WM8731SED_13 Datasheet, PDF (41/65 Pages) Wolfson Microelectronics plc – Portable Internet Audio CODEC with Headphone Driver and Programmable Sample Rates

WM8731 / WM8731L
Production Data
AUDIO DATA SAMPLING RATES
The WM8731/L provides for two modes of operation (normal and USB) to generate the required DAC
and ADC sampling rates. Normal and USB modes are programmed under software control according
to the table below.
In Normal mode, the user controls the sample rate by using an appropriate MCLK or crystal frequency
and the sample rate control register setting. The WM8731/L can support sample rates from 8ks/s up
to 96ks/s.
In USB mode, the user must use a fixed MLCK or crystal frequency of 12MHz to generate sample
rates from 8ks/s to 96ks/s. It is called USB mode since the common USB (Universal Serial Bus) clock
is at 12MHz and the WM8731/L can be directly used within such systems. WM8731/L can generate
all the normal audio sample rates from this one Master Clock frequency, removing the need for
different master clocks or PLL circuits.
Uniquely, the WM8731/L offers the user the ability to sample the ADC and DAC at different rates
under software control in both Normal and USB modes. This reduces the burden on any controlling
DSP. However, the signal processing in the ADC and DAC over-sampling filters is tightly coupled
together in order to minimise power consumption. To this end, only the combinations of sample rates
listed in the following sections are supported. Note that these rates supported are anticipated to be
the likely combinations used in typical audio systems.
REGISTER
ADDRESS
0001000
Sampling
Control
BIT
LABEL
0
USB/
NORMAL
1
BOSR
5:2 SR[3:0]
Table 17 Sample Rate Control
DEFAULT
DESCRIPTION
0
0
0000
Mode Select
1 = USB mode (250/272fs)
0 = Normal mode (256/384fs)
Base Over-Sampling Rate
USB Mode
0 = 250fs
1 = 272fs
Normal Mode
96/88.2kHz
0 = 256fs
0 = 128fs
1 = 384fs
1 = 192fs
ADC and DAC sample rate control;
See USB Mode and Normal Mode
Sample Rate sections for operation
NORMAL MODE SAMPLE RATES
In normal mode MCLK/crystal oscillator is set up according to the desired sample rates of the ADC
and DAC. For ADC or DAC sampling rates of 8, 32, 48 or 96kHz, MCLK frequencies of either
12.288MHz (256fs) or 18.432MHz (384fs) can be used. For ADC or DAC sampling rates of 8, 44.1 or
88.2kHz from MCLK frequencies of either 11.2896MHz (256fs) or 16.9344MHz (384fs) can be used.
Table 18 should be used to set up the device to work with the various sample rate combinations. For
example if the user wishes to use the WM8731/L in normal 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 with a 12.288MHz MCLK or with BOSR = 1, SR3 = 0, SR2 = 0, SR1
= 0 and SR0 = 0 with a 18.432MHz MCLK. The ADC and DAC will then operate with a Digital Filter of
type 1, refer to Digital Filter Characteristics section for an explanation of the different filter types.
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PD, Rev 4.9, October 2012
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