m41t64 STMicroelectronics, m41t64 Datasheet - Page 18

• Manufacturer Part Number: m41t64
• Description: Serial Access Real-time Clock With Alarms
• Manufacturer: STMicroelectronics

M41T62/63/64/65
Calibrating the Clock
The M41T6x is driven by a quartz controlled oscil-
lator with a nominal frequency of 32,768Hz. The
accuracy of the Real-Time Clock depends on the
frequency of the quartz crystal that is used as the
time-base for the RTC. The accuracy of the clock
is dependent upon the accuracy of the crystal, and
the match between the capacitive load of the oscil-
lator circuit and the capacitive load for which the
crystal was trimmed. The M41T6x oscillator is de-
signed for use with a 6pF crystal load capacitance.
When the Calibration circuit is properly employed,
accuracy improves to better than ±2 ppm at 25°C.
The oscillation rate of crystals changes with tem-
perature (see
M41T6x design employs periodic counter correc-
tion. The calibration circuit adds or subtracts
counts from the oscillator divider circuit at the di-
vide by 256
23., page
are blanked (subtracted, negative calibration) or
split (added, positive calibration) depends upon
the value loaded into the five Calibration Bits found
in the Calibration Register. Adding counts speeds
the clock up, subtracting counts slows the clock
down.
The Calibration Bits occupy the five lower order
bits (D4-D0) in the Calibration Register (08h).
These bits can be set to represent any value be-
tween 0 and 31 in binary form. Bit D5 is a Sign Bit;
'1' indicates positive calibration, '0' indicates nega-
tive calibration. Calibration occurs within a 64
minute cycle. The first 62 minutes in the cycle
may, once per minute, have one second either
shortened by 128 or lengthened by 256 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
adding 512 or subtracting 256 oscillator cycles for
every 125,829,120 actual oscillator cycles, that is
18/33
19. The number of times pulses which
Figure 22., page
stage, as shown in
19). Therefore, the
Figure
+4.068 or –2.034 PPM of adjustment per calibra-
tion step in the calibration register.
Assuming that the oscillator is running at exactly
32,768 Hz, each of the 31 increments in the Cali-
bration byte would represent +10.7 or –5.35 sec-
onds per day which corresponds to a total range of
+5.5 or –2.75 minutes per month (see
23., page
Two methods are available for ascertaining how
much calibration a given M41T6x may require:
–
–
Any deviation from 512Hz indicates the degree
and direction of oscillator frequency shift at the test
temperature.
512.010124 Hz would indicate a +20 ppm oscilla-
tor frequency error, requiring a –10 (XX001010) to
be loaded into the Calibration Byte for correction.
Note that setting or changing the Calibration Byte
does not affect the Frequency test or Square
Wave output frequency.
The first involves setting the clock, letting it run
for a month and comparing it to a known
accurate reference and recording deviation
over a fixed period of time. Calibration values,
including the number of seconds lost or gained
in a given period, can be found in Application
Note AN934, “TIMEKEEPER
CALIBRATION.” This allows the designer to
give the end user the ability to calibrate the
clock as the environment requires, even if the
final product is packaged in a non-user
serviceable enclosure. The designer could
provide a simple utility that accesses the
Calibration byte.
The second approach is better suited to a
manufacturing environment, and involves the
use of either the SQW pin (M41T62/63/64) or
the IRQ/FT/OUT pin (M41T65). The SQW pin
will toggle at 512Hz when RS3 = '0,' RS2 = '1,'
RS1 = '1,' RS0 = '0,' SQWE = '1,' and ST = '0.'
Alternatively, for the M41T65, the IRQ/FT/
OUT pin will toggle at 512Hz when FT and
OUT Bits = '1' and ST = '0.'
19).
For
example,
®
a
reading
Figure
of