M41T56

Clock operation

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3

Clock operation

The eight byte clock register (see Table 3) is used to both set the clock and to read the date

and time from the clock, in a binary coded decimal format. Seconds, Minutes, and Hours are

contained within the first three registers. Bits D6 and D7 of Clock Register 2 (Hours

Register) contain the CENTURY ENABLE Bit (CEB) and the CENTURY Bit (CB). Setting

CEB to a '1' will cause CB to toggle, either from '0' to '1' or from '1' to '0' at the turn of the

century (depending upon its initial state). If CEB is set to a '0,' CB will not toggle. Bits D0

through D2 of Register 3 contain the Day (day of week). Registers 4, 5, and 6 contain the

Date (day of month), Month, and Years. The final register is the Control Register (this is

described in the Clock Calibration section). Bit D7 of Register 0 contains the STOP Bit (ST).

Setting this bit to a '1' will cause the oscillator to stop.

If the device is expected to spend a significant amount of time on the shelf, the oscillator

may be stopped to reduce current drain. When reset to a '0' the oscillator restarts within one

second.

The seven Clock Registers may be read one byte at a time, or in a sequential block. The

Control Register (Address location 7) may be accessed independently. Provision has been

made to assure that a clock update does not occur while any of the seven clock addresses

are being read. If a clock address is being read, an update of the clock registers will be

delayed by 250ms to allow the READ to be completed before the update occurs. This will

prevent a transition of data during the READ.

Note:

This 250ms delay affects only the clock register update and does not alter the actual clock

time.

Table 3.

1.

Keys:

S = Sign bit

FT = Frequency test bit

ST = Stop bit

OUT = Output level

X = Don't care

CEB = Century enable bit

CB = Century bit

Address

Data

Function/range

BCD format

D7

D6

D5

D4

D3

D2

D1

D0

0

ST

10 Seconds

Seconds

Seconds

00-59

1

X

10 Minutes

Minutes

Minutes

00-59

2.

When CEB is set to '1,' CB toggles from '0' to '1' or from '1' to '0' every 100 years (dependent upon the

initial value set). When CEB is set to '0,' CB does not toggle.

CB

10 hours

Hours

Century/hours

0-1/00-23

3X

X

X

X

X

Day

Day

01-07

4

X

X

10 date

Date

Date

01-31

5

X

X

X

10 M.

Month

Month

01-12

6

10 years

Years

Year

00-99

7

OUT

FT

S

Calibration

Control