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WM8904ECS/R 数据表(PDF) 81 Page - Wolfson Microelectronics plc |
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WM8904ECS/R 数据表(HTML) 81 Page - Wolfson Microelectronics plc |
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81 / 188 page  Pre-Production WM8904 w PP, Rev 3.3, September 2012 81 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) VMID Control (0) 6 VMID_BUF_ ENA 0 Enable VMID buffer to unused Inputs/Outputs 0 = Disabled 1 = Enabled 2:1 VMID_RES [1:0] 00 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) 0 VMID_ENA 0 Enable VMID master bias current source 0 = Disabled 1 = Enabled R4 (04h) Bias Control (0) 3:2 ISEL [1:0] 10 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. 0 BIAS_ENA 0 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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