Datasheet

DocID12615 Rev 8 25/56
M41T93 Clock operation
56
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 38 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 11 on page 26) when a 12.5 pF crystal is connected. Referring to
Figure 12 on page 28, the device has two load capacitors, C
XI
and C
XO
, 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 C
XI
= C
XO
= 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 C
XI
and C
XO
. 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 11) 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.
b7 b6 b5 b4 b3 b2 b1 b0
sign 16 8 4 2 1 0.5 0.25 pF
C
LOAD
(pF)
C
XI
, C
XO
(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)
C
LOAD
C
XI
C
XO

C
XI
C
XO
+
---------------------------=
C
LOAD
25 25
25 25+
------------------- 12.5pF==