M48T59 datasheet(14/21 Pages) STMICROELECTRONICS

14 / 21 page

M48T59, M48T59Y, M48T59V

14/21

The most significant bit of the Watchdog Register

is the Watchdog Steering Bit. When set to a ’0’, the

watchdog will activate the IRQ/FT pin when timed-

out. When WDS is set to a ’1’, the watchdog will

output a negative pulse on the RST pin for a dura-

tion of 40ms to 200ms. The Watchdog register and

the FT bit will reset to a ’0’ at the end of a watchdog

time-out when the WDS bit is set to a ’1’.

The watchdog timer resets when the microproces-

sor performs a re-write of the Watchdog Register.

The time-out period then starts over. The watch-

dog timer is disabled by writing a value of

00000000 to the eight bits in the Watchdog Regis-

ter. The watchdog function is automatically dis-

abled upon power-down and the Watchdog

to output to the IRQ/FT pin and the frequency test

function is activated, the watchdog or alarm func-

tion prevails and the frequency test function is de-

nied.

BATTERY LOW FLAG

The M48T59/59Y/59V automatically performs pe-

riodic battery voltage monitoring upon power-up

and at factory-programmed time intervals of 24

hours (at day rollover) as long as the device is

powered and the oscillator is running. The Battery

Low flag (BL), Bit D4 of the flags Register 1FF0h,

will be asserted high if the internal or SNAPHAT

battery is found to be less than approximately

2.5V. The BL flag will remain active until comple-

tion of battery replacement and subsequent bat-

tery low monitoring tests, either during the next

power-up sequence or the next scheduled 24-hour

interval.

If a battery low is generated during a power-up se-

quence, this indicates that the battery voltage is

below 2.5V (approximately), which may be insuffi-

cient to maintain data integrity. Data should be

considered suspect and verified as correct. A fresh

battery should be installed.

If a battery low indication is generated during the

24-hour interval check, this indicates that the bat-

tery is near end of life. However, data has not been

compromised due to the fact that a nominal VCC

is supplied. In order to insure data integrity during

subsequent periods of battery back-up mode, it is

recommended that the battery be replaced. The

SNAPHAT top may be replaced while VCC is ap-

plied to the device.

Note: Battery monitoring is a useful technique only

when performed periodically. The M48T59/59Y/

59V only monitors the battery when a nominal

VCC is applied to the device. Thus applications

which require extensive durations in the battery

back-up mode should be powered-up periodically

(at least once every few months) in order for this

technique to be beneficial. Additionally, if a battery

low is indicated, data integrity should be verified

upon power-up via a checksum or other technique.

POWER-ON DEFAULTS

Upon application of power to the device, the fol-

lowing register bits are set to a ’0’ state: WDS;

BMB0-BMB4; RB0-RB1; AFE; ABE; W; R; FT.

(See Table 13).

Table 13. Default Values

Note: 1. WDS, BMB0-BMB4, RBO, RB1.

2. State of other control bits undefined.

3. State of other control bits remains unchanged.

4. Assuming these bits set to ’1’ prior to power-down.

Condition

AFE

ABE

WATCHDOG

Initial Power-up

(Battery Attach for SNAPHAT) (2)

00000

Subsequent Power-up / RESET (3)

00000

Power-down (4)

00011

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