What is Digital Watchdog?

The concept of an artificial creature that obeys your directions and never steps out of line, is always alert and on the job, always ready to step in no matter what is happening in the environment is something that security professionals and directories have wanted for years. It looks like we are going to continue to be disappointed for the time being, unless you count such things as Sony's Aibo™ or the various DIY robot dogs you can build by following instructions you find on the internet. Most people don't. So until Japan designs the equivalent of a cybernetic Doberman that can keep an eye on all of our appliances and take necessary action when required, there won't be any real artificial dog patrolling property or monitoring equipment.

But the dream of a digital watchdog (DWD) has arrived in one form, and it works in the background to keep advanced electronics functioning without a hiccup. Almost every modern electronic product contains a CPU of some kind (even if it is a reduced instruction set CPU - RISC - it's still a CPU). It may be a surprise to some to know that CPU's make mistakes all the time. Internal clocks malfunction, memory allocations fail, data paths make intermittent connection, information is lost or mistakenly recalled, and more. (Of course I am simplifying this.) How then do they continue to work? Error correction, of course. CPU's have built-in error correction on the chip or through software. So the concept of DWD (and implementation) have been with us for a long time.

When an electronic product fails it is usually obvious to a human observer. In the case of a DVR or computer, the human will notice it lock-up or execute instructions incorrectly. In the most basic corrective action, the human turns off the DVR (or computer) and turns it back on - a reset. Security and surveillance products, specifically DVRs, are made to operate without a human being involved so they have to have a way to know when to perform the equivalent of a "hard" reset. This can be done in a number of ways. We're going to look at one method.

Refer to the simplified diagram of a security DVR at right. Note the relationship between the CPU and the Watchdog component (normally these two circuits would be located on the same circuit board, they are shown separately for the sake of this discussion). Everything gets power from the power supply section where input voltage is filtered and split to supply various parts. The CPU (and/or support circuits) generate a clock pulse that goes to the DWD. The DWD is separate from the CPU, which insures that an elaborate malfunction of the CPU won't cascade to the DWD for any reason.

The DWD monitors a clock pulse from the CPU section. The clock pulse is generated through a series of steps. Each step relates to a process that is necessary for the product to function. If a process fails, the clock pulse is effected. The failure may not be obvious to a human observer, but the clock generating circuits - CPU based or not -  will detect it and the clock pulse will deviate from the norm. The DWD simply compares information it generates from the clock signal it is getting to the information it has stored in memory and when  there is a discrepancy between the two it knows there is a problem. The nature of the problem is not its concern - it just knows an error exists. In the simplest implementations the DWD merely asserts the CPU reset line, which then reset the CPU. All processes are brought to a halt and the product is re-booted just as if a human operator had turned it off/on (after waiting for 10 Mississippis, Chimpanzees, or whatever superstitious ritual the human prefers to observe). In this way the digital watchdog circuit restarts the product.

This is a very simple discussion of DWD. Different products will implement the DWD function in different ways, including complex shutdown and restart procedures, and multiple monitoring levels and fault tolerance. It's good to know that our DVRs have this built-in function when you are watching your store or home on your iPhone from a remote location after a power failure occurred on your property. You can be confident your video recorder is up and running and monitoring your property thanks to a digital watchdog.