WM8904ECSSLASHR datasheet(81/188 Pages) WOLFSON

81 / 188 page

Pre-Production

WM8904

PP, Rev 3.3, September 2012

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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R5 (05h)

VMID

Control (0)

VMID_BUF_

ENA

Enable VMID buffer to unused Inputs/Outputs

0 = Disabled

1 = Enabled

2:1

VMID_RES

[1:0]

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)

VMID_ENA

Enable VMID master bias current source

0 = Disabled

1 = Enabled

R4 (04h)

Bias Control

(0)

3:2

ISEL [1:0]

Master Bias Control

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.

BIAS_ENA

Enables the Normal bias current generator

(for all analogue functions)

0 = Disabled

1 = Enabled

Table 47 Reference Voltages and Master Bias Enable

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