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+---
+title: How to configure ATmega328P microcontrollers to run at 3.3V and 5V
+date: 2025-04-10
+layout: post
+---
+
+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. 
+
+The steps that follow refer to the following pinout.
+
+<table style="border: none; width: 100%;">
+  <tr style="border: none;">
+    <td style="border: none; width: 50%; vertical-align: top;">
+      <img src="pinout.png" alt="Pinout" style="width: 100%">
+      <p style="text-align: center;">Pinout</p>
+    </td>
+    <td style="border: none; width: 50%; vertical-align: top;">
+      <img src="breadboard.jpeg" alt="Circuit" style="width: 100%">
+      <p style="text-align: center;">Breadboard</p>
+    </td>
+  </tr>
+</table>
+
+## 5V-16MHz configuration 
+
+Powering ATmega328P microcontrollers with 5V is the most common setup. This is
+also how Arduino Uno boards are wired.
+
+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. 
+
+[Here's](Makefile) a sample Makefile for compiling C programs for ATmega328P
+microcontrollers using avr-gcc/avrdude toolchain.
+
+## 3.3V-8MHz configuration 
+
+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.
+
+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). 
+
+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.
+
+Power up the the ATmega328P by connecting its V<sub>CC</sub> 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.
+
+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](3v3.Makefile) a sample Makefile with adjusted parameters (e.g., baud
+rate) for an 8MHz clock.
+
+## Remarks
+
+In both configurations, if you intend to use the ATmega328P's analog-to-digital
+converter with the internal 1.1V or AV<sub>cc</sub> voltage as reference, do
+not connect AREF (pin 21) to V<sub>cc</sub>. Refer to section 23.5.2 in the
+datasheet for more information.
+