M48T02, M48T12

Clock operations

13/24

Table 5.

Register map

Keys:

S = SIGN Bit

FT = FREQUENCY TEST Bit (Set to '0' for normal clock operation)

R = READ Bit

W = WRITE Bit

ST = STOP Bit

0 = Must be set to '0'

3.3

Stopping and starting the oscillator

The oscillator may be stopped at any time. If the device is going to spend a significant

amount of time on the shelf, the oscillator can be turned off to minimize current drain on the

battery. The STOP Bit is the MSB of the seconds register. Setting it to a '1' stops the

oscillator. The M48T02/12 is shipped from STMicroelectronics with the STOP Bit set to a '1.'

When reset to a '0,' the M48T02/12 oscillator starts within one second.

3.4

Calibrating the clock

The M48T02/12 is driven by a quartz-controlled oscillator with a nominal frequency of

32,768 Hz. A typical M48T02/12 is accurate within 1 minute per month at 25°C without

calibration. The devices are tested not to exceed ± 35 ppm (parts per million) oscillator

frequency error at 25°C, which equates to about ±1.53 minutes per month.

The oscillation rate of any crystal changes with temperature. Figure 8 on page 15 shows the

frequency error that can be expected at various temperatures. Most clock chips compensate

for crystal frequency and temperature shift error with cumbersome “trim” capacitors. The

M48T02/12 design, however, employs periodic counter correction. The calibration circuit

adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as

shown in Figure 9 on page 15. The number of times pulses are blanked (subtracted,

negative calibration) or split (added, positive calibration) depends upon the value loaded into

the five-bit Calibration Byte found in the Control Register. Adding counts speeds the clock

up, subtracting counts slows the clock down.

The Calibration Byte occupies the five lower order bits in the Control register. This byte can

be set to represent any value between 0 and 31 in binary form. The sixth bit is the Sign Bit;

Addres

s

Data

Function/Range

BCD Format

D7

D6

D5

D4

D3

D2

D1

D0

7FF

10 Years

Year

Year

00-99

7FE

0

0

0

10 M

Month

Month

01-12

7FD

0

0

10 Date

Date

Date

01-31

7FC

0

FT

0

0

0

Day

Day

01-07

7FB

0

0

10 Hours

Hours

Hours

00-23

7FA

0

10 Minutes

Minutes

Minutes

00-59

7F9

ST

10 Seconds

Seconds

Seconds

00-59

7F8

W

R

S

Calibration

Control