Datasheet

M48T02, M48T12 Clock operations
Doc ID 2410 Rev 9 13/25
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;
Address
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