Arduino is an open-source platform based on flexible, easy-to-use hardware and software.
In case of PIC or 89S51 Microcontroller you are limited with a size of 2K programming code. As , Arduino is open source ,there is no such limitations and you get support from Arduino Forums.
Simply, you load on some code and it can read sensors, perform actions based on inputs from buttons, control motors, and accept shields to further expand it’s capabilities.
The Arduino IDEis the software that allows you to write and upload code.
The Arduino hardware comes in several flavors , like
Duemilanove ,LilyPad , Arduino Mega ,Arduino Mega 2560 ,Arduino Uno ,Arduino NG
The Arduino Unoboard is a good choice for students.
The Arduino Uno is a microcontroller board based on the ATmega328. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 (in version R3) programmed as a USB-to-serial converter.
The Programs you write are called Sketches and stored with extension .ino in a folder called SketchBook.The previous versions of Arduino IDE stores files with .pde extension.You can open a .pde sketch in new IDE,it will be automatically converted to a .ino file.
Shields are boards to be mounted on top of the Arduino board and that extend the functionality of Arduino to control different devices, acquire data, etc. There are wide variety of shields to support Ethernet , Wireless, Xbee , Servo Motor,etc
Watch this video on Arduino Setup in WIN7/XP platform :
For further details :
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