Pre-Production
WM8904
REFERENCE VOLTAGES AND MASTER BIAS
This section describes the analogue reference voltage and bias current controls. Note that, under the
recommended usage conditions of the WM8904, these features will be configured by scheduling 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.
ANALOGUE REFERENCE AND MASTER BIAS
The analogue circuits in the WM8904 require a mid-rail analogue reference voltage, VMID. This
reference is generated from AVDD via a programmable resistor chain.
VMID is enabled by setting the VMID_ENA register bit. The programmable resistor chain is
configured by VMID_RES [1:0], and can be used to optimise the reference for normal operation, low
power standby or for fast start-up as described in Table 47. For normal operation, the VMID_RES
field should be set to 01.
The VMID_BUF_ENA bit allows the buffered VMID reference to be connected to unused
inputs/outputs.
The analogue circuits in the WM8904 require a bias current. The bias current is enabled by setting
BIAS_ENA. Note that the bias current source requires VMID to be enabled also.
The Bias current is controlled using the ISEL register field. Note that the ISEL register should only be
changed as part of the ‘Low Power Mode Enable’ sequence described in Table 48. In all other cases,
it is recommended that the ISEL register is not changed from the default setting.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R5 (05h)
Enable VMID buffer to unused Inputs/Outputs
0 = Disabled
6
VMID_BUF_
ENA
0
VMID
Control (0)
1 = Enabled
VMID Divider Enable and Select
00 = VMID disabled (for OFF mode)
01 = 2 x 50k divider (for normal operation)
10 = 2 x 250k divider (for low power standby)
11 = 2 x 5k divider (for fast start-up)
Enable VMID master bias current source
0 = Disabled
2:1
VMID_RES
[1:0]
00
0
VMID_ENA
ISEL [1:0]
0
1 = Enabled
R4 (04h)
Master Bias Control
3:2
10
Bias Control
(0)
00 = Low power bias
01 = Reserved
10 = High performance bias (default)
11 = Reserved
Note that the ISEL register should only be
changed as part of the Low Power Mode
Enable/Disable sequences.
Enables the Normal bias current generator
(for all analogue functions)
0
BIAS_ENA
0
0 = Disabled
1 = Enabled
Table 47 Reference Voltages and Master Bias Enable
PP, Rev 3.3, September 2012
81
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
