M41T93 datasheet(24/49 Pages) STMICROELECTRONICS

24 / 49 page

Clock operation

M41T93

24/49

The on-chip capacitance can be calculated as follows:

For example:

CLOAD (12h = x0000000) = 12.5pF,

CLOAD (12h =11001000) = 3.5pF, and

CLOAD (12h = 00100111) = 17.4pF.

The oscillator sees a minimum of 3.5pF with no programmable load capacitance selected.

Note:

These are typical values, and the total load capacitance seen by the crystal will include

approximately 1-2pF of package and board capacitance in addition to the Analog Calibration

Any invalid value of Analog Calibration will result in the default capacitance of 25pF.

The combination of analog and digital trimming can give up to –93 to +156 ppm of the total

adjustment.

Figure 11 on page 24 represents a typical curve of clock ppm adjustment versus the Analog

Calibration value. This curve may vary with different crystals, so it is good practice to

evaluate the crystal to be used with an M41T93 device before establishing the adjustment

values for the application in question.

Figure 11.

Clock accuracy vs. on-chip load capacitors

LOAD

AC6 AC0 value

–

decimal

() 0,25pF

[] 7pF

–20.0

–10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

–10.0

–5.0

0.0

5.0

10.0

15.0

20.0

Analog Calibration Value

AI12293

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