The Arduino Nano is a small, breadboard friendly version of the popular Duemilanove

It has everything that Diecimila has (electrically) with more analog input pins and onboard +5V AREF jumper.

The Nano has the breadboard-ability of the Boarduino and the Mini+USB with smaller footprint than either, so users have more breadboard space. It has a pin layout that works well with the Mini or the Basic Stamp (TX, RX, ATN, GND on one top, power and ground on the other). It is constructed from a four-layer board with power and ground planes to help provide ICs with sufficient charge during switching and reduce noise (EMC) on high speed switching I/O pins. Whilst the ground plane helps reduce radiation (EMI), the power plane is low in inductance to reduce transients that may develop on the power.

Arduino Nano Manual (pdf)
Eagle files note: since the free version of Eagle does not handle more than 2 layers,and the Nano is currently 4-layer board, Gravitech publishes it here as unrouted, so users can at least open it and use it in the free version of Eagle

• Atmel ATmega168 MCU
• Operating Voltage (logic level) 5 V
• Input Voltage (recommended)7-12 V
• Input Voltage (limits)6-20V
• 14 Digital I/O Pins (of which 6 provide PWM output)
• 8 Analog Input Pins
• DC Current per I/O Pin 40 mA
• 16Kb Flash Memory (of which 2KB used by bootloader)
• 1 KB SRAM
• 512 bytes EEPROM
• 16MHz Clock Speed
• Dimensions 0.73� x 1.70�

• Automatic reset during program download
• Power OK blue LED on the bottom
• Green (TX), red (RX) and orange (L) LED
• +5V to AREF jumper
• Auto sensing/switching power input
• Small mini-B USB for programming and serial monitor
• ICSP header for direct program download
• Power OK blue LED on the bottom
• Standard 0.1� spacing DIP (breadboard friendly)
• Manual reset switch

The Arduino Nano can be powered via the mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source.

The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

The ATmega168 on the Nano has 16 KB of flash memory for storing code (of which 2 KB is used for the bootloader). It has 1 KB of SRAM and 512 bytes of EEPROM (which can be read and written with the EEPROM library).

Each of the 14 digital pins on the Nano can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:

• Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the corresponding pins of the FTDI USB-to-TTL Serial chip.
• External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value, a rising or falling edge, or a change in value. See the attachInterrupt() function for details.
• PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function.
• SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication, which, although provided by the underlying hardware, is not currently included in the Arduino language.
• LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value, the LED is on, when the pin is LOW, it's off.