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Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 1926
Keywords: watchdog timer, watchdog circuit, windowed, reset, adjustable watchdog, capacitor adjustable
watchdog
APPLICATION NOTE 1926
Watchdogs Improve System Reliability—How to
Choose the Right Part
Mar 26, 2003
Abstract: Watchdog timers are used to monitor and minimize code execution errors. Internal watchdog
timers are subject to code execution problems, making external watchdog circuits invaluable in
preventing system lockup.
This document provides assistance in selecting the right time of watchdog/supervisory product for
different type of applications and how to apply the circuit without the software code.
Many circuit functions previously realized with dedicated hardware are now implemented in software, due
in part to today's broad choice of low-cost microprocessors (µPs). While software is often the lowest cost
and most flexible way to solve a problem, it forces the designer to take extra measures to ensure system
reliability. While there is no such thing as a program without code errors, careful testing can reduce the
number of errors to one to ten per 1000 lines of code. Therefore, designers must expect a minimum of
10 code errors in a typical control software program with 10,000 lines of code.
Desktop application software errors that cause a system crash are not critical since the user can reboot
the system with only a minor loss of data. However, for industrial control software, the system must be
able to recover from code errors without human intervention. This feature is critical for two main
categories: systems that have high availability, such as servers, telephone systems, and production lines;
and systems that must be highly reliable because a crash could lead to injuries, as with automobiles,
medical instruments, industrial control, robots, and automatic doors. Even if neither of these criteria
apply, system crash/recovery without user intervention (pressing reset or power cycling) is preferred. If a
device recovers from an error without human intervention, the perceived quality of this device is good, as
the user is unaware that something went wrong inside the device. A simple and effective method of
achieving such improved system reliability is to use a watchdog.
The Watchdog
The watchdog is a counter that must be cleared within the watchdog timeout period. If clearing does not
occur, the watchdog generates a reset to cause system reboot or creates a non-maskable interrupt
(NMI), causing a program branch to a fault-recovery subroutine. Most watchdogs are edge triggered.
Therefore, either a rising or a falling edge on the watchdog input (WDI) will clear the counter. The WDI
pin is connected to a processor I/O pin, which is toggled by the software (Figure 1).
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