M41T83RM6F datasheet(31/64 Pages) STMICROELECTRONICS

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M41T82, M41T83

Clock operation

3.4.2

Analog calibration (programmable load capacitance)

A second method of calibration employs the use of programmable internal load capacitors

to adjust (or trim) the oscillator frequency. As discussed in Section 3.4.1, the 512 Hz

frequency test output can be used to determine the amount of frequency error in the

oscillator. Changes in the analog calibration value will affect the frequency test output, thus

the user can immediately see the effects of these changes (see Section 3.14 on page 44 for

more information on enabling the FT output).

By design, the oscillator is intended to be 0 ppm (± crystal accuracy) at room temperature

(25 °C, see Figure 18 on page 32) when a 12.5 pF crystal is connected. Referring to

Figure 19 on page 34

, the device has two load capacitors, CXI and CXO, connected from the

XI and XO pins to ground. These are nominally 25 pF each. The effective load capacitance

is the series equivalent of these two:

For the nominal case of CXI = CXO = 25 pF,

Thus, the nominal effective load capacitance matches the crystal specification of 12.5 pF.

The analog calibration register can be digitally adjusted, up or down, in increments of

0.25 pF, to change the capacitance of CXI and CXO. The default value is 25 pF. The

maximum is 34.75 pF, to slow the clock, and the minimum is 7 pF, to speed up the clock.

The analog calibration value is in sign-magnitude format with the most significant bit the sign

bit. The table below shows the approximate weighting for each of the bits.

While the 7 bits plus sign suggest a total adjustment range of ±31.75 pF, the logic inside the

device limits this to the range +9.75 pF / –18 pF. The table below summarizes the nominal,

upper and lower limits of the load capacitance and the expected effect on the operating

frequency of the oscillator.

The asymmetrical nature of the adjustment range (+9.75 pF / –18 pF) is due to the nature of

the frequency versus temperature curve (Figure 18) of 32.768 kHz watch crystals. The

oscillator will slow down at temperatures both above and below room level (~25 °C). Hence,

it usually needs to be sped up, so more adjustment range is provided to remove capacitance

than to increase it.

sign

0.5

0.25

CLOAD

(pF)

CXI, CXO

(pF)

ACAL

(Addr 0x12)

Oscillator frequency

12.5

25 (default)

0x00

0 ppm

17.4

34.75 (+9.75)

0x27

–15 ppm (slow)

3.5

7 (–18)

0xC8

+95 ppm (fast)

LOAD

•

---------------------------

LOAD

•

-------------------

12.5pF

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