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WM8581
w
PP Rev 1.0 March 2006
69
Figure 34
and Figure 35 show typical power up scenarios in a real system. Both AVDD and DVDD
must be established, and VMID must have reached the threshold Vporr before the device is ready
and can be written to. Any writes to the device before Device Ready will be ignored.
Figure 34 shows DVDD powering up before AVDD. Figure 35 shows AVDD powering up before
DVDD. In both cases, the time from applying power to Device Ready is dominated by the charge time
of VMID.
A 4.7µF capacitor (minimum) is recommended for decoupling on VMID. The charge time for VMID
will dominate the time required for the device to become ready after power is applied. The time
required for VMID to reach the threshold is a function of the VMID resistor string and the decoupling
capacitor. To reduce transient audio effects during power on, the stereo DACs on the WM8581 have
their outputs clamped to VMID at power-on. This increases the capacitive loading of the VMID
resistor string, as the DAC output AC coupling capacitors must be charged to VMID, and hence the
required charge time. To ensure minimum device startup time, the VMIDSEL bit is set by default,
thus reducing the impedance of the resistor string. If required, the VMID string can be restored to a
high impedance state to save power once the device is ready.
REGISTER ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R29
ADC CONTROL 1
1Dh
8 VMIDSEL
1
VMID
Impedance
Selection
0 = High impedance, power
saving
1 = Low impedance, fast power-
on
DEVICE ID READBACK
Reading from registers R0, R1 and R2 returns the device ID and revision number. R0 returns 80h, R1
returns 85h, R2 returns the device revision number. Device ID readback is not possible in continuous
readback mode (CONTREAD=1).