STK17TA8-R45I datasheet(15/22 Pages) ETC

15 / 22 page

STK17TA8

February 2004

Document Control # ML0023 rev 0.3

counter load register. Counting begins on power up

and restarts from the loadable value any time the

Watchdog Strobe (WDS) bit is set to 1. The counter

is compared to the terminal value of 0. If the counter

reaches this value, it causes an internal flag and an

optional interrupt output. The user can prevent the

timeout interrupt by setting WDS bit to 1 prior to the

counter reaching 0. This causes the counter to be

reloaded with the watchdog timeout value and to be

restarted. As long as the user sets the WDS bit prior

to the counter reaching the terminal value, the inter-

rupt and flag never occurs.

New timeout values can be written by setting the

watchdog write bit to 0. When the WDW is 0 (from

the previous operation), new writes to the watchdog

timeout value bits D5-D0 allow the timeout value to

be modified. When WDW is a 1, then writes to bits

D5-D0 will be ignored. The WDW function allows a

user to set the WDS bit without concern that the

watchdog timer value will be modified. A logical dia-

gram of the watchdog timer is shown below. Note

that setting the watchdog timeout value to 0 would

be otherwise meaningless and therefore disables

the watchdog function.

The output of the watchdog timer is a flag bit WDF

that is set if the watchdog is allowed to timeout. The

flag is set upon a watchdog timeout and cleared

when the Flags/Control register is read by the user.

The user can also enable an optional interrupt

source to drive the INT pin if the watchdog timeout

occurs.

POWER MONITOR

The STK17TA8 provides a power management

scheme with power-fail interrupt capability. It also

controls the internal switch to backup power for the

clock and protects the memory from low-Vcc

access. The power monitor is based on an internal

band-gap reference circuit that compares the Vcc

voltage to various thresholds.

As descibed in the AutoStore™ section previously,

when Vswitch is reached as Vcc decays from power

loss, a data store operation is initiated from SRAM

to the nonvolatile elements, securing the last SRAM

data state. Power is also switched from Vccx to the

backup supply (battery or capacitor) to operate the

RTC oscillator.

When operating from the backup source no data

may be read or written and the clock functions are

not available to the user. The clock continues to

operate in the background. Updated clock data is

available to the user 10 msec after Vcc has been

restored to the device.

INTERRUPTS

The STK17TA8 provides three potential interrupt

sources. They include the watchdog timer, the

power monitor, and the clock/calendar alarm. Each

can be individually enabled and assigned to drive

the INT pin. In addition, each has an associated flag

bit that the host processor can use to determine the

cause of the interrupt.

Some of the sources have additional control bits that

determine functional behavior. In addition, the pin

driver has three bits that specify its behavior when

an interrupt occurs. A functional diagram of the

interrupt logic is shown below.

The three interrupts each have a source and an

enable. Both the source and the enable must be

active (true high) in order to generate an interrupt

Figure 5. Watchdog Timer Block Diagram

Figure 6. Interrupt Block Diagram

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