FrugalWatchdog

FrugalWatchdog is a simple and cheap DIY watchdog for your computer, i.e. a small device that will automatically reset your computer if it stops responding. It consists of a cheap USB to RS232 converter and an ATtiny45 microcontroller. Because you can easily buy a converter that does RS232 at TTL voltage levels, you don't even need a PCB. Check the img/ directory for an example build; the resistor and the LED are entirely optional. The header that sticks out from the USB connector is used to plug the watchdog directly to the motherboard. Total cost is between 2€ and 5€.

This is not an original idea, but draws inspiration from other similar projects. This one, however, has all the code available and is very easy to extend if neccessary. It is also compatible with the watchdog daemon for linux, working out of the box (the metaphorical box, of course: you still have to do the soldering ;-) ).

Building

The hardware build is intentionally underspecified because it is very simple and because you should adapt it to your needs. The prototype shown in the images is made to plug onto the motherboard of a small home server, but you may want to use a cable and have the watchdog plugged into an external USB port.

The pin numbers can be changed freely. The LED and computer reset pins can be set in main.cpp whereas serial Rx and Tx pins can be chosen in softuart.h. The ATtiny runs on its internal oscillator and it seems that it runs best at 3.3V. At 5V, the timings for serial communication are a bit off, so you may need to do some calibration.

To build the firmware, you need avr-gcc and avr-libc. Simply run make in the microcontroller/ directory. Upload the generated FrugalWatchdog.hex file to the microcontroller however you want; the make upload and make fuses commands use avrdude and ArduinoISP, see the Makefile.

Using manually

The watchdog is configured for serial communication at 2400 baud. It recognizes the following commands:

• timeout: sets the timeout in seconds. The default is 60 seconds to match the behaviour of the watchdog utility, see below.

• start: starts the watchdog.

• stop: stops the countdown.

• reset: resets the countdown, i.e. it postpones the machine reset. Implies start. Sets the timeout string, see below.

• status: prints the elapsed time since the last reset and prints the last timeout string.

• clearmem: clears the last timeout string from memory.

The timeout and reset commands take an argument. It is not passed on the same line, but on the next one. For example, a testing session might look like this (input lines prefixed with >, printed lines prefixed with <, comments begin with #):

> status   # request the state
< 0        # watchdog is not running, timer is at 0
<          # we just flashed the firmware, no timeout string present
> timeout  # set the timeout
> 10       # provide the argument on a separate line
# nothing is printed back unless we mistyped a command
> start    # start the countdown, LED starts blinking
# wait a second or two
> status
< 3        # we obviously waited three seconds
<          # still no timeout string is set
> reset    # reset the timer
> TESTING  # arbitrary text at most 14 characters in length
# wait a second
> status
< 1
<          # still no timeout string
# wait until the timeout expires; the LED remains on
> status
< 9        # the watchdog is stopped, counter is at timeout (rounded down)
< TESTING  # the timeout string given at the last reset is printed back

As you can see, the status command prints the string that was given at the last reset command before the timeout. This string is stored in persistent memory and will not disappear if power is lost. This way, you can store useful information (such as a timestamp) that will tell you when the reset happened.

A script called frugal_watchdog is provided to make it easier to use the watchdog. Run it with the -h argument to see how it is used. It will take care of writing the timestamp and printing the date of the last reset. Set the path to the watchdog serial device at the top of the script.

Using with the system daemon

There are two ways to use the watchdog. The simplest way is to set a periodic job to run before the timeout. On unix-like systems, cron will do this happily. It only provides one minute resolution though, so you will want to increase the timeout to several minutes.

An even simpler, yet more comprehensive solution, is the watchdog daemon for linux. It can monitor all sorts of things on the system to determine its health. Refer to its documentation for more information. For hardware watchdogs, it normally relies on the /dev/watchdog interface provided by a linux driver. There is no such driver for FrugalWatchdog as it would overcomplicate matters. Instead, the frugal_watchdog script behaves like a test/repair script for the watchdog daemon. Simply drop it in the /etc/watchdog.d/ directory and it will happily reset the watchdog hardware in every interval of the watchdog daemon. If a timeout occurs, use frugal_watchdog status to learn when it happened.

Licence

Copyright 2015 Jure Varlec jure@varlec.si.

The package is available under the terms of GPL3. See the COPYING file for more information.