Wednesday, 23. September 2009 5:18
There are several decisions you need to make when you purchase an Arduino. Arduino is a trademarked name that strictly speaking applies to only those parts made by the Arduino team. But, the design is open and they encourage people to adapt it and make varieties. Most of the products from third parties are completely compatible with the software that a branded Arduino would use. Strictly speaking, and as a class, those clones are known as Freeduinos.
First, what is an Arduino? Note that I am going to use the term Arduino to refer to the branded products and their clones. An Arduino is an Atmel AVR microcontroller loaded with a small amount of firmware(the bootloader) which gives it several characteristics. The bootloader makes the chip listen on certain pins for a remote computer to download programs. This saves on the amount of hardware needed to program the Arduino. It also make the chip execute the downloaded program in a certain way. The communications to the outside world occur on what is essentially a simplified serial port. The communications take place using only 2 voltages, unlike standard RS-232(see http://en.wikipedia.org/wiki/RS-232). The word microcontroller is also used in two ways on this web site. The Atmel AVR chip is strictly speaking the microcontroller. The Arduino is mostly a way of making the Atmel chip easier to use and interface to, and so I sometimes mean the microcontroller board.
The main decisions you need to make are:
1. how to communicate with the board
2. how to power the board
3. how to connect to the board
4. how much memory do you need
1. There are two main options you see on Arduino boards. Some have 9 pin serial port connections. Those have circuitry on the board to convert from standard Serial to the version the Arduino uses. If your computer has a serial port, I recommend a serial port connected Arduino as the simplest to use. Some Arduinos have USB connections. That must also be converted to the signaling the microcontroller uses and that is normally accomplished using a FTDI FT232 chip. Since that FTDI chip is a little expensive, some Arduino boards don’t put it on the board, but require you to buy a special USB cable that includes it. That saves you money, since you can use that cable on all of your Arduinos that need one.
2. One option here is to power the Arduino from the USB cable. The Arduino needs very little power and the USB cable has enough, as long as you don’t make the Arduino drive too much stuff connected to it(like 30 LEDs). But, you probably don’t want to leave a computer powered on and connected to the Arduino all the time. That kind of defeats the purpose of having a stand alone microcontroller. So, there is usually a way to plug in an external battery or power adapter. The simplest Arduino clones have no voltage regulation built in and need a 5 volt regulated supply. The type of regulator built in will govern what range of input voltages the board will run with. A common range is 7.5V to 18V. Anything in that range will work for a plug in power adapter. You are better off using a voltage near the bottom of the range, so the voltage regulator on the Arduino does not work as hard and get as hot. Don’t go lower than they specify, or the voltage regulator on the Arduino won’t be able to keep pumping out 5V to the circuits on the Arduino board. One thing to be careful about is that on some boards, you can cause a problem if the external board is connected at the same time as a USB cable. Sometimes there is a jumper that allows it. Be careful and check.
3. Some boards have rows of female pins along two sides of the board. You can then plug jumper wires from these to your breadboard. Some have pins on the bottom with the right spacing to plug right into breadboards. I personally like that style a lot. I would use them all the time, except I have not found any that also use a real serial port to talk to the computer. The ones with two rows of female connectors on top have a bonus feature. There are little daughter boards you can plug in there. They are called shields. More on shields later. I’d love to have a Danger Shield
4. The two main microcontrollers you will find on Arduino boards at the moment are the 168(Atmega168) chip and the 328(Atmega328). The 168 has 16k of program memory, 1K of ram, and 512 bytes of eeprom. The 328 has twice as much of all three. They both run at the same speed. There are bigger and smaller microcontrollers available, but start with one of those. I have not filled up a 168 yet, so don’t worry if that is all you can get.
A few recommended boards:
The current standard Arduino is the Duemilanove, which means 2009 in Italian. Several venders sell it. It is a good board and buying it supports further development from the people who started the Arduino community and opened their design. SparkFun can probably get it to you fairly soon.
Modern Device Company http://moderndevice.com/ has two boards that are great for using with breadboards. They both are designed to connect to your computer over a USB cable and need the FTDI adapter on the USB. Modern Device does that with the USB BUB. I recommend paying the extra 75 cents and getting the larger regulator, unless you know you won’t need it.
Spiffie http://store.fundamentallogic.com/ecom/ has a nice board that interfaces with a real serial port. It takes the same shields that the standard Arduino does, but connects with a serial port. Spiffie also has another product that is breadboardable, but I don’t like it as much as Modern Device’s. Spiffie does not put the voltage regulator on the breadboardable version.
Seeed Studio http://www.seeedstudio.com/blog/?page_id=110 has a nice board. They use a smaller version of the Atmel chip and have some extra space on their board. They give you an on/off switch and are still compatible with the Arduino shields.
liquidware http://www.liquidware.com/shop/show/ILL/Illuminato%3A%3AGenesis has a board using the next step up in microcontroller from Atmel. It has more memory, more IO pins, and runs at the same speed as the Atmega168 and AtMega328. Don’t be in a hurry to get it. He is keeping his costs down by producing in batches as enough orders come in. Somebody ought to get one and bring it to a meeting. The one limitation I see on this board is it’s limited power options. You appear to be limited to the .5 amp you can get from USB..