Burning Uno Bootloader
to Arduino Pro Mini 3.3V 8MHz

Why This Is Needed

The Arduino Pro Mini ships with an Optiboot bootloader that does not disable the watchdog timer (WDT) on startup. When firmware uses wdt_enable() to trigger a software reset, the watchdog persists through the reset cycle. The bootloader starts but does not disable the WDT, the watchdog fires again, and the board is trapped in a reset loop — indicated by the onboard LED flashing continuously.

The Arduino Uno version of Optiboot handles WDT correctly — it saves and restores the MCUSR register and disables the watchdog before handing off to the sketch. Burning the Uno bootloader onto the Pro Mini fixes this while preserving 3.3V 8MHz operation.

Hardware Required

Arduino Uno          (as ISP programmer)
Arduino Pro Mini     3.3V 8MHz (target board)
6x jumper wires

Wiring — Uno to Pro Mini

Step 1 — Load ArduinoISP onto Uno

Load the ArduinoISP sketch onto the Uno first. This turns the Uno into an in-system programmer.

File → Examples → 11.ArduinoISP → ArduinoISP

Select Uno as target board.
Upload to Uno.

Step 2 — Burn Bootloader

Tools → Board     → Arduino Uno        ← MUST be Uno, not Pro Mini
Tools → Programmer → Arduino as ISP
Tools → Burn Bootloader

Wait approximately 30 seconds for completion.

Step 3 — Upload Sketch

After bootloader burn, disconnect the ISP wires from the Uno and connect the FTDI adapter to the Pro Mini programming header.

Tools → Board     → Arduino Pro or Pro Mini
Tools → Processor → ATmega328P (3.3V, 8MHz)  ← back to Pro Mini
Tools → Port      → FTDI port
Upload sketch normally.

Step 4 — Firmware WDT Pattern

With the Uno bootloader installed, the following WDT pattern works correctly in firmware:

#include <avr/wdt.h>

void setup() {
  wdt_disable();  // MUST be first line — disables WDT left from previous reset
  Serial.begin(57600);
  // rest of setup...
}

// Software reset via watchdog:
if (input == "z") {
    Serial.println("OK RESETTING");
    delay(100);              // flush serial buffer
    wdt_enable(WDTO_15MS);  // enable watchdog, 15ms timeout
    while(1) {}              // spin — watchdog fires, hardware reset
}

Expected Behavior

With the correct bootloader and firmware pattern, the reset command produces a clean single reset:

z
OK RESETTING
  → 15ms pause
  → hardware reset fires
  → bootloader disables WDT correctly
  → sketch starts
  → wdt_disable() called as first line
  → rest of setup...

No reset loop. No LED flashing. Clean single reset and restart.

Identifying Pro Mini Voltage Version

Pro Mini boards are not always clearly labeled. To identify 3.3V vs 5V:

• Look for the crystal/resonator — 8.000 marking = 8MHz = 3.3V version
• Look for the voltage regulator SOT-23-5 — marking S2RA = 3.3V LDO
• Measure VCC pin with multimeter after powering via RAW pin with 5.0V - reading 3.3V confirms 3.3V version, 5.0V indicates 5V version

Pro Mini Power in Final Installation

For permanent installation the Pro Mini VCC pin vs RAW pin distinction matters:

• RAW pin → feeds onboard LDO regulator → outputs 3.3V to ATmega. Accepts up to 16V input.
• VCC pin → bypasses LDO → feeds ATmega directly. Must be 3.3V.

For the WesTek ProtIoT RF coprocessor board, 5V is fed to the RAW pin. The onboard LDO provides clean 3.3V to the ATmega. The 3.3V output from the LDO powers the crystal 433MHz receiver module, keeping its data output at 3.3V logic — directly compatible with the Pi GPIO without level shifting.

WesTek Lab · Pensacola FL · April 2026
Procedure developed during ProtIoT RF Coprocessor build.