This is an old revision of the document!
There's so many Arduino's out there, it may get a little confusing. We wanted to clarify for people some of the changes in the new version
NB this is just our opinion and interpretation of some of the decisions made by Arduino. We aren't associated with Arduino, and don't speak for them! If you have to get an Official Response to your Arduino question please contact them directly. Thx!
But first…some history!
First there was the serial Arduino (whats the name of it?) with RS232 which was not used outside of the Arduino team & friends.
The first popularly manufactured Arduino was called the NG (New Generation, like Star Trek, yknow?) The NG used the Atmega8 chip running at 16 MHz and an FT232 chip for the USB interface. The bootloader takes up 2KB of space and runs at 19200 baud
The next version was the Diecimila. The Diecimila updated the chip from the Atmega8 to the Atmega168. The great thing here is double the space and memory (16K instead of 8K). It still ran at 16MHz. The Diecimila also added two extra header pins for 3.3V (from the FTDI chip) and the reset pin which can be handy when a shield is covering up the Reset button. The bootloader takes up 2KB of space and runs at 19200 baud
In 2009, the Duemilanove was released. This one also upgraded the chip again, to the Atmega328. Yet another doubling of space and memory! Another upgrade is now the power is automagically switched between USB and DC-jack which removed the previous jumper. This makes it easier and faster to move from programming to standalone and got rid of some confusion. The bootloader takes up 2KB of space and runs at 57600 baud
In 2010, we have the Uno! The Uno still uses the 328P chip and the power switcher. It has a smaller bootloader called OptiBoot (more space for users' projects) that runs at 115K. So even though the chip is the same, you get another 1.5K of extra flash space that was previously used by the bootloader. The FTDI chip has also been replaced with a atmega8u2 which allows for different kinds of USB interfaces. Finally, there's an extra 3.3V regulator (LP2985) for a better 3.3V supply. whew!
So! All of the older Arduinos (NG, Diecimila and Duemilanove) have used an FTDI chip (the FT232RL) to convert the TTL serial from the Arduino chip (Atmel ATmega). This allows for printable debugging, connecting to software like PureData/Max, Processing, Python, etc. etc. It also allows updating the firmware via the serial bootloader.
The good news about the FT232RL has royalty-free drivers and pretty much just works. The bad news is that it can -only- act as a USB/Serial port. It can't act like a keyboard, mouse, disk drive, MIDI device, etc.
The Uno has changed that by exchanging the FT232RL chip with an atmega8u2 chip. There are a few things that are possible with this new chip but before we discuss that lets make it clear that by default, this chip acts identically to the FTDI chip that it replaces. It's just a USB-serial port!
One improvement in updating the chip is that, previously, Mac users needed to install FTDI drivers. The 8u2 imitates a 'generic' CDC serial device. So now, Mac users do not have to install a driver. Windows users still need to install the .INF file but luckily there are no drivers. This means there will be fewer problems with new versions of windows. There is no way to have a serial USB device that doesn't require an INF file in windows, sadly :(
The big thing that is nice about the 8u2 is that advanced users can turn it into a different kind of USB device. For example it can act like a keyboard or mouse. Or a disk driver. Or a MIDI interface, etc. Right now there are no examples of how to do this, but we hope to shortly.
And, finally, going with the 8u2 reduced the price of the board which made up for some of the other extras.
The Arduino team has indicated they thought about this but preferred that hackability of a DIP chip. There may be future Arduino's with 32u4 or other chips.
The 8u2 can be programmed by soldering a 6-pin ISP header and using a standard AVR programmer. You can also use the bootloader (DFU) in the 8u2 by soldering the jumper right underneath the board. Again, we don't have any examples or tutorials but hope to shortly.
The code for the 8u2 is based on LUFA, Dean Cameras totally awesome USB-AVR library that has great examples and documentation. Its also completely open source.
The FT232RL had an internal oscillator whereas the 8u2 does not. That means there is a 16mhz crystal next to the 8u2 to allow it to keep up with precise USB timing.
On the other hand, the Atmega328p chip that is the core processor in the Arduino now has a 16mhz ceramic resonator. Ceramic resonators are slightly less precise than crystals but we have been assured that this one was specified and works quite well.
The short answer is: yes. The long answer is that most things that people are doing with Arduino do not rely on 20ppm precision timing where 100ppm would fail. For people who want long term precise timekeeping we suggest going with a TCXO (temperature compensation crystal oscillator) - but you would know if you needed that.
Good question, technically you can. However, in practice the board did not make it through FCC certification with one crystal (long traces with fast squarewaves = lots of noise)
You can absolutely connect the CLKO out the crystal from the '8u2 to the '328p but you're on your own as we don't think there will be any tutorials about that.
There's a new bootloader. It works just like the old one - being an STK500-protocol compatible but its a quarter of the size! Down from 2K, the new bootloader is a tiny 512b. This gives you more space for your project code! Yay! Its also faster - 115K instead of 57.6k so you'll be uploading code in 3 seconds.
The Bad News is that you must make sure to select Uno in the Boards menu!!! If you don't things will be confusing because the bootloader speed is wrong, and you won't get that extra 1.5K!
Overall, its a good direction, and the chips can be used in older Arduinos just fine (so you can upgrade your Diecimila or Duemilanove to the Uno by simply replacing the chip)
All previous shields should still work perfectly fine as the header spacing is the same, the core chip is the same and the location of parts is the same. In fact, some should work better because the 3V supply has been upgraded (see next point)
One sad thing about older boards is that they had a 3.3v power supply but it was really just whatever the FTDI chip's internal 3.3v regulator could give. You -could- get 50mA out of it, maybe. But high power stuff like XBees, SD cards, some fast ADC or DACs could easily drag down the FTDI chip and reset the USB connection. The Uno solves this problem by adding a new 3.3V regulator the LP2985 which can easily provide 150mA
The LP2985 is a very high quality regulator, and will work great for powering stuff and as a nice solid 1% analog reference.
This is a common question. The reason is that the first Arduino used the Atmega8 which could not run faster than 16Mhz. As the chip has been upgraded they wanted to make the boards speed compatible. Arduino is also not really intended for fast-processing (its only 8-bit anyways) so the chips are running at 16MHz