STK17TA8-R datasheet(16/22 Pages) ETC

16 / 22 page

STK17TA8

February 2004

Document Control # ML0023 rev 0.3

output. Only one source is necessary to drive the

pin. The user can identify the source by reading the

Flags/Control register, which contains the flags

associated with each source. All flags are cleared to

0 when the register is read. The cycle must be a

complete read cycle (WE high); otherwise the flags

will not be cleared. The power monitor has two pro-

grammable settings that are explained in the power

monitor section.

Once an interrupt source is active, the pin driver

determines the behavior of the output. It has two

programmable settings as shown below. Pin driver

control bits are located in the Interrupts register.

According to the programming selections, the pin

can be driven in the backup mode for an alarm inter-

rupt. In addition, the pin can be an active low (open-

drain) or an active high (push-pull) driver. If pro-

grammed for operation during backup mode, it can

only be active low. Lastly, the pin can provide a one-

shot function so that the active condition is a pulse

or a level condition. In one-shot mode, the pulse

width is internally fixed at approximately 200 ms.

This mode is intended to reset a host microcontrol-

ler. In level mode, the pin goes to its active polarity

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

This mode is intended to be used as an interrupt to

a host microcontroller. The control bits are summa-

rized as follows:

Watchdog Interrupt Enable - WIE. When set to 1,

the watchdog timer drives the INT pin as well as an

internal flag when a watchdog timeout occurs.

WhenWIE is set to 0, the watchdog timer affects

only the internal flag.

Alarm Interrupt Enable - AIE. When set to 1, the

alarm match drives the INT pin as well as an internal

fla. When set to 0, the alarm match only affects to

internal flag.

Power Fail Interrupt Enable - PFE. When set to 1,

the power fail monitor drives the pin as well as an

internal flag. When set to 0, the power fail monitor

affects only the internal flag.

Alarm Battery-backup Enable - ABE. When set to 1,

the clock alarm interrupt (as controlled by AIE) will

function even in battery backup mode. When set to

0, the alarm will occur only when Vcc>Vswitch. AIE

should only be set when the INT pin is programmed

for active low operation. In addition, it only functions

with the clock alarm, not the watchdog. If enabled,

the power monitor will drive the interrupt during all

normal Vcc conditions regardless of the ABE bit.

The application for ABE is intended for power con-

trol, where the system powers up at a predeter-

mined time. Depending on the application, it may

require dedicating the INT pin to this function.

High/Low - H/L. When set to a 1, the INT pin is

active high and the driver mode is push-pull. The

INT pin can drive high only when Vcc>Vswitch.

When set to a 0, the INT pin is active low and the

drive mode is open-drain. Active low (open drain) is

operational even in battery backup mode.

Pulse/Level - P/L. When set to a 1 and an interrupt

occurs, the INT pin is driven for approximately 200

ms. When P/L is set to a 0, the INT pin is driven high

or low (determined by H/L) until the Flags/Control

When an enabled interrupt source activates the INT

pin, as external host can read the Flags/Control reg-

ister to determine the cause. Remember that all

flags will be cleared when the register is read. If the

INT pin is programmed for Level mode, then the

condition will clear and the INT pin will return to its

inactive state. If the pin is programmed for Pulse

mode, then reading the flag also will clear the flag

and the pin. The pulse will not complete its specified

duration if the Flags/Control register is read. If the

INT pin is used as a host reset, then the Flags/Con-

trol register cannot be read during a reset.

During a power-on reset with no battery, the inter-

rupt register is automatically loaded with the value

24h. This causes power-fail interrupt to be enabled

with an active-low pulse.

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