ATmega328P at 3.3V and 5V

ATMEGA328P AT 3.3V AND 5V

10 JUNE 2025

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

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Pinout

Breadboard

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5V-16MHz

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Standard setup. How Arduino Uno boards are wired.

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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.

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Sample Makefile: Makefile

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3.3V-8MHz

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

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

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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.

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

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Sample Makefile (8MHz): Makefile.3v3

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Remarks

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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.

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by W. D. Sadeep Madurange