Arduino Uno R3 Hardware Platform

Core Specifications

• •MCU: ATmega328P
• •Operating Voltage: 5V
• •Input Voltage: 7-12V (recommended), 6-20V (limits)
• •Clock Speed: 16 MHz
• •Flash Memory: 32 KB (0.5 KB used by bootloader)
• •SRAM: 2 KB
• •EEPROM: 1 KB

GPIO Pin Capabilities

Digital I/O Pins

Available: D0-D13 (14 pins)

• •Can be INPUT, OUTPUT, or INPUT_PULLUP
• •Each pin can source/sink up to 40 mA (20 mA recommended)
• •Total current across all pins: 200 mA max

PWM-Capable Pins (marked with ~)

PWM Pins: 3, 5, 6, 9, 10, 11

• •8-bit PWM (0-255 via analogWrite())
• •Pins 5, 6: ~980 Hz frequency
• •Pins 3, 9, 10, 11: ~490 Hz frequency
• •⚠️ Pins 5 and 6 share a timer with millis() and delay()

Analog Input Pins

Analog Pins: A0-A5 (6 pins)

• •10-bit resolution (0-1023)
• •0-5V input range
• •Can also be used as digital I/O (pins 14-19)
• •Reference voltage: AREF pin (default 5V)

Interrupt-Capable Pins

External Interrupts: Pin 2 (INT0), Pin 3 (INT1)

• •Triggered on: RISING, FALLING, CHANGE, LOW
• •Use attachInterrupt(digitalPinToInterrupt(pin), ISR, mode)
• •Pin Change Interrupts available on all pins (more complex)

Communication Interfaces

UART (Serial)

Pins: RX=D0, TX=D1

• •Connected to USB-to-Serial chip
• •Baud rates: 300 to 115200 (higher rates unreliable)
• •Standard: 9600 baud
• •⚠️ D0 and D1 should not be used while Serial communication is active

I2C (Wire)

Pins: SDA=A4, SCL=A5

• •Master or slave mode
• •Standard (100 kHz) and Fast (400 kHz) modes
• •Internal pull-up resistors available
• •External 4.7kΩ pull-ups recommended for reliable operation

SPI

Pins:

• •MOSI=D11
• •MISO=D12
• •SCK=D13
• •SS=D10 (user-defined, typically D10)

Special Pin Functions

Pin 13 (Built-in LED)

• •Has built-in LED and 1kΩ series resistor
• •LED is on when pin is HIGH
• •May interfere with SPI (shares SCK)
• •Good for basic testing without external components

Pin 0 (RX)

• •Serial receive
• •Avoid using during Serial communication
• •Can cause upload failures if externally loaded

Pin 1 (TX)

• •Serial transmit
• •Avoid using during Serial communication
• •Can cause upload failures if externally loaded

AREF Pin

• •Analog reference voltage for ADC
• •Default: connected to 5V internally
• •Can use external reference (1.1V internal, 5V, or external via AREF pin)
• •⚠️ Never connect voltage to AREF if using internal reference

IOREF Pin

• •Provides reference voltage (5V on Uno)
• •Used by shields to detect board voltage level

Pin Selection Best Practices

For Digital Output (LED, Relay, etc.)

Best choices: 2, 4, 7, 8, 12, 13 Avoid: 0, 1 (Serial), 10-13 if using SPI

For Digital Input (Button, Sensor)

Best choices: 2, 3, 4, 7, 8 Recommended for interrupts: 2, 3

For PWM (Servo, Motor, LED dimming)

Available: 3, 5, 6, 9, 10, 11 Best for servo: 9, 10 (490 Hz is better for servos) Best for LED dimming: 5, 6 (980 Hz reduces flicker)

For Analog Input (Sensors)

Available: A0-A5 Can also use as digital: Refer to as pins 14-19 in code

For I2C

Fixed pins: SDA=A4, SCL=A5 Cannot be remapped on Uno

For SPI

Fixed pins: MOSI=11, MISO=12, SCK=13 SS (Slave Select): User-defined, typically D10

Timing Considerations

millis() and delay()

• •Resolution: 1 millisecond
• •Based on Timer0
• •⚠️ Affected by PWM on pins 5 and 6

micros() and delayMicroseconds()

• •Resolution: 4 microseconds
• •Also uses Timer0

Timer Usage

• •Timer0: millis(), delay(), PWM on pins 5 and 6
• •Timer1: Servo library, PWM on pins 9 and 10
• •Timer2: tone(), PWM on pins 3 and 11

Servo Library

• •Uses Timer1
• •Disables PWM on pins 9 and 10
• •Can control up to 12 servos (Uno has only 2 timers available)

Power Specifications

Per-Pin Current

• •Maximum: 40 mA per pin
• •Recommended: 20 mA per pin
• •Total: 200 mA across all I/O pins

Current-Limiting Resistors

For LEDs:

• •Red LED: 220Ω to 1kΩ
• •Blue/White LED: 100Ω to 220Ω
• •Always use resistors to prevent pin damage

Power Supply

• •USB: 5V, up to 500 mA
• •DC Jack: 7-12V (9V recommended), regulated to 5V
• •5V Pin: Can supply 5V if powered via DC jack
• •3.3V Pin: 50 mA maximum (from onboard regulator)

Pin Conflict Checker

Before assigning a pin, verify:

1. •✅ Pin supports required function (PWM/Analog/Digital)
2. •✅ Not pins 0 or 1 if using Serial
3. •✅ Not pins 10-13 if using SPI (except for SPI itself)
4. •✅ Not A4 or A5 if using I2C (except for I2C itself)
5. •✅ Not already used by another peripheral
6. •✅ If using Servo library, PWM on pins 9 and 10 will be disabled

Common Pin Assignments

Safe General Purpose Pins

These are the safest pins for most projects:

• •D2, D3: Good for interrupts (buttons, encoders)
• •D4, D7, D8: Good for general digital I/O
• •D5, D6: Best for high-frequency PWM (LED dimming)
• •D9, D10: Best for servo control

Pins to Avoid or Use Carefully

• •D0, D1: Used by Serial (USB), avoid for general I/O
• •D13: Has built-in LED and resistor, may affect sensitive circuits
• •A4, A5: Used for I2C, can be repurposed but not recommended

Hardware Constraints

Memory Limitations

• •

  Flash: 32 KB total, ~30.5 KB available

  • •Use F() macro for string literals to save RAM
  • •Use PROGMEM for large constant arrays
• •

  SRAM: Only 2 KB

  • •Avoid large arrays or strings
  • •Use String class carefully (causes fragmentation)
  • •Static allocation preferred over dynamic
• •

  EEPROM: 1 KB

  • •100,000 write cycles per location
  • •Use for configuration storage, not frequent logging

Timing Limitations

• •No floating-point unit (FPU) - float math is slow
• •Integer math preferred
• •16 MHz clock limits processing speed

Upload Process

Reset Pin

• •Automatically resets via DTR signal from USB
• •Can be manually reset via button
• •Reset required before upload

Bootloader

• •Occupies 0.5 KB of flash
• •Allows USB upload without external programmer
• •Timeout: ~1 second after reset