80C186EC/80C188EC Microprocessor User's Manual

CHAPTER 12
WATCHDOG TIMER UNIT
System upsets can come from a variety of sources. Errant software can work its way into an endless
loop, waiting for an event that never occurs. An unanticipated radiation source can couple
into improperly shielded circuitry. Not all sources of system upsets can be anticipated and guarded
against. The Watchdog Timer Unit provides a graceful method for recovery from unexpected
hardware and software upsets.
Watchdog timers are designed to reset the system unless the timer is periodically reloaded with a
new value (this is also known as “kicking the watchdog”). The system software is responsible for
reloading the watchdog timer. It is assumed that errant code or a system lockup will prevent the
watchdog timer from being reloaded, resulting in a system reset. A special instruction sequence,
a sequence that errant code would be very unlikely to produce, is typically used to reload the timer.
The Watchdog Timer Unit (WDT) can function either as a system watchdog or as a general-purpose
timer, or it can be disabled for systems that do not wish to use it.
12.1 FUNCTIONAL OVERVIEW
A block diagram of the Watchdog Timer Unit is shown in Figure 12-1. The 32-bit down counter
decrements every CLKOUT cycle. The WDTOUT pin is driven low for four CLKOUT cycles
when the down counter reaches zero (a WDT timeout). The WDTOUT signal may be used to reset
the device or as an interrupt request.
The down counter is reloaded with the 32-bit reload value under two conditions:
• when a special LOCKed instruction sequence is issued to the Protection and Control
Circuitry
• when the down counter reaches zero
The Protection and Control Circuitry is responsible for enabling and disabling the Watchdog
Timer as well as preventing unauthorized modification of count values.
12.2 USING THE WATCHDOG TIMER AS A SYSTEM WATCHDOG
There are two methods for recovery following a software upset: a full system reset or an interrupt
request. Both methods can be implemented with the Watchdog Timer Unit.
12-1