arduino-uno

name: arduino-uno description: Arduino Uno R3 hardware platform specifications including ATmega328P pin layout, PWM capabilities, analog inputs, communication interfaces, and timing constraints. Reference this when generating code for Arduino Uno. user-invocable: false

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