WM8721 / WM8721L

Production Data

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PD Rev 4.0 November 2004

16

Recommended values are C1 = 220uF (10V electrolytic), R1 = 47k

C1 forms a DC blocking capacitor to isolate the dc of the HPOUT from the headphones. R1 form a

pull down resistor to discharge C1 to prevent the voltage at the connection to the headphones from

rising to a level that may damage the headphones.

DEVICE OPERATION

DEVICE RESETTING

The WM8721 contains a power on reset circuit that resets the internal state of the device to a known

condition. The power on reset is applied as DCVDD powers on and released only after the voltage

level of DCVDD crosses a minimum turn off threshold. If DCVDD later falls below a minimum turn on

threshold voltage then the power on reset is re-applied. The threshold voltages and associated

hysteresis are shown in the Electrical Characteristics table.

The user also has the ability to reset the device to a known state under software control as shown in

the table below.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001111

Reset Register

8:0

RESET

not reset

Reset Register

Writing 000000000 to register resets

device

Table 5 Software Control of Reset

When using the software reset. In 3-wire mode the reset is applied on the rising edge of CSB and

released on the next rising edge of SCLK. In 2-wire mode the reset is applied for the duration of the

ACK signal (approximately 1 SCLK period, refer to Figure 19).

CLOCKING SCHEMES

In a typical digital audio system there is only one central clock source producing a reference clock to

which all audio data processing is synchronised. This clock is often referred to as the audio system’s

Master Clock. The external master system clock can be applied directly through the MCLK input pin

with no software configuration necessary.

Note that on the WM8721, MCLK is used to derive clocks for the DAC path. The DAC path consists

of DAC sampling clock, DAC digital filter clock and DAC digital audio interface timing. In a system

where there are a number of possible sources for the reference clock it is recommended that the

clock source with the lowest jitter be used to optimise the performance of the DAC.

CORE CLOCK

The WM8721 DSP core can be clocked either by MCLK or MCLK divided by 2. This is controlled by

software as shown in Table 6 below.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001000

Sampling

Control

6

CLKIDIV2

0

Core Clock divider select

1 = Core Clock is MCLK divide by 2

0 = Core Clock is MCLK

Table 6 Software Control of Core Clock

Having a programmable MCLK divider allows the device to be used in applications where higher

frequency master Clocks are available. For example the device can support 512fs master clocks

whilst fundamentally operating in a 256fs mode.

DIGITAL AUDIO INTERFACES

WM8721 may be operated in either one of the 4 offered audio interface modes. These are:

•

Right justified

•

Left justified

•

I

•

DSP mode