ATmega328P 3.3V post.

1 files changed, 23 insertions, 50 deletions

diff --git a/_log/arduino-uno.md b/_log/arduino-uno.md
index dfdbdce..8521725 100644
--- a/_log/arduino-uno.md
+++ b/_log/arduino-uno.md
@@ -1,17 +1,11 @@
 ---
-title: How to configure ATmega328P microcontrollers to run at 3.3V and 5V
+title: ATmega328P at 3.3V and 5V
 date: 2025-06-10
 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.
+Quick reference for wiring ATmega328P ICs at 5V and 3.3V. 5V uses 16MHz
+crystal, 3.3V uses 8MHz.
 
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   <tr style="border: none;">
@@ -26,57 +20,36 @@ The steps that follow refer to the following pinout.
   </tr>
 </table>
 
-## 5V-16MHz configuration 
+## 5V-16MHz
 
-Powering ATmega328P microcontrollers with 5V is the most common setup. This is
-also how Arduino Uno boards are wired.
+Standard setup. 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. 
+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.
 
-[Here's](Makefile) a sample Makefile for compiling C programs for ATmega328P
-microcontrollers using avr-gcc/avrdude toolchain.
+Sample Makefile: [Makefile](Makefile)
 
-## 3.3V-8MHz configuration 
+## 3.3V-8MHz
 
-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.
+Electrical connections identical to 5V circuit. Replace 5V with 3.3V supply,
+16MHz crystal with 8MHz.
 
-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). 
+Problem: ATmega328P ships configured for 5V. Must modify fuses (BOD level,
+etc.) and replace bootloader. Standard Arduino bootloader expects 16MHz—needs
+8MHz version.
 
-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.
+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.
 
-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.
+Arduino IDE: Select 'ATmega328P (3.3V, 8MHz)' processor, 'Arduino as ISP'
+programmer. Burn bootloader via 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.
+Sample Makefile (8MHz): [Makefile.3v3](3v3.Makefile)
 
 ## 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.
+Don't connect AREF (pin 21) to V<sub>cc</sub> if using ADC with internal 1.1V
+or AV<sub>cc</sub> reference. See datasheet section 23.5.2.