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7.0

WATCHDOG Timer (WDT)

7.1

OVERVIEW

The WATCHDOG Timer prompts the system via SMI or interrupt when no system activity is detected on a predefined selec-

tion of system events for a predefined period of time of between 1 and 255 minutes or seconds. The numeric value (1-255)

is configured in the WDTO register (WDTO); the time unit used for counting down (minutes or seconds) is configured in the

Status register (WDST).

The WATCHDOG Timer monitors four maskable system events: the interrupt request lines of the Keyboard, Mouse and the

two serial ports (UART1 and UART2). The system prompt is performed by asserting a special-purpose output pin (WDO),

which can be attached to external SMI. Alternatively, this indication can be routed to any arbitrary IRQ line and is also avail-

able on a status bit that can be read by the host.

This chapter describes the generic WATCHDOG Timer functional block. A device may include a different implementation.

For device specific implementation, see the

Device Architecture and Conﬁguration chapter.

7.2

FUNCTIONAL DESCRIPTION

This section describes how the WATCHDOG Timer works. By default, the WATCHDOG Timer counts down in minutes. To

change the time unit from minutes to seconds, reconfigure the WDST register as defined in Section 7.4 on page 139.

The WATCHDOG Timer consists of an 8-bit counter and three registers: Timeout register (WDTO), Mask register (WDMSK)

and Status register (WDST). The counter is an 8-bit down counter that is clocked every minute or second and is used for

the timeout period countdown. The WDTO register holds the programmable timeout, which is the period of inactivity after

which the WATCHDOG Timer prompts the system (1 to 255 minutes or seconds). The WDMSK register determines which

system events are enabled as WATCHDOG Timer trigger events to restart the countdown. The WDST register holds the

WATCHDOG Timer status bit that reflects the value of the WDO pin and indicates that the timeout period has expired. In

addition, it sets the time unit (minutes or seconds). Figure 22 shows the functionality of the WATCHDOG Timer.

Figure 22. WATCHDOG Timer Functional Diagram

Upon reset, the Timeout register (WDTO) is initialized to zero, the timer is deactivated, the WDO is inactive (high) and all

trigger events are masked.

Upon writing to the WDTO register, the timer is activated while the counter is loaded with the timeout value and starts count-

ing down every minute or second. If a trigger event (unmasked system event) occurs before the counter has expired

(reached zero), the counter is reloaded with the timeout period (from WDTO register) and restarts the countdown. If no trig-

ger event occurs before the timeout period expires, the counter reaches zero and stops counting. Consequently, the WDO

pin is asserted (pulled low) and the WDO Status bit is cleared to 0.

Writing to the WDTO register de-asserts the WDO output (released high) and sets the WDO Status bit to 1. If a non-zero

value is written, a new countdown starts as described above. If 00h is written, the timer is deactivated.

To summarize, the WDO output is de-asserted (high) and the Status bit is set to 1 (inactive) upon:

Reset, or

Activating the WATCHDOG Timer or

Writing to the WDTO register.

The WDO output is asserted (low) and the WDO status is set to zero (active) when the counter reaches zero.

When an IRQ is assigned to the WATCHDOG Timer (through the WATCHDOG Timer device configuration), the selected

IRQ level is active as long as the WDO status bit is low (active).

WDTO Register

Timer

Zero Detector

WDO

Interrupt

WDMSK

Write

Load

Reload

Enable Bits

Status Bit

Data Bus

Serial Port 2 IRQ

Serial Port 1 IRQ

Mouse IRQ

Keyboard IRQ

1 Minute/Second Clock

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