Pre-Production
WM8850
A low-noise microphone bias generator is provided as part of Port-B, which is suitable for use as a
bias supply for analogue microphones. The control of the microphone bias level is via the Pin Widget
Control Verb.
Port-B supports impedance sensing when using the microphone bias generator; this can be read
directly from the Pin Sense Verb. Port-B also supports presence detect, using the Pin Sense Verb.
If nothing is plugged into Port-B it is possible to use an automute function which will automatically
replace the samples in the stream with zeros, thus avoiding any noise being transmitted when there is
no input device connected. This function can be enabled using the vendor-specific Auto-Mute Control
Verb.
SET VERB
BIT
BITFIELD
NAME
DEFAULT
DESCRIPTION
7B0h
0
PD_
AUTOMUTE
0h
Auto-Mute Control:
0 = Automute disabled
1 = Automute enabled
Note: When PD_AUTOMUTE=1 and
Presence Detect (bit 31 in verb F09h)=1 the
WM8850 will replace the samples in the
stream with zeros
PGA2 (NID = 0Bh)
Table 12 gives a summary of the PGA2 node:
NODE SUMMARY INFORMATION
0Ch
NID
Widget Type
Audio Selector
Supported Get Verbs
Supported Set Verbs
Unsolicited Responses
Vendor-Specific Verbs
Bh, F00h, F02h, FB2h
3h, 7B2h, 7B3h
None
FB2h, 7B2h, 7B3h : PGA Control Verb
Table 12 PGA2 Node Summary Information
PGA2 provides analogue gain for the inputs to Port-B, allowing the signal level to be adjusted by up to
+/-12dB in 0.5dB steps. Gain can be applied to left and right channels independently if required.
Additionally, a mute function is available – again this can be applied to separate channels as required.
The gain of PGA2 is controlled using the Amplifier Gain/Mute Verb.
PGA2 uses an internal zero cross detect circuit to ensure that all gain changes occur while the signal
passes through the zero point. This function eliminates any potential DC steps that can occur if gain
changes are applied at other times, and therefore the potential for zipper noise is removed. If no zero
cross occurs within a specified time after the gain change is requested via a register write a timeout
period will elapse and the gain will be changed regardless. This timeout period can be changed using
the vendor-specific PGA Control Verb:
PP, April 2011, Rev 3.2
45
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
