WM8918 Datasheet, PDF (27/176 Pages) Cirrus Logic – Ultra Low Power DAC for Portable Audio Applications

Production Data
WM8918
DEVICE DESCRIPTION
INTRODUCTION
The WM8918 is a high performance ultra-low power stereo CODEC optimised for portable audio
applications. Flexible analogue interfaces and powerful digital signal processing (DSP) make it ideal
for small portable devices.
The WM8918 supports up to 4 analogue audio inputs. One pair of single-ended or differential
microphone/line inputs is selected as the analogue input source. An integrated bias reference is
provided to power standard electret microphones. The analogue inputs can be mixed into the
headphone or line output signal paths.
A two-channel digital microphone interface is also supported, with direct input to the DSP core. The
digital microphone can be routed to the digital audio interface output and/or mixed into the DAC
output signal path.
One pair of ground-reference Class-W headphone outputs is provided; these are powered from an
integrated Charge Pump, enabling high quality, power efficient headphone playback without any
requirement for DC blocking capacitors. A DC Servo circuit is available for DC offset correction,
thereby suppressing pops and reducing power consumption. Two line outputs are provided; these are
also capable of driving ear speakers and stereo headsets. Ground loop feedback is available on the
headphone outputs and the line outputs, providing rejection of noise on the ground connections. All
outputs use Wolfson SilentSwitch™ technology for pop and click suppression.
The stereo DACs are of hi-fi quality, using a 24-bit low-order oversampling architecture to deliver
optimum performance. A flexible clocking arrangement supports many common audio sample rates,
whilst an integrated ultra-low power FLL provides additional flexibility. A high pass filter is available in
the digital microphone path for suppressing low frequency noise such as mechanical vibration and
wind noise. A digital mixing path provides a digital microphone sidetone of enhanced quality during
voice calls. DAC soft mute and un-mute is available for pop-free music playback.
The integrated Dynamic Range Controller (DRC) and ReTune^TM Mobile 5-band parametric equaliser
(EQ) provide further processing capability of the digital audio paths. The DRC provides compression
and signal level control to improve the handling of unpredictable signal levels. ‘Anti-clip’ and ‘quick
release’ algorithms improve intelligibility in the presence of transients and impulsive noises. The EQ
provides the capability to tailor the audio path according to the frequency characteristics of an
earpiece or loudspeaker, and/or according to user preferences.
The WM8918 has a highly flexible digital audio interface, supporting a number of protocols, including
I2S, DSP, MSB-first left/right justified, and can operate in master or slave modes. PCM operation is
supported in the DSP mode. A-law and -law companding are also supported. Time division
multiplexing (TDM) is available to allow multiple devices to stream data simultaneously on the same
bus, saving space and power.
The system clock SYSCLK provides clocking for the DACs, DSP core, digital audio interface and
other circuits. SYSCLK can be derived directly from the MCLK pin or via an integrated FLL, providing
flexibility to support a wide range of clocking schemes. Typical portable system MCLK frequencies,
and sample rates from 8kHz to 96kHz are all supported. The clocking circuits are configured
automatically from the sample rate (fs) and from the SYSCLK / fs ratio.
The integrated FLL can be used to generate SYSCLK from a wide variety of different reference
sources and frequencies. The FLL can accept a wide range of reference frequencies, which may be
high frequency (e.g. 13MHz) or low frequency (eg. 32.768kHz). The FLL is tolerant of jitter and may
be used to generate a stable SYSCLK from a less stable input signal. The integrated FLL can be
used as a free-running oscillator, enabling autonomous clocking of the Charge Pump and DC Servo if
required.
The WM8918 uses a standard 2-wire control interface, providing full software control of all features,
together with device register readback. An integrated Control Write Sequencer enables automatic
scheduling of control sequences; commonly-used signal configurations may be selected using ready-
programmed sequences, including time-optimised control of the WM8918 pop suppression features.
It is an ideal partner for a wide range of industry standard microprocessors, controllers and DSPs.
Unused circuitry can be disabled under software control, in order to save power; low leakage currents
enable extended standby/off time in portable battery-powered applications.
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PD, Rev 4.1, January 2012
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