17-3
WATCHDOG TIMER UNIT
17.1.1 WDT Signals
Table 17-1 describes the signals associated with the WDT.
17.2 WATCHDOG TIMER UNIT OPERATION
After a device reset, the WDT begins counting down in general-purpose timer mode. Unless you
change the mode, change the reload value, or disable it, the WDT times out and asserts WDTOUT
after 4 million (222) processor clock cycles (PH1 or CLKOUT cycles).
The 32-bit down-counter decrements on every processor clock cycle. When the down-counter
reaches zero, the 8-state binary counter drives the WD TOUT pin high for e ight proc ess or clock
cycles (16 CLK2 cycles) to signal the timeout. An interna l signal carries the inverted value of the
WDTOUT pin to the interrupt control unit (the slave’s IR7 line). A WDT timeout can reset the
system or generate an interrupt request, depending on how WDTOUT is used in your system.
Table 17-1. WDT Signals
Signal Device Pin or
Internal Signal Description
ADS# Device pin Address Status (from the bus interface uni t):
Indicates that the processor is driving a valid bus-cycle definition
and address onto its pins. Bus monitor mode reloads and starts the
down-counter each time ADS# is asserted.
IDLE Internal signal Idle (from the clock and power management unit):
Indicates that the device is in idle mode (core clocks stopped and
peripheral clocks running). In watchdog mode, the down-counter
stops when the core is idle. In bus monitor or general-purpose
timer mode, the WDT continues to run while the core is idle.
READY# Device pin Ready (from the bus interface unit):
Indicates that the current bus cycle has completed. Bus monitor
mode stops the down-counter when READY# is asserted.
WDTOUT Device pin Watchdog Timer Output:
Indicates that the down-counter has timed out. If you want a WDT
timeout to reset the device, connect WDTOUT to the RESET input.
If you want a WDT timeout t o generate a nonmaskable interrupt,
connect WDTOUT to the NMI input.
An internal signal carries the inverted value of WDTOUT to the
interrupt control unit (the slave’s I R7 line). If you want a WDT
timeout to cause a maskable interrupt, enable the interrupt.
(Chapter 8, “Interrupt Control Unit,” explains how to do this. )