M41T93_11 STMICROELECTRONICS [STMicroelectronics], M41T93_11 Datasheet - Page 21

• Manufacturer Part Number: M41T93_11
• Description: Serial SPI bus real-time clock with battery switchover
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

M41T93
3.3
Real-time clock accuracy
The M41T93 is driven by a quartz controlled oscillator with a nominal frequency of
32,768 Hz. 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 M41T93 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 10. 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
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 512 Hz divider stage. This approach provides
periodic compensation of approximately –63 ppm to +126 ppm (see
(periodic counter correction) on page
XO
(see
Figure
10), are selectable from a range of –18 pF to +9.75 pF in steps of
XI
XO
Figure 11 on page
Doc ID 12615 Rev 6
C XO
C XI
Crystal Oscillator
22).
25).
24).
Digital calibration
Clock operation
XI
AI11804
and
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