WM8321
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 WM8321. The oscillator requires an external crystal on the
XTI and XTO pins, as well as two capacitors, connected as shown in Figure 17.
32kHz
WM8321
Crystal Oscillator
XOSCGND
Crystal Model
XTI
C1
XTO
C2
Cp
Lxt
Rxt
Cxt
Figure 17 Crystal Oscillator
A suitable crystal oscillator should be selected in accordance with the following requirements:
PARAMETER
Nominal frequency
MIN
MAX
UNITS
kHz
k
32.768
Series resistance
50
70
Maximum driving level
0.5
W
Table 18 Selection of Crystal Oscillator Component
The load capacitors C1 and C2 should be selected according to the recommended load capacitance,
CL of the crystal, which is given by the following equation:
Assuming C1 = C2 and CSTRAY = 2.75pF (typical pad i/o capacitance), then:
C1 = C2 = 2 x (CL - 2.75pF).
For example, if the crystal has a load capacitance CL = 9pF, then C1 = C2 = 12.5pF.
If a suitable 32.768kHz clock is already present elsewhere in the system, it is possible for the
WM8321 to use that external clock instead. The external clock should be applied to pin XTI, and the
XTO pin left floating in this case.
PD, February 2012, Rev 4.0
48
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
