Quick reference for wiring ATmega328P ICs at 5V and 3.3V. 5V uses 16MHz crystal, 3.3V uses 8MHz.

Pinout

Breadboard

5V-16MHz

Standard setup. How Arduino Uno boards are wired.

Connections: Pin 1 → 5V via 10kΩ resistor. Pins 9/10 → 16MHz crystal via 22pF capacitors to ground. Pins 7/20/21 → 5V supply. Pins 8/22 → ground. Add 0.1μF decoupling caps between pins 7/20/21 and ground.

Sample Makefile: Makefile

3.3V-8MHz

Electrical connections identical to 5V circuit. Replace 5V with 3.3V supply, 16MHz crystal with 8MHz.

Problem: ATmega328P ships configured for 5V. Must modify fuses (BOD level, etc.) and replace bootloader. Standard Arduino bootloader expects 16MHz—needs 8MHz version.

Solution: Use Arduino Uno as ISP. Upload ‘ArduinoISP’ sketch from Arduino IDE. Connect SPI pins (ATmega328P ↔ Uno), Uno’s SS pin → ATmega328P RESET pin. Power ATmega328P from Uno’s 5V pin during programming.

Arduino IDE: Select ‘ATmega328P (3.3V, 8MHz)’ processor, ‘Arduino as ISP’ programmer. Burn bootloader via tools menu.

Sample Makefile (8MHz): Makefile.3v3

Remarks

Don’t connect AREF (pin 21) to V_cc if using ADC with internal 1.1V or AV_cc reference. See datasheet section 23.5.2.