13-12 Intel® PXA255 Processor Developer’s Manual
AC’97 Controller Unit
The ACUNIT only supports a 16-bit resolution from the microphone.

13.4.2.8 Slots 7-11: Reserved

Slots 7-11 are reserved for future use. The ACUIN T igno res them.

13.4.2.9 Slot 12: I/O Status

GPIOs which are configured as inputs return their status in Slot 12 of every frame. Only the 16
MSBs are used to return GPIO status. Bit 0 in the LSBs indicates a GPI Input Interrupt event. See
the AC’97 revision 2.0 spec for more i n formation.
The data returned on the latest frame is also accessible to software through the CODEC register at
address 0x54 in the modem CODEC I/O space. Data received in Slot 12 is stored internally in the
ACUNIT. So when software initiates a read of the CODEC register at address 0x54 in the modem
CODEC I/O space, the read data is already inside the ACUNIT.
13.5 AC-link Low Power Mode
Software must set the ACLINK_OFF bit of the GCR to one before the processor enters Sleep
Mode. The ACUNIT then drives SYNC and SDATA_OUT to a logic low level . The ACUNIT
maintains nACRESET high when ACLINK_OFF is set to one.

13.5.1 Powering Down the AC-link

The AC-link signals enter a low power mode after the PR4 bit of the AC’97 CODEC Powerdown
Register (0x26) is set to a 1 (by writing 0x1000). Then, the Primary CODEC drives both BITCLK
and SDATA_IN to a logic low voltage level. The sequence follows the timing diagram shown in
Figure13-7.
Figure 13-7. AC-link Powerdown Timing
SDATA
_
OUT TAG
SYNC
BITCLK
Write to
0x26 Data
PR4
slot 12
prev. frame
TAG
slot 12
prev. frame
SDATA_IN
Note: BITCLK not to scale