DATA RETENTION MODE

a conventionalBYTEWIDE static RAM. Should the

supply voltage decay, the RAM will automatically

outputsbecome high impedance, and all inputs are

treated as ”don’t care.”

Note: A power failure during a write cycle may

corrupt data at the currently addressed location,

but does not jeopardize the rest of the RAM’s

be assured the memory will be in a write protected

The M48T02/12 may respond to transient noise

fore, decoupling of the power supply lines is rec-

ommended.

is checked. If the voltage is too low, an internal

Battery Not OK (BOK) flag will be set. The BOKflag

can be checked after power up. If the BOK flag is

set, the first write attempted will be blocked. The

flag is automatically cleared after the first write, and

normal RAM operation resumes. Figure 9 illus-

trates how a BOK check routine could be struc-

tured.

For more information on a Battery Storage Life

refer to the Application Note AN1012.

CLOCK OPERATIONS

Reading the Clock

Updates to the TIMEKEEPER registers should be

halted before clock data is read to prevent reading

data in transition. Because the BiPORT TIME-

KEEPER cells in the RAM array are only data

registers, and not the actual clock counters, updat-

ing the registers can be halted without disturbing

the clock itself.

Updatingis halted when a ’1’ is written to the READ

bit, the seventh bit in the control register. As long

as a ’1’ remains in that position, updating is halted.

After a halt is issued, the registersreflect the count;

that is, the day, date, and the time that were current

at the moment the halt command was issued.

All of the TIMEKEEPER registers are updated si-

multaneously. A halt will not interrupt an update in

progress.Updating is withina secondafter the read

bit is reset to a ’0’.

Setting the Clock

The eighth bit of the control register is the WRITE

bit. Setting the WRITE bit to a ’1’, like the READ bit,

halts updates to the TIMEKEEPER registers. The

user can then load them with the correct day, date,

and time data in 24 hour BCD format (see Table

10). Resetting the WRITE bit to a ’0’ then transfers

the values of all time registers (7F9h-7FFh) to the

actual TIMEKEEPER counters and allows normal

operationto resume.The FT bit and the bits marked

as ’0’ in Table 10 must be written to ’0’ to allow for

normal TIMEKEEPER and RAM operation.

See the Application Note AN923 ”TIMEKEEPER

rolling into the 21st century” for more information

on Century Rollover.

READ DATA

AT ANY ADDRESS

AI00607

IS DATA

COMPLEMENT

OF FIRST

READ?

(BATTERY OK)

POWER-UP

YES

NO

WRITE DATA

COMPLEMENT BACK

TO SAME ADDRESS

READ DATA

AT SAME

ADDRESS AGAIN

NOTIFY SYSTEM

OF LOW BATTERY

(DATA MAY BE

CORRUPTED)

WRITE ORIGINAL

DATA BACK TO

SAME ADDRESS

(BATTERY LOW)

CONTINUE

Figure 9. Checking the BOK Flag Status

9/15

M48T02, M48T12