Datasheet

M48T08, M48T08Y, M48T18 Clock operations
Doc ID 2411 Rev 11 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