Massmind : CPUs : Atmel : AVR

RISC core running single cycle instructions. 32 general purpose registers. On chip hardware includes: Internal oscillators, timers, UART, SPI, USB, pull-up resistors, pulse width modulation, ADC, analog comparator, up to 4x40 LCD drivers and watch-dog timers. On chip in-system programmable Flash and EEPROM. 1.8-5.5 Volt power options. Software controlled frequency and sleep modes with fast wakeup. Used in popular Arduino system.

About

The Atmel AVR and the Microchip PIC are very similar (Harvard architecture) lines of microcontrollers.

Advantages of Atmel AVR:

Cost. At the moment, the very lowest-price microcontroller available from any manufacturer is the $0.54 Atmel AVR ATtiny11 6 MHz FLASH (same price for 8 pin DIP and 8 pin SOIC)(price in singles from Digikey as of 2004-07-06).
Speed: Not only are most AVRs capable of 20MHz (even really cheap ones like the ATtiny25/45/85 and ATmega48), but they actually run at near 20 MIPS; the PIC chips I'm aware of at a higher price (for example, PIC16F88) only run at 5 MHz with a 20 MHz oscillator frequency. In addition, with the better addressing modes and registers of the AVRs, most operation can be done in only one instruction, where it often takes more than one instruction to do the same thing on a PIC.+
Peripherals: Many Atmel AVR µcontrollers, like many Microchip PIC µcontrollers, have a built-in 10 bit ADC. Some have LCD or USB drivers.
emaste-sandvine says: "One big advantage of AVRs is that they are supported by the GNU Compiler Collection (GCC)."

Disadvantages of Atmel AVR:

Single source: only available from Atmel.
Power: Atmel AVRs have fairly low power, but the TI MSP430 series has the lowest power of any microcontroller.
Speed: near 20MIPS is great, but the SX chips regularly run at near 50MIPS and can run at 75 or even 100 (in rare cases). The Xtensa uC on the ESP-8266 runs at 80Mhz and can burst to 160Mhz

Common misperceptions: Some people think the fastest Microchip PIC (20 MHz 4:1 pipeline) is as fast as the fastest Atmel AVR (20 MHz 1:1 pipeline). They are wrong. The PIC is actually processing instructions at 5 MIPS and the AVR is at just under 20 MIPS. Also, the AVR instruction set is larger, so in many cases, it is getting more done in fewer cycles.

In System Development: The device is its own emulator which ensures actual behavior and electrical characteristics. You will have all resources available while debugging, and it is available on the tiniest 8-pin parts to the large 100-pin devices. AVR devices also support self-programming, allowing them to upgrade their own firmware. A single, free tool chain supports the entire AVR line. Assembler, Simulator, ICE, C and C++ support included at no cost. FORTH, Pascal, BASIC and many flavors of C and C++ are available from 3rd party vendors.

Groups: The AVR line is divided into to major groups; Tiny and Mega, with some additional special function versions for specific applications such as lighting, automotive, battery management, rf comms, and LCD drivers.

Tiny: 1 to 8KB FLASH, 32 to 512 Byte RAM, 0 to 512 Byte EEPROM, 6 yo 18 IO pins.
Mega: 4 to 256KB FLASH, 0.5 to 8KB RAM, 1/4 to 4KB EEPROM, 18 to 86 IO pins.

Development Tools

Code Library

See http://playground.arduino.cc/Main/GeneralCodeLibrary

Input / Output
- Virtual Peripherals
Math / Functions / Conversions
Conditionals / Program flow
AVR Interrupts +
Time / Clocks / Delays
Memory / Tables / Data

A small robot board with the AVR + by Dimitar Dimitrov

ATMEL AVR USB Programmer and Rapid Prototyping Board by Tobias Frost

Free IDE http://www.atmel.com/atmel/products/prod203.htm