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Arduino IO pin Mapping

Summary http://arduino.cc/en/Main/ArduinoBoardUno  
Microcontroller ATmega328  
Operating Voltage 5V  
Input Voltage (recommended) 7-12V  
Input Voltage (limits) 6-20V  
Digital I/O Pins 14 (of which 6 provide PWM output)  
Analog Input Pins 6  
DC Current per I/O Pin 40 mA  
DC Current for 3.3V Pin 50 mA  
Flash Memory 32 KB (ATmega328) of which 0.5 KB used by bootloader  
SRAM 2 KB (ATmega328)  
EEPROM 1 KB (ATmega328)  
Clock Speed 16 MHz  
Input and Output    
Each of the 14 digital pins on the Uno 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 ATmega8U2 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 using the SPI library.  
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.  
TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the Wire library. 
ATmega328 Pin Mapping   I/O
Pin 1 Reset  
Pin 2 PD0 (Rs232 - Rx) 0
Pin 3 PD1 (Rs232 - Tx) 1
Pin 4 PD2 (Int / Rs232 - Rx.1) 2
Pin 5 PD3 (Int / Rs232 - Tx.1 / PWM_3) 3
Pin 6 PD4 4
Pin 7 VCC (5v)  
Pin 8 GND  
Pin 9 TOSC1  
Pin 10 TOSC2  
Pin 11 PD5 (PWM_5) 5
Pin 12 PD6 (PWM_6) 6
Pin 13 PD7 7
Pin 14 PB0 8
Pin 15 PB1 (PWM_9) 9
Pin 16 PB2 (PWM_10) 10
Pin 17 PB3 (PWM_11) 11
Pin 18 PB4 12
Pin 19 PB5 (LED) 13
Pin 20 AVCC  
Pin 21 AREF  
Pin 22 GND  
Pin 23 PC0 (ADC0) ADC0
Pin 24 PC1 (ADC1) ADC1
Pin 25 PC2 (ADC2) ADC2
Pin 26 PC3 (ADC3) ADC3
Pin 27 PC4 (ADC4) ADC4
Pin 28 PC5 (ADC5) ADC5