m41t83 STMicroelectronics, m41t83 Datasheet - Page 27

• Manufacturer Part Number: m41t83
• Description: Serial I 2c Bus Rtc With Battery Switchover
• Manufacturer: STMicroelectronics

3.3
Real-time clock accuracy
The M41T8x is driven by a quartz controlled oscillator with a nominal frequency of
32,768Hz. The accuracy of the real-time clock is dependent upon the accuracy of the
crystal, and the match between the capacitive load of the oscillator circuit and the capacitive
load for which the crystal was trimmed. Temperature also affects the crystal frequency,
causing additional error (see
The M41T8x provides the option of clock correction through either manufacturing calibration
or in-application calibration. The total possible compensation is typically –93 ppm to +156
ppm. The two compensation circuits that are available are:
1.
2.
Figure 16. Internal load capacitance adjustment
An Analog Calibration register (12h) can be used to adjust internal (on-chip) load
capacitors for oscillator capacitance trimming. The individual load capacitors C
C
0.25pF. This translates to a calculated compensation of approximately ±30 ppm (see
Section 3.4.2: Analog calibration (programmable load capacitance) on page
A Digital Calibration register (08h) can also be used to adjust the clock counter by
adding or subtracting a pulse at the 512Hz divider stage. This approach provides
periodic compensation of approximately –63 ppm to +126 ppm (see
Digital calibration (periodic counter correction) on page
XO
(see
Figure
16), are selectable from a range of –18pF to +9.75pF in steps of
Figure 17 on page
XI
XO
C XO
C XI
Crystal Oscillator
31).
28).
Section 3.4.1:
AI11804
30).
XI
and
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