WM0011 datasheet(45/233 Pages) CIRRUS

45 / 233 page

WM0011

Production Data

PD, Rev 4.1, August 2013

CRYSTAL OSCILLATOR

The external clock reference connected to the XTI pin may be either a digital logic input, or may be

provided using an external crystal. The typical connection details for an external crystal are illustrated

in Figure 17.

Figure 17 Crystal Oscillator External Components

Selection of the correct external components for the crystal oscillator is important. Recommended

guidelines are provided below. Users should also refer to the crystal component datasheet for

applicable guidelines.

The feedback resistor (Rf) biases the internal inverter in the high gain region. A typical resistance of

1MΩ is recommended.

The damping resistor (Rd) increases stability, and reduces power consumption, suppressing the high

frequency gain. Note that, if this resistance is too large, the loop could fail to oscillate. Some circuits

may omit the Rd resistor altogether.

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 recommended load capacitance CL = 9pF, then C1 = C2 = 12.5pF.

Table 13 shows the recommended load capacitance and maximum ESR values for a range of suitable

WM0011 clocking frequencies.

FREQUENCY

CAPACITANCE (CL)

MAXIMUM ESR

2MHz to 6MHz

20pF

1000Ω

6MHz to 10MHz

16pF

160Ω

10MHz to 20 MHz

12pF

90Ω

20MHz to 30MHz

8pF

40Ω

Table 13 Crystal Selection Guide

WM0011 Datasheet(PDF) 45 Page - Cirrus Logic