WM8731 datasheet(40/56 Pages) WOLFSON

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WM8731

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 February 2001

USB MODE SAMPLE RATES

In USB mode the MCLK/crystal oscillator input is 12MHz only.

SAMPLING

RATE

ADC

DAC

MCLK

FREQUENCY

SAMPLE

RATE

DIGITAL

FILTER

TYPE

kHz

MHz

BOSR

SR3

SR2

SR1

SR0

12.000

44.1

(Note 2)

44.1

(Note 2)

12.000

44.1

(Note 2)

(Note 1)

12.000

(Note 1)

44.1

(Note 2)

12.000

(Note 1)

12.000

88.2

(Note 3)

88.2

(Note 3)

12.000

Table 20 USB Mode Sample Rate Look-up Table

Notes:

8k not exact, actual = 8.021kHz

44.1k not exact, actual = 44.118kHz

88.1k not exact, actual = 88.235kHz

All other combinations of BOSR and SR[3:0] that are not in the truth table are invalid

The table above can be used to set up the device to work with various sample rate combinations. For

example if the user wishes to use the WM8731 in USB mode with the ADC and DAC sample rates at

48kHz and 48kHz respectively then the device should be programmed with BOSR = 0, SR3 = 0, SR2

= 0, SR1 = 0 and SR0 = 0. The ADC and DAC will then operate with a Digital Filter of type 0, refer to

Digital Filter Characteristics section for an explanation of the different filter types.

The BOSR bit represents the base over-sampling rate. This is the rate that the WM8731 digital signal

processing is carried out at and the sampling rate will always be a sub-multiple of this. In USB mode,

with BOSR = 0, the base over-sampling rate is defined at 250fs, with BOSR = 1, the base over-

sampling rate is defined at 272fs. This can be used to determine the actual audio sampling rate

produced by the ADC and required by the DAC.

Example scenarios are, :-

with a requirement that the ADC data sampling rate is 8kHz and DAC data sampling rate is

48kHz the device is programmed with BOSR = 0 (250fs), SR3 = 0, SR2 = 0, SR1 = 1, SR0 =

0.The ADC will then be exactly 8kHz ( derived from 12MHz/250 x 1/6 ) and the DAC expects

data at exactly 48kHz ( derived from 12MHz/250 ).

with a requirement that ADC data rate is 8kHz and DAC data rate is 44.1kHz the device is

programmed with BOSR = 0 (272fs), SR3 = 0, SR2 = 0, SR1 = 1, SR0 = 0. The ADC will not

output data at exactly 8kHz, instead it will be 8.021kHz ( derived from 12MHz/272 x 2/11 ) and

the DAC at 44.118kHz ( derived from 12MHz/272 ). A slight (sub 0.5%) pitch shift will therefore

results in the 8kHz and 44.1kHz audio data and (more importantly) the user must ensure that

the data across the digital interface is correctly synchronised at the 8.021kHz and 44.117kHz

rates.

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