m41t93 STMicroelectronics, m41t93 Datasheet - Page 22

• Manufacturer Part Number: m41t93
• Description: Serial Spi Bus Rtc With Battery Switchover
• Manufacturer: STMicroelectronics

3.4.2
22/51
Analog calibration (programmable load capacitance)
A second method of calibration employs the use of programmable internal load capacitors to
adjust (or trim) the oscillator frequency.
By design, the oscillator is intended to be 0 ppm ± crystal accuracy at room temperature
(25°C, see
the crystal will be 25pF. For incrementing or decrementing the calibration value, capacitance
will be added or removed in increments of 0.25pF to each side of the crystal.
Internally, C
C
on-chip series load capacitance, C
of 12.5pF (AC0-AC6 = ‘0’).
The effective series load capacitance (C
Seven analog calibration bits, AC0 to AC6, are provided in order to adjust the on-chip load
capacitance value for frequency compensation of the RTC. Each bit has a different weight
for capacitance adjustment. An Analog Calibration Sign (ACS) bit determines if capacitance
is added (ACS Bit = ‘0,’ negative calibration) or removed (ACS Bit = ‘1,’ positive calibration).
The majority of the calibration adjustment is positive (i.e. to increase the oscillator frequency
by removing capacitance) due to the typical characteristic of quartz crystals to slow down
due to changes in temperature, but negative calibration is also available.
Since the Analog Calibration Register adjustment is essentially “pulling” the frequency of the
oscillator, the resulting frequency changes will not be linear with incremental capacitance
changes. The equations which govern this mechanism indicate that smaller capacitor values
of Analog Calibration adjustment will provide larger increments. Thus, the larger values of
Analog Calibration adjustment will produce smaller incremental frequency changes. These
values typically vary from 6-10 ppm/bit at the low end to <1 ppm/bit at the highest
capacitance settings. The range provided by the Analog Calibration Register adjustment
with a typical surface mount crystal is approximately ±30 ppm around the AC6-AC0 = 0
default setting because of this property (see
XO
, connected from the XI and XO pins to ground (see
Figure 10 on page
LOAD
of the oscillator is changed via two digitally controlled capacitors, C
23). For a 12.5pF crystal, the default loading on each side of
C
LOAD
LOAD
=
, ranges from 3.5pF to 17.4pF, with a nominal value
LOAD
1
Table 5 on page
1 C
) is the combination of C
XI
+
1 C
Figure 9 on page
XO
23).
XI
and C
19). The effective
XO
:
XI
and