WM8351 datasheet(46/330 Pages) WOLFSON

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WM8351

Production Data

PD, April 2012, Rev 4.5

12.1.1

CLOCKING THE AUDIO CODEC

The WM8351 audio CODEC core requires an accurate, low-jitter clock. Clocks for the ADCs, DACs,

DSP core functions, and the digital audio interface are all derived from a common internal clock

source, SYSCLK. This clock may be derived directly from MCLK, or may be generated from an FLL

using MCLK or alternate sources as an external reference. The SYSCLK source is selected by

MCLK_SEL. Many commonly-used audio sample rates can be derived directly from typical MCLK

frequencies.

The ADC and DAC sample rates are independently selectable, relative to SYSCLK, using

ADC_CLKDIV and DAC_CLKDIV. Refer to Section 12.3 for more details

12.1.2

CLOCKING THE DC-DC CONVERTERS

During a system start-up, no external clock signals are available. The WM8351 therefore generates

all internal clocks required for the DC-DC converters, system control and housekeeping functions.

These clocks are derived from the on-chip RC oscillator. The DC-DC converters’ nominal switching

rate is 2.0MHz and 1.0MHz.

12.1.3

INTERNAL RC OSCILLATOR

The internal RC Oscillator generates the system clock 2.0MHz as well as the clock for the DCDC

converters. The period of the generated clock is defined by the time needed for a fixed value

capacitor to be charged up to the reference voltage by a constant current source.

12.2 CRYSTAL OSCILLATOR

The on-chip crystal oscillator generates a 32.768kHz reference clock, which can be used to provide

reference clock for the Real Time Clock (RTC) in the WM8351. It may also be used as a reference

input to the FLL, for the purpose of generating the CODEC clocks. The oscillator is powered from

VRTC, so that it can keep running when no other power source is available. It requires an external

crystal on the X1 and X2 pins, as well as two capacitors and a resistor, connected as shown in Figure

34.

Figure 34 WM8351 Crystal Oscillator

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