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Library for Arduino that logs the program address where a watchdog timeout occurred.
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Arduino library that logs the program address where a watchdog timeout occurred to EEPROM before the microcontroller resets.

Who watches the watchdog? This library helps to find the cause of watchdog timeouts by saving the program address where the timeout occurred. This program address can be used with a disassembly of the compiled code to identify the location of the problem in your sketch. Another use for the library is to identify the cause of a reset.

Based on:


There is a bug in the bootloader used on some Arduino boards that causes them to go into an endless reset loop after a watchdog reset(, which makes it impossible to upload sketches. Due to this bug watchdog resets(including the ones in the examples included with this library) should not be done on the following boards unless you have replaced their bootloaders with one that has the bug fixed:

  • Duemilanove
  • Diecimila
  • Nano
  • Arduino Mega w/ ATmega1280 (ATmega2560 does not have the bug)
  • Mini
  • Fio
  • BT
  • LilyPad
  • Pro
  • Pro Mini
  • NG

It's easy to update the bootloader on these boards by either burning the Uno bootloader in the case of the ATmega328P@16MHz boards, then using the board as an Uno thereafter, or using MiniCore for ATmega168 and/or 8MHz boards. If you have "bricked" your board by doing a watchdog reset with the buggy bootloader you can fix it by burning the bootloader with an ISP(in-system programming) programmer.

The Watchdog Timer

The main purpose of the watchdog timer is to automatically reset the Arduino if it locks up. The timeout duration of watchdog timer is set using wdt_enable(). If the timer is not reset using wdt_reset() before timing out it will trigger a reset of the microcontroller. Often the cause of unintended watchdog timeouts is a bug in the code. This library helps you to identify the location of the bug.


  • Download the most recent version of WatchdogLog here:
  • Using Arduino IDE 1.0.x:
    • Sketch > Import Library... > Add Library... > select the downloaded file > Open
  • Using Arduino IDE 1.5+:
    • Sketch > Include Library > Add ZIP Library... > select the downloaded file > Open


See the example sketches in File > Examples > WatchdogLog for demonstration of usage.


Put the watchdog into interrupt+reset mode and set the base EEPROM address.

  • Parameter: baseEEPROMaddress - The base EEPROM address for storage of the program address where the watchdog timeout occurred to EEPROM. The saved program address will take 4 bytes of EEPROM starting with this address.
    • Type: unsigned int
  • Returns: true = success, false = invalid EEPROM base address. The base address must allow for the 4 byte program address value to fit within the capacity of the EEPROM.
    • Type: boolean


When you upload a sketch to your Arduino the IDE compiles your program to assembly code. The program address logged by WatchdogLog relates to this code so you must produce a disassembly of the compiled code. This is accomplished by using the avr-objdump program included with the Arduino IDE in the *Arduino\hardware\tools\avr\bin* folder:

avr-objdump -I[sketch path] -d -S -j .text [.elf file path] > disassembly.txt

The location of the compiled .elf file can be found by turning verbose ouput on in File > Preferences > Show verbose output during: compilation(check). If you are using Arduino IDE versions 1.0.2 up to and including 1.6.5-r5 then avr-objdump needs to know the location of your sketch so that it can include the source code with the assembly code.

I have written a Windows batch file that automates the process of creating a disassembly dump of the last compiled sketch:

Serial output from WatchdogLogExample.ino:

Logged program address: 224

Relevant section of the disassembly dump:

  Serial.println("Waiting for watchdog timeout reset...");
 20a:  0f b6         in  r0, 0x3f  ; 63
 20c: f8 94         cli
 20e: a8 95         wdr
 210: 80 93 60 00   sts 0x0060, r24
 214: 0f be         out 0x3f, r0  ; 63
  wdt_enable(WDTO_500MS);  //enable the watchdog timer with a 500MS second timeout duration
 216: 20 93 60 00   sts 0x0060, r18
 21a: 60 e0         ldi r22, 0x00 ; 0
 21c: 70 e0         ldi r23, 0x00 ; 0
 21e: 88 e8         ldi r24, 0x88 ; 136
 220: 92 e0         ldi r25, 0x02 ; 2
 222: 02 d0         rcall .+4       ; 0x228 <_ZN16WatchdogLogClass5beginEj>
 224: ff cf         rjmp  .-2       ; 0x224 <setup+0xe0>

00000226 <loop>:
 226: 08 95         ret

In this simple example it is fairly easy to decipher the disassembly but in many cases the address will point to the code of a function from a library that was running when the reset occurred. So you will need to find where in your code that function was called but it can still provide a valuable clue to track down an elusive bug.


Pull requests or issue reports are welcome! Please see the contribution rules for instructions.

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