WM8281ECSSLASHR datasheet(262/392 Pages) CIRRUS

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WM8281

262

Rev 4.0

CLOCKING AND SAMPLE RATES

The WM8281 requires a clock reference for its internal functions and also for the input (ADC) paths,

output (DAC) paths and digital audio interfaces. Under typical clocking configurations, all commonly-

used audio sample rates can be derived directly from the external reference; for additional flexibility,

the WM8281 incorporates two Frequency Locked Loop (FLL) circuits to perform frequency conversion

and filtering.

External clock signals may be connected via MCLK1 and MCLK2. (These inputs are referenced to the

DBVDD1 power domain.) In AIF Slave modes, the BCLK signals may be used as a reference for the

system clocks. The SLIMbus interface can provide the clock reference, when used as the input to one

of the FLLs. To avoid audible glitches, all clock configurations must be set up before enabling

playback.

SYSTEM CLOCKING

The WM8281 supports two independent clock domains, referenced to the SYSCLK and ASYNCCLK

system clocks respectively.

Up to five different sample rates may be independently selected for specific audio interfaces and other

input/output signal paths. Each selected sample rate must be synchronised either to SYSCLK or to

ASYNCCLK, as described later.

The two system clocks are independent (ie. not synchronised). Stereo full-duplex sample rate

conversion is supported, allowing asynchronous audio data to be mixed and to be routed between

independent interfaces

. See “Digital Core” for further details.

Each subsystem within the WM8281 digital core is clocked at a dynamically-controlled rate, limited by

the SYSCLK (or ASYNCCLK) frequency, as applicable. For maximum signal mixing and processing

capacity, it is recommended that the highest possible SYSCLK and ASYNCCLK frequencies are

configured.

SAMPLE RATE CONTROL

The WM8281 supports two independent clock domains, referenced to SYSCLK and ASYNCCLK

respectively.

Different sample rates may be selected for each of the audio interfaces (AIF1, AIF2, AIF3, SLIMbus),

and for the input (ADC) and output (DAC) paths. Each of these must be referenced either to SYSCLK

or to ASYNCCLK. (Note that the SLIMbus interface supports multiple sample rates, selected

independently for each input or output channel.)

The WM8281 can support a maximum of five different sample rates at any time. The supported

sample rates range from 4kHz to 192kHz.

Up to three different sample rates can be selected using the SAMPLE_RATE_1, SAMPLE_RATE_2

and SAMPLE_RATE_3 registers. These must each be numerically related to each other and to the

SYSCLK frequency (further details of these requirements are provided in Table 99 and the

accompanying text).

The remaining two sample rates can be selected using the ASYNC_SAMPLE_RATE_1 and

ASYNC_SAMPLE_RATE_2 registers. These sample rates must be numerically related to each other

and to the ASYNCCLK frequency (further details of these requirements are provided in Table 100 and

the accompanying text),

Each of the audio interfaces, input paths and output paths is associated with one of the sample rates

selected by the SAMPLE_RATE_n or ASYNC_SAMPLE_RATE_n registers.

Note that if any two interfaces are operating at the same sample rate, but are not synchronised, then

one of these must be referenced to the ASYNCCLK domain, and the other to the SYSCLK domain.

Note that, when any of the SAMPLE_RATE_n or ASYNC_SAMPLE_RATE_n registers is written to,

the activation of the new setting is automatically synchronised by the WM8281 to ensure continuity of

all active signal paths. The SAMPLE_RATE_n_STS and ASYNC_SAMPLE_RATE_n_STS registers

provide readback of the sample rate selections that have been implemented.

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