M48T18 datasheet(6/27 Pages) STMICROELECTRONICS

6 / 27 page

M48T08, M48T08Y, M48T18

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OPERATION MODES

As Figure 6., page 5 shows, the static memory ar-

ray and the quartz-controlled clock oscillator of the

M48T08/18/08Y are integrated on one silicon chip.

The two circuits are interconnected at the upper

eight memory locations to provide user accessible

BYTEWIDE™ clock information in the bytes with

addresses 1FF8h-1FFFh.

The clock locations contain the year, month, date,

day, hour, minute, and second in 24 hour BCD for-

mat. Corrections for 28, 29 (leap year - valid until

2100), 30, and 31 day months are made automat-

ically. Byte 1FF8h is the clock control register. This

byte controls user access to the clock information

and also stores the clock calibration setting.

The eight clock bytes are not the actual clock

counters themselves; they are memory locations

consisting of BiPORT™ READ/WRITE memory

cells. The M48T08/18/08Y includes a clock control

circuit which updates the clock bytes with current

information once per second. The information can

be accessed by the user in the same manner as

any other location in the static memory array.

The M48T08/18/08Y also has its own Power-fail

Detect circuit. The control circuitry constantly mon-

itors the single 5V supply for an out of tolerance

condition. When VCC is out of tolerance, the circuit

write protects the SRAM, providing a high degree

of data security in the midst of unpredictable sys-

tem operation brought on by low VCC. As VCC falls

below the Battery Back-up Switchover Voltage

(VSO), the control circuitry connects the battery

which maintains data and clock operation until val-

id power returns.

Table 2. Operating Modes

Note: X = VIH or VIL; VSO = Battery Back-up Switchover Voltage.

1. See Table 11., page 20 for details.

Mode

VCC

DQ0-DQ7

Power

Deselect

4.75 to 5.5V

4.5 to 5.5V

VIH

High Z

Standby

Deselect

VIL

High Z

Standby

WRITE

VIL

VIH

VIL

DIN

Active

READ

VIL

VIH

VIL

VIH

DOUT

Active

READ

VIL

VIH

High Z

Active

Deselect

VSO to VPFD(min)

(1)

High Z

CMOS Standby

Deselect

≤ VSO(1)

High Z

Battery Back-up Mode

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