FM31L278/L276/L274/L272 - 3V I2C Companion

Rev. 2.1

Sept. 2011

Page 15 of 26

01h

CAL/Control

D7

D6

D5

D4

D3

D2

D1

D0

OSCEN

Reserved

CALS

CAL.4

CAL.3

CAL.2

CAL.1

CAL.0

/OSCEN

/Oscillator Enable. When set to 1, the oscillator is halted. When set to 0, the oscillator runs. Disabling the

oscillator can save battery power during storage. On a power-up without battery, this bit is set to 1. Battery-

backed, read/write.

Reserved

Reserved bits. Do not use. Should remain set to 0.

CALS

Calibration sign. Determines if the calibration adjustment is applied as an addition to or as a subtraction from

the time-base. Calibration is explained on page 7. Nonvolatile, read/write.

CAL.4-0

These five bits control the calibration of the clock. Nonvolatile, read/write.

00h

Flags/Control

D7

D6

D5

D4

D3

D2

D1

D0

Reserved

CF

Reserved

Reserved

Reserved

CAL

W

R

CF

Century Overflow Flag. This bit is set to a 1 when the values in the years register overflows from 99 to 00. This

indicates a new century, such as going from 1999 to 2000 or 2099 to 2100. The user should record the new

century information as needed. This bit is cleared to 0 when the Flag register is read. It is read-only for the user.

Battery-backed.

CAL

Calibration Mode. When set to 1, the clock enters calibration mode. When CAL is set to 0, the clock operates

normally, and the CAL/PFO pin is controlled by the power fail comparator. Battery-backed, read/write.

W

Write Time. Setting the W bit to 1 freezes the clock. The user can then write the timekeeping registers with

updated values. Resetting the W bit to 0 causes the contents of the time registers to be transferred to the

timekeeping counters and restarts the clock. Battery-backed, read/write.

R

Read Time. Setting the R bit to 1 copies a static image of the timekeeping core and place it into the user

registers. The user can then read them without concerns over changing values causing system errors. The R bit

going from 0 to 1 causes the timekeeping capture, so the bit must be returned to 0 prior to reading again.

Battery-backed, read/write.

Reserved

Reserved bits. Do not use. Should remain set to 0.