3.8

Century bits

The two century bits, CB1 and CB0, are bits D7 and D6, respectively, in the century/month register at address

06h. Together, they comprise a 2-bit counter which increments at the turn of each century. CB1 is the most

significant bit.

The user may arbitrarily assign the meaning of CB1:CB0 to represent any century value, but the simplest way of

using these bits is to extend the year register (07h) by mapping them directly to bits 9 and 8. (The reader is

reminded that the year register is in BCD format.) Higher order year bits can be maintained in the application

software.

00

CB1:CB0

01

10

11

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Example: 16-bit year value

MAINTAIN

ADDITIONAL

YEAR BITS IN

SOFTWAR

LOWER 8 BITS

CONTAINED IN

YEAR REGISTER

(07h)

LET CB1:CB0 REPRESENT

BITS 9 AND 8 TO EXTEND

THE YEAR REGISTER

CB1

CB0

CENTURY

0

0

2000

0

1

2100

1

0

2200

1

1

2300

3.9

Leap year

Leap year occurs every four years, in years which are multiples of 4. For example, 2012 was a leap year. An

exception to that is any year which is a multiple of 100. For example, the year 2100 is not a leap year. A further

exception is that years which are multiples of 400 are indeed leap years. Hence, while 2100 is not a leap year,

2400 is.

During any year which is a multiple of 4, the M41T6x RTC will automatically insert leap day, February 29.

Therefore, the application software must correct for this during the exception years (2100, 2200, etc.) as noted

above.

3.10

Output driver pin (M41T62/65)

When the OFIE bit, AFE bit, and watchdog register are not set to generate an interrupt, the IRQ /OUT pin

becomes an output driver that reflects the contents of D7 of the calibration register. In other words, when D7

(OUT bit) is a '0,' then the IRQ /OUT pin will be driven low.

Note:

The IRQ /OUT pin is an open drain which requires an external pull-up resistor.

M41T62, M41T64, M41T65

Century bits

DS3840 - Rev 24

page 20/42