WM8326 Datasheet, PDF (48/255 Pages) Wolfson Microelectronics plc – Processor Power Management Subsystem

WM8326
Production Data
13.3 CRYSTAL OSCILLATOR CONNECTIONS
The crystal oscillator generates a 32.768kHz reference clock, which is used to provide reference
clock for the Real Time Clock (RTC) in the WM8326. The oscillator requires an external crystal on the
XTI and XTO pins, as well as two capacitors, connected as shown in Figure 17.
Figure 17 Crystal Oscillator
A suitable crystal oscillator should be selected in accordance with the following requirements:
PARAMETER
MIN
MAX
Nominal frequency
32.768
Series resistance
50
70
Maximum driving level
0.5
Table 18 Selection of Crystal Oscillator Component
UNITS
kHz
k
W
The load capacitors C1 and C2 should be selected according to the recommended load capacitance,
C_L of the crystal, which is given by the following equation:
Assuming C1 = C2 and C_STRAY = 2.75pF (typical pad i/o capacitance), then:
C1 = C2 = 2 x (C_L - 2.75pF).
For example, if the crystal has a load capacitance C_L = 9pF, then C1 = C2 = 12.5pF.
If a suitable 32.768kHz clock is already present elsewhere in the system, it is possible for the
WM8326 to use that external clock instead. The external clock should be applied to pin XTI, and the
XTO pin left floating in this case.
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PD, June 2012, Rev 4.0
48