M48T12(1998) 查看數據表（PDF） - STMicroelectronics
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M48T12 Datasheet PDF : 15 Pages
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M48T02, M48T12
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 M48T02/12is shipped
from STMicroelectronics with the STOP bit set to a
’1’. When reset to a ’0’, the M48T02/12 oscillator
starts within 1 second.
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 minuteper
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 (see
Figure 10).Most clockchips compensatefor crystal
frequencyand temperatureshift error with cumber-
some trim capacitors. The M48T02/12 design,
however, employs periodic counter correction.The
calibration circuit adds or subtractscounts from the
oscillator divider circuit at the divide by 256 stage,
as shown in Figure 11. 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 a sign bit; ’1’ indicates positive
calibration, ’0’ indicates negative calibration. Cali-
bration occurswithin a 64 minute cycle. The first 62
minutes in the cycle may, once per minute, have
one second either shortened or lengthened by 128
oscillator cycles. If a binary ’1’ is loaded into the
register, only the first 2 minutes in the 64 minute
cycle will be modified; if a binary 6 is loaded, the
first 12 will be affected, and so on.
Therefore, each calibration step has the effect of
addingor subtracting256 oscillator cycles for every
125,829,120actual oscillator cycles, that is +4.068
or –2.034 ppm of adjustment per calibration step in
the calibration register. Assuming that the oscillator
is in fact running at exactly 32,768 Hz, each of the
31 increments in the Calibration byte would repre-
sent +10.7 or –5.35 seconds per month which
corresponds to a total range of +5.5 or –2.75
minutes per month.
Two methods are available for ascertaining how
much calibration a given M48T02/12 may require.
The first involves simply setting the clock, letting it
run for a month and comparing it to a known
accurate reference (like WWV broadcasts). While
that may seem crude, it allows the designer to give
the end user the ability to calibrate his clock as his
environment may require, even after the final prod-
uct is packaged in a non-user serviceable enclo-
sure. All the designer has to do is provide a simple
utility that accesses the Calibration byte.
Table 11. Register Map
Address
Data
D7
D6
D5
D4
D3
D2
D1
D0
7FFh
10 Years
Year
7FEh
0
0
0 10 M.
Month
7FDh
0
0
10 Date
Date
7FCh
0
FT
0
0
0
Day
7FBh
0
0
10 Hours
Hours
7FAh
0
10 Minutes
Minutes
7F9h
ST
10 Seconds
Seconds
7F8h
W
R
S
Calibration
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’
Function/Range
BCD Format
Year
Month
Date
Day
Hour
Minutes
Seconds
Control
00-99
01-12
01-31
01-07
00-23
00-59
00-59
10/15
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