WM8731 Wolfson Microelectronics Ltd., WM8731 Datasheet - Page 31

• Manufacturer Part Number: WM8731
• Description: WM8731 : Low Power Audio Codec With Integrated Headphone Driver
• Manufacturer: Wolfson Microelectronics Ltd.

WM8731
DEVICE OPERATION
The side tone mode and attenuation is selected under software control using the SIDETONE bit as
shown in Table 10. In true side tone the output from the DAC (DACSEL) and line inputs (BYPASS)
should be deselected from the line output block. However, this can also be used to sum the DAC
output, line inputs and microphone inputs together.
headphone output volume control and mutes are still operational in side tone mode. The maximum
signal at any point in the side tone path must be no greater than 1.0V rms at VDD = 3.3V, to avoid
distortion. This amplitude tracks linearly with AVDD.
DEVICE RESETTING
The WM8731 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.
Table 11 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 32).
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. To allow WM8731 to be used in a centrally clocked system, the WM8731 is capable of
either generating this system clock itself or receiving it from an external source as will be discussed.
For applications where it is desirable that the WM8731 is the system clock source, then clock
generation is achieved through the use of a suitable crystal connected between the XTI/MCLK input
and XTO output pins (see CRYSTAL OSCILLATOR section).
For applications where a component other than the WM8731 will generate the reference clock, the
external system can be applied directly through the XTI/MCLK input pin with no software
configuration necessary. Note that in this situation, the oscillator circuit of the WM8731 can be safely
powered down to conserve power (see POWER DOWN section).
CORE CLOCK
The WM8731 DSP core can be clocked either by MCLK or MCLK divided by 2. This is controlled by
software as shown in Table 12 below.
Table 12 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.
0001111
Reset Register
0001000
Sampling
Control
REGISTER
ADDRESS
REGISTER
ADDRESS
6
BIT
8:0
BIT
CLKIDIV2
RESET
LABEL
LABEL
0
DEFAULT
not reset
DEFAULT
The microphone boost gain control and
Reset Register
Writing 00000000 to register resets
device
Core Clock divider select
1 = Core Clock is MCLK divided by 2
0 = Core Clock is MCLK
DESCRIPTION
DESCRIPTION
AI Rev 2.3 August 2001
Advanced Information
31