WM8912
Production Data
DC SERVO
The WM8912 provides four DC servo circuits, two on the headphone outputs HPOUTL and HPOUTR
and two on the line outputs LINEOUTL and LINEOUTR, to remove DC offset from these ground-
referenced outputs. When enabled, the DC servos ensure that the DC level of these outputs remains
within 1mV of ground. Removal of the DC offset is important because any deviation from GND at the
output pin will cause current to flow through the load under quiescent conditions, resulting in
increased power consumption. Additionally, the presence of DC offsets can result in audible pops
and clicks at power up and power down.
The recommended usage of the DC Servo is initialised by scheduling the default Start-Up sequence
as described in the “Control Write Sequencer” section. The default Start-Up sequence executes a
series of DC offset corrections, after which the measured offset correction is maintained on the
headphone output channels. If a different usage is required, e.g. if a periodic DC offset correction is
required, then the default Start-Up sequence may be modified according to specific requirements.
The relevant control fields are described in the following paragraphs and are defined in Table 31.
DC SERVO ENABLE AND START-UP
The DC Servo circuits are enabled on HPOUTL and HPOUTR by setting DCS_ENA_CHAN_0 and
DCS_ENA_CHAN_1 respectively. Similarly, the DC Servo circuits are enabled on LINEOUTL and
LINEOUTR by setting DCS_ENA_CHAN_2 and DCS_ENA_CHAN_3 respectively When the DC
Servo is enabled, the DC offset correction can be commanded in a number of different ways,
including single-shot and periodically recurring events.
Writing a logic 1 to DCS_TRIG_STARTUP_n initiates a series of DC offset measurements and
applies the necessary correction to the associated output; (‘n’ = 3 for LINEOUTR channel, 2 for
LINEOUTL channel, 1 for HPOUTR channel, 0 for HPOUTL channel). On completion, the output will
be within 1mV of AGND. This is the DC Servo mode selected by the default Start-Up sequence.
Completion of the DC offset correction triggered in this way is indicated by the
DCS_STARTUP_COMPLETE field, as described in Table 31. Typically, this operation takes 86ms
per channel.
Writing a logic 1 to DCS_TRIG_DAC_WR_n causes the DC offset correction to be set to the value
contained in the DCS_DAC_WR_VAL_n fields in Registers R73 to R76. This mode is useful if the
required offset correction has already been determined and stored; it is faster than the
DCS_TRIG_STARTUP_n mode, but relies on the accuracy of the stored settings. Completion of the
DC offset correction triggered in this way is indicated by the DCS_DAC_WR_COMPLETE field, as
described in Table 31. Typically, this operation takes 2ms per channel.
When using either of the DC Servo options above, the status of the DC offset correction process is
indicated by the DCS_CAL_COMPLETE field; this is the logical OR of the
DCS_STARTUP_COMPLETE and DCS_DAC_WR_COMPLETE fields.
The DC Servo control fields associated with start-up operation are described in Table 31. It is
important to note that, to minimise audible pops/clicks, the Start-Up and DAC Write modes of DC
Servo operation should be commanded as part of a control sequence which includes muting and
shorting of the headphone outputs; a suitable sequence is defined in the default Start-Up sequence.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R68 (44h)
7
DCS_TRIG_STAR
TUP_3
0
Writing 1 to this bit selects Start-Up
DC Servo mode for LINEOUTR.
DC Servo 1
In readback, a value of 1 indicates
that the DC Servo Start-Up correction
is in progress.
6
DCS_TRIG_STAR
TUP_2
0
Writing 1 to this bit selects Start-Up
DC Servo mode for LINEOUTL.
In readback, a value of 1 indicates
that the DC Servo Start-Up correction
is in progress.
PD, Rev 4.0, September 2010
48
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
