How to configure ATmega328P microcontrollers to run at 3.3V and 5V

HOW TO CONFIGURE ATMEGA328P MICROCONTROLLERS TO RUN AT 3.3V AND 5V

10 APRIL 2025

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This is a quick reference for wiring up ATmega328P ICs to run at 5V and 3.3V. +While the 5V configuration is common, the 3.3V configuration can be useful in +low-power applications and when interfacing with parts that themselves run at +3.3V. In this guide, the 5V setup is configured with a 16MHz crystal +oscillator, while the 3.3V configuration makes use of an 8MHz crystal +oscillator.

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The steps that follow refer to the following pinout.

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Pinout

Breadboard

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5V-16MHz configuration

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Powering ATmega328P microcontrollers with 5V is the most common setup. This is +also how Arduino Uno boards are wired.

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In this configuration, the microcontroller’s pin 1 is connected to 5V via a +10kΩ resistor. Pins 9 and 10 are connected to a 16MHz crystal oscillator via +two 22pF capacitors connected to ground. The microcontroller is powered by +connecting pins 7, 20, and 21 to a 5V DC power supply. Lastly, pins 8 and 22 +are connected to ground. In addition to the these connections, which are +required, it’s a good idea to add 0.1μF decoupling capacitors between pins 7, +20, and 21 and ground.

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Here’s a sample Makefile for compiling C programs for ATmega328P +microcontrollers using avr-gcc/avrdude toolchain.

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3.3V-8MHz configuration

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Electrical connections for running an ATmega328P at 3.3V are identical to that +of the 5V circuit. The only differences are that all the 5V connections are +replaced with a 3.3V power source and a 8MHz crystal oscillator takes the place +of the 16MHz crystal.

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However, standard ATmega328P chips are preconfigured to run at 5V. To run one +at 3.3V, we must first modify its fuses that control characteristics like the +BOD level. If a bootloader that expects a 16MHz clock (e.g., Arduino +bootloader) is pre-installed on the ATmega328P, it must be swapped with one +that accepts an 8MHz clock. To accomplish that, we need an in-system programmer +(ISP).

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Fortunately, we can turn an ordinary Arduino Uno board into an ISP by uploading +the ‘ArduinoISP’ sketch found in the Arduino IDE. The ISP communicates with the +microcontroller using a Serial Peripheral Interface (SPI). So, connect the SPI +port of the ATmega328P to that of the Arduino Uno, and the Uno’s SS pin +to the ATmega328P’s RESET pin.

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Power up the the ATmega328P by connecting its V_CC to a 5V supply (we +can use Arduino Uno’s 5V pin). From the Arduino IDE, select ‘ATmega328P (3.3V, +8MHz)’ for processor from the tools menu. Also from the tools menu, select +‘Arduino as ISP’ as programmer. Finally, upload the new bootloader by selecting +‘Burn Bootloader’ from the tools menu.

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The ATmega328P is now ready to run at 8MHz with a 3.3V power supply. You can +upload programs to the ATmega328P as you normally would using avrdude. +Here’s a sample Makefile with adjusted parameters (e.g., baud +rate) for an 8MHz clock.

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Remarks

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In both configurations, if you intend to use the ATmega328P’s analog-to-digital +converter with the internal 1.1V or AV_cc voltage as reference, do +not connect AREF (pin 21) to V_cc. Refer to section 23.5.2 in the +datasheet for more information.

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by W. D. Sadeep Madurange