Pre-Production
WM8904
POP SUPPRESSION CONTROL
The WM8904 incorporates Wolfson’s SilentSwitch™ technology which enables pops normally
associated with Start-Up, Shutdown or signal path control to be suppressed. To achieve maximum
benefit from these features, careful attention is required to the sequence and timing of these controls.
Note that, under the recommended usage conditions of the WM8904, these features will be
configured by running the default Start-Up and Shutdown sequences as described in the “Control
Write Sequencer” section. In these cases, the user does not need to set these register fields directly.
The Pop Suppression controls relating to the Headphone / Line Output drivers are described in the
“Output Signal Path” section.
DISABLED INPUT CONTROL
The analogue inputs to the WM8904 are biased to VMID in normal operation. In order to avoid
audible pops caused by a disabled signal path dropping to AGND, the WM8904 can maintain these
connections at VMID when the relevant input stage is disabled. This is achieved by connecting a
buffered VMID reference to the input. The buffered VMID reference is enabled by setting
VMID_BUF_ENA; when the buffered VMID reference is enabled, it is connected to any unused input
pins.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R5 (05h)
VMID buffer to unused Inputs/Outputs
0 = Disabled
6
VMID_BUF_ENA
0
VMID
Control 0
1 = Enabled
VMID Buffer Enable
0 = Disabled
0
VMID_ENA
0
1 = Enabled
Table 50 Disabled Line Input Control
PP, Rev 3.3, September 2012
83
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
