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---
title: Programming ATmega328P chips
date: 2025-04-10
author: Wickramage Don Sadeep Madurange
layout: post
---
This post is a step-by-step guide for wiring up ATmega328P ICs to run at 5 V
with a 16 MHz crystal and 3.3 V with an 8 MHz crystal. While the 5 V
configuration is common, the 3.3 V configuration can be advantageous in
low-power applications and when interfacing with parts that run at 3.3 V.
## 5 V - 16 MHz configuration
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>
1. Connect pin 1 to 5 V via a 10 kΩ resistor.
2. Connect a 16 MHz crystal oscillator across pins 9 and 10.
3. Connect each pin of the crystal to ground via 22 pF capacitors.
4. Connect pins 7, 20, and 21 to 5 V.
5. Connect pins 8 and 22 to ground.
In addition to the connections described above, it's a good idea to add 0.1 μF
decoupling capacitors between pins 7, 20, and 21 and ground.
[Here's](Makefile) a sample Makefile for avr-gcc and avrdude.
## 3.3 V - 8 MHz configuration
The following steps use Arduino Uno as an ISP and Arduino utilities to program
ATmega328P's bootloader and the fuses (e.g., BOD level) for a 3.3 V supply.
1. Upload the 'ArduinoISP' sketch to the Uno.
2. Wire up the ATmega328P as described in the previous section. Replace the 5 V
supply with a 3.3 V supply and use an 8 MHz crystal instead of the 16 MHz
crystal.
3. Connect the SPI ports (SCK, MISO, and MOSI) of the two MCUs.
4. Connect Uno's SS pin to the IC's pin 1 (RESET).
5. The IC can be powered by the Arduino Uno's 5 V pin.
6. Burn the bootloader to the ATmega328P:
- Select 'ATmega328P (3.3 V, 8 MHz)' from Tools > Processor.
- Select 'Arduino as ISP' from Tools > Programmer.
- Select Tools > Burn Bootloader.
The ATmega328P is now ready to run at 8 MHz with a 3.3 V power supply. You can
upload programs to the ATmega328P as you usually would using avrdude.
[Here's](3v3.Makefile) a sample Makefile with adjusted parameters (e.g., baud
rate) for an 8 MHz clock.
In both configurations, if you intend to use the ATmega328P's analog-to-digital
converter with the internal 1.1 V 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 ADC
Voltage Reference in the datasheet for more information.
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