21-06-2012, 05:54 PM
DESIGN OF AN IMPROVED WATCHDOG CIRCUIT FOR MICROCONTROLLER-BASED SYSTEMS
DESIGN OF AN IMPROVED WATCHDOG.pdf (Size: 284.43 KB / Downloads: 41)
Introduction
Watchdog timers are a form of active hardware
redundancy that is extremely useful for detecting system
faults [ 1-31. These timers must be reset by software on a
repetitive basis. If the software is performing as expected
the watchdog timer will be reset before producing a
system reset. But if a hardware or software failure causes
the software to deflect from its normal sequence. the
watchdog timer will not be reset in time and the error is
detected The watchdog timer may cause the system to
reset for protection agunst software bugs and temporary
failures or it may cause the system to be replaced by an
active spare for protection against permanent failures. In
most modem microcontrollers, the watchdog timer is
built-in [4].
Astable Multivibrator
The astable multivibrator is one of the components of the
improved watchdog circuit. Following is a brief
descripQon of its operation. Figure 2 shows the block
chag” of the circuit it consists of a 555 IC with two
resistances Fb and Rb and two capacitors [6]. If pin 4
(Reset) of the 555 timer is high, the timer is enabled and
produces a square wave at the output @m 3).
PULSE-TO-LEVEL Circuit
The function of this circuit is to convert a train of positive
pulses into a constant high voltage. Also a low input
produces a low output. Figure 4 shows the block diagram
of the pulse-to-Level h i t . The basic idea of this circuit
stems from the fact thal R2 >> R1. Consequently.
capacitor C will charge veryquicklyduringthepositive
pulse and discharge very slowly till the arrival of the next
positive pulse.
Design of the Improved Watchdog Circuit
a block diagram of the improved
watchdog circuit. Remember that the on@ watchdog
circuit from [5] only detected refresh inputs with a period
higher than a certain predetermined value. The improved
watchdog circuit described below will detect refresh
inputs with a period less than TI or greater than T2.
Obviously T2 > TI. WDI detects refresh inputs with
period greater than TI while WD2 detects refksh inputs
with period greater than T2. The circuit will be analyzed
in the following three cases: 1) The period of the reinput
is less than TI, 2) The period of the refresh q u t is
greater than TI but less than T2 and 3) The penod of the
refresh input is greater thanT2.
Conclusions
Watchdog circuits are very valuable tools for protecting
micrmntroller-based circuits against permanent andor
temporary failures. This pper has presented the design of
an improved watchdog that can detect failurescausing
faster than normal as well as slower than normal refresh
inputs. The circuit consists of watchdogs based on the 555
timer, circuits (DRC) that convert positive pulses into a
constant high level. the XNOR logic gte, inverters and
an astable multivibrator based agam on the 555 timer. It is
shown that refresh inputs with a period less than T1 or
greater than T2 will be interpreted by the improved
watchdog circuit as incorrect inputs