M48T212A datasheet(11/20 Pages) STMICROELECTRONICS

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M48T212A

The RST signal also remains active during this

time (see Figure 5).

Note: Most low power SRAMs on the market to-

day can be used with the M48T212A TIMEKEEP-

ER Controller. There are, however some criteria

which should be used in making the final choice of

an SRAM to use. The SRAM must be designed in

a way where the chip enable input disables all oth-

er inputs to the SRAM. This allows inputs to the

M48T212A and SRAMs to be Don’t Care once

VCC falls below VPFD(min). The SRAM should also

guarantee data retention down to VCC =2.0V. The

chip enable access time must be sufficient to meet

the system needs with the chip enable output

propagation delays included.

If the SRAM includes a second chip enable pin

(E2), this pin should be tied to VOUT.

If data retention lifetime is a critical parameter for

the system, it is important to review the data reten-

tion

current

specifications

for

the

particular

SRAMs being evaluated. Most SRAMs specify a

data retention current at 3.0V. Manufacturers gen-

erally specify a typical condition for room temper-

ature along with a worst case condition (generally

at elevated temperatures). The system level re-

quirements will determine the choice of which val-

ue to use.

The data retention current value of the SRAMs can

then be added to the IBAT value of the M48T212A

to determine the total current requirements for

data retention. The available battery capacity can

then be divided by this current to determine the

amount of data retention available.

For a further more detailed review of lifetime calcu-

lations, please see Application Note AN1012.

Table 13. TIMEKEEPER Register Map

Address

Function/Range

BCD Format

10 Years

Year

00-99

10M

Month

01-12

10 Date

Date: Day of Month

Date

01-31

Day of Week

Day

01-7

10 Hours

Hours (24 Hour Format)

Hour

00-23

10 Minutes

Minutes

Min

00-59

10 Seconds

Seconds

Sec

00-59

Calibration

Control

WDS

BMB4

BMB3

BMB2

BMB1

BMB0

RB1

RB0

Watchdog

AFE

ABE

Al 10M

Alarm Month

A Month

01-12

RPT4

RPT5

AI 10 Date

Alarm Date

A Date

01-31

RPT3

AI 10 Hour

Alarm Hour

A Hour

00-23

RPT2

Alarm 10 Minutes

Alarm Minutes

A Min

00-59

RPT1

Alarm 10 Seconds

Alarm Seconds

A Sec

00-59

1000 Year

100 Year

Century

00-99

WDF

Flag

Keys:

S = Sign Bit

FT = Frequency Test Bit

R = Read Bit

W = Write Bit

ST = Stop Bit

0 = Must be set to zero

BL = Battery Low Flag

BMB0-BMB4 = Watchdog Multiplier Bits

AFE = Alarm Flag Enable Flag

RB0-RB1 = Watchdog Resolution Bits

WDS = Watchdog Steering Bit

ABE = Alarm in Battery Back-Up Mode Enable Bit

RPT1-RPT5 = Alarm Repeat Mode Bits

WDF = Watchdog flag

AF = Alarm flag

Y = ’1’ or ’0’

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