M48T08, M48T08Y, M48T18

Clock operations

Doc ID 2411 Rev 10

15/31

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 (ST) is the MSB of the seconds register. Setting it to a '1' stops the

oscillator. The M48T08/18/08Y (in the PCDIP28 package) is shipped from

STMicroelectronics with the STOP bit set to a '1.' When reset to a '0,' the M48T08/18/08Y

oscillator starts within one second.

Note:

To guarantee oscillator startup after initial power-up, first write the STOP bit (ST) to '1,' then

reset to '0.'

3.4

Calibrating the clock

The M48T08/18/08Y is driven by a quartz-controlled oscillator with a nominal frequency of

32,768 Hz. A typical M48T08/18/08Y 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. With the

calibration bits properly set, the accuracy of each M48T08/18/08Y improves to better than

+1/–2 ppm at 25 °C.

The oscillation rate of any crystal changes with temperature. Figure 8 on page 17 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

Table 5.

Register map

Address

Data

Function/range

BCD format

D7

D6

D5

D4

D3

D2

D1

D0

1FFFh

10 years

Year

Year

00-99

1FFEh

0

0

0

10 M

Month

Month

01-12

1FFDh

0

0

10 date

Date

Date

01-31

1FFCh

0

FT

0

0

0

Day

Day

01-07

1FFBh

0

0

10 hours

Hours

Hours

00-23

1FFAh

0

10 minutes

Minutes

Minutes

00-59

1FF9h

ST

10 seconds

Seconds

Seconds

00-59

1FF8h

W

R

S

Calibration

Control