Making your projects more reliable

Arduino Internal Watchdog

The Arduino is a much simpler machine than a Raspberry Pi. However, it is actually easier to hang an Arduino than it is a Raspberry Pi, because all the code is single threaded on the Arduino. Single threading means that only one program runs at a time. An interesting project at kwartzlab.ca [3] implements a degree of multithreading on the Arduino.

If you only have one thread running at a time, any hangup on that thread will stop the computer. Naturally, other problems can cause your code to crash and the Arduino to lock up: timeouts on peripherals, power issues, RFI, and so on. Bad code using the millis() function is a classic problem. You need to handle the rollover at 49.5 days if you aren't using a real-time clock [4].

Arduino Internal Watchdog

In this section, I will show you how to use the Arduino internal watchdog (if you can make it work). I'm not trying to be sarcastic; I just mean to say that many times something in the Arduino will keep the internal watchdog from working, so beware.

Assuming Jupiter is rising and Saturn is in ascension (now that IS sarcasm), here is a way to work with the internal Arduino WDT. First, the Wto of all the Arduino models is a maximum of eight seconds. Keep that in mind. Having a longer Wto covers a lot more sins in my opinion (Wto is 16 seconds on the internal Raspberry Pi watchdog, which still isn't long enough for my taste), so the Arduino Wto is a bit short. I often have serial processes that run longer than eight seconds in my design. Yes, you can incorporate patting into the code, but when you are using external libraries, that is a pain.

To experiment with your Arduino WDT, build a new sketch in the Arduino IDE (Listing 1); however, If you are using an ArduinoMega 2560 or similar device, you could "soft brick" your device (see the "Arduino Watchdog Problems" box). My Arduino Uno worked fine with this sketch.

01 #include <avr/wdt.h>
02
03 #define RESETWATCHDOG
04 void setup()
05 {
06   Serial.begin(57600);
07       Serial.println("");
08       Serial.println ("------->Arduino Rebooted");
09       Serial.println("");
10       wdt_enable(WDTO_8S);
11
12
13 }
14
15
16 void loop()
17 {
18
19 #ifdef RESETWATCHDOG
20   wdt_reset();
21 #endif
22
23   Serial.println("Arduino Running");
24   delay(1000);
25
26 }

Let the sketch run for 30 seconds or so: You should never see the Arduino Rebooted message after startup. Then, comment out the RESETWATCHDOG with:

// #define RESETWATCHDOG

Now when you run the sketch (if your watchdog is working), you should see the Arduino Rebooted every eight seconds or so in the serial monitor. The solution to all of the potential problems discussed here (but see the "RFI Interference" box) is to use an external WDT.

As I was exploring this set of problems with Project Curacao, I decided to build my own external WDT. This gave me a fixed set of parameters not dependent on other software processes, in the case of the Raspberry Pi, or on which model I used, in the case of the Arduino. I decided to build an external dual watchdog board so I didn't have to rely on the internal WDTs (Project Curacao has a Raspberry Pi and Arduino that monitors the power system). With the use of standard 555 timers, I got the flexibility I needed.