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M41T93

Serial SPI bus RTC with battery switchover

STMicroelectronics 

M41T93

Clock operation

3.3

Real time clock accuracy

The M41T93 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 Figure 10 on page 23).

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. 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 9), are selectable from a range of –18pF to +9.75pF in steps of

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

Analog calibration (programmable load capacitance) on page 22).

2. 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 20).

Figure 9. Internal load capacitance adjustment

XI

CXI

XO

CXO

Crystal Oscillator

AI11804

19/51

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