M41T82 datasheet(25/61 Pages) STMICROELECTRONICS

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M41T82-M41T83

Clock operation

Doc ID 12578 Rev 12

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3.3

Real-time clock accuracy

The M41T8x 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 Figure 17 on page 30).

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:

An analog calibration register (12h) can be used to adjust internal (on-chip) load

capacitors for oscillator capacitance trimming. The individual load capacitors CXI and

CXO (see Figure 16), are selectable from a range of –18 pF to +9.75 pF in steps of

0.25 pF. This translates to a calculated compensation of approximately ±30 ppm (see

Section 3.4.2: Analog calibration (programmable load capacitance) on page 29).

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 Section 3.4.1:

Digital calibration (periodic counter correction) on page 26).

Figure 16.

Internal load capacitance adjustment

AI11804

Crystal Oscillator

CXI

CXO

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