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+---
+title: Notes on programming ATmega328P chips
+date: 2025-04-10
+author: W. D. 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.
+