Datasheet

DocID12615 Rev 8 21/56

M41T93 Clock operation

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 Figure 11 on page 26).

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

capacitors for oscillator capacitance trimming. There are two load capacitors C

and

(see Figure 10), nominally 25 pF each, one on either side of the crystal. The

effective load capacitance is the series equivalent of C

and C

. For the nominal

25 pF, the effective load capacitance is 12.5pF.

Writing to the analog calibration register adjusts both capacitors by the same amount.

That is, the two capacitors will always have the same value. They can be adjusted up

or down in 0.25 pF steps. The maximum adjustment up is +9.75 pF for a total of

34.75 pF (17.4 pF effective load) to slow the oscillator. The maximum downward

adjustment is –18 pF for a total of 7 pF (3.5 pF effective load) to speed up the oscillator.

2. 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 Digital calibration

(periodic counter correction) on page 22).

This range of load values translates to an approximate frequency range adjustment of

–15 to +95 ppm (see Analog calibration (programmable load capacitance) on page 25).

Figure 10. Internal load capacitance adjustment

AI11804

Crystal Oscillator