From 2ac486ca5cb6b81c3755ec0bb8194e88952dcb40 Mon Sep 17 00:00:00 2001
From: Sadeep Madurange <sadeep@asciimx.com>
Date: Fri, 24 Apr 2026 14:17:11 +0800
Subject: Improve Uno configurations post.

---
 _log/arduino-uno.md | 55 +++++++++++++++++++++++++++++++++++------------------
 1 file changed, 36 insertions(+), 19 deletions(-)

diff --git a/_log/arduino-uno.md b/_log/arduino-uno.md
index 84ecb69..d726c1f 100644
--- a/_log/arduino-uno.md
+++ b/_log/arduino-uno.md
@@ -1,11 +1,11 @@
 ---
-title: ATmega328P at 3.3 V and 5 V 
+title: ATmega328P 3.3 V and 5 V configurations
 date: 2025-06-10
 layout: post
 ---
 
 Quick reference for wiring ATmega328P ICs at 5 V and 3.3 V. 5 V uses 16 MHz
-crystal, 3.3 V uses 8 MHz.
+crystal. 3.3 V uses 8 MHz.
 
 <table style="border: none; width: 100%;">
   <tr style="border: none;">
@@ -22,34 +22,51 @@ crystal, 3.3 V uses 8 MHz.
 
 ## 5 V - 16 MHz
 
-Standard setup. How Arduino Uno boards are wired.
+Standard setup. How Arduino Uno boards are wired:
 
-Connections: Pin 1 → 5 V via 10 kΩ resistor. Pins 9/10 → 16 MHz crystal via 22
-pF capacitors to ground. Pins 7/20/21 → 5 V supply. Pins 8/22 → ground. Add 0.1
-μF decoupling caps between pins 7/20/21 and ground.
+<pre class="pre-no-style">
+    Pin 1   --> [ 10k Resistor ] --> 5V
+    Pin 7   --> 5V (w/ 0.1uF Cap to GND)
+    Pin 8   --> GND
+    Pin 9   --> [ 16MHz Crystal ] --> [ 22pF Cap ] --> GND
+    Pin 10  --> [ 16MHz Crystal ] --> [ 22pF Cap ] --> GND
+    Pin 20  --> 5V (w/ 0.1uF Cap to GND)
+    Pin 21  --> 5V (w/ 0.1uF Cap to GND)
+    Pin 22  --> GND
+</pre>
 
 Sample Makefile: [Makefile](Makefile)
 
 ## 3.3 V - 8 MHz
 
-Electrical connections identical to 5 V circuit. Replace 5 V with 3.3 V supply,
+Electrical connections are identical to 5 V circuit. Replace 5 V with 3.3 V supply,
 16 MHz crystal with 8 MHz.
 
-Problem: ATmega328P ships configured for 5 V. Must modify fuses (BOD level,
-etc.) and replace bootloader. Standard Arduino bootloader expects 16 MHz—needs
-8 MHz version.
+ATmega328P ships with its fuses(BOD level, etc.) configured for 5 V. Need to
+modify them. The standard Arduino bootloader requires a 16 MHz clock.  Replace
+the default bootloader with one that works witha 8 MHz clock.
 
-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.
+Upload the 'ArduinoISP' sketch from the Arduino IDE to an Arduino Uno to use it
+as an ISP.
 
-Arduino IDE: Select 'ATmega328P (3.3 V, 8 MHz)' processor, 'Arduino as ISP'
-programmer. Burn bootloader via tools menu.
+<pre class="pre-no-style">
+    ARDUINO UNO (ISP)                       ATMEGA328P
+    +-------------------+               +-------\/-------+
+    |               D13 |-------------->| 19  PB5 (SCK)  |
+    |               D12 |<--------------| 18  PB4 (MISO) |
+    |               D11 |-------------->| 17  PB3 (MOSI) |
+    |               D10 |-------------->| 1   RESET      |
+    |                   |               |                |
+    |                5V |-------------->| 7   VCC        |
+    |               GND |-------------->| 8   GND        |
+    +-------------------+               +----------------+
+</pre>
 
-Sample Makefile (8 MHz): [Makefile.3v3](3v3.Makefile)
+From the Arduino IDE, select 'ATmega328P (3.3 V, 8 MHz)' processor, 'Arduino as
+ISP' programmer. Upload the bootloader from the tools menu.
 
-## Remarks
+Sample Makefile (8 MHz): [Makefile.3v3](3v3.Makefile)
 
-Don't connect AREF (pin 21) to V<sub>cc</sub> if using ADC with internal 1.1 V
-or AV<sub>cc</sub> reference. See datasheet section 23.5.2.
+Note: Don't connect AREF (pin 21) to V<sub>cc</sub> if using ADC with internal
+1.1 V or AV<sub>cc</sub> reference. See datasheet section 23.5.2.
 
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