will run with no problems. Don´t forget that all arduino ide uses 16 mhz and not 8 mhz....
bmlbytes ipfone3 years ago
The ATMega chip will run on a 8MHz crystal though, it will just process information slower.
Agree.. but the problem is the arduino IDE .. not the fact that it does run or does not.
A.O.D3 years ago
This might be a silly question, but one thing I wasn't quite clear on was whether or not I need to connect the crystal to the ground.
bmlbytes A.O.D3 years ago
No the crystal itself does not get connected to the ground. The crystal is connected to the pins on the ATMEGA chip, and also to the capacitors. The capacitors are connected to ground, but the crystal itself is not.
btw if you own an arduino its super easy to bit bang to the chip