SigmaTel, Inc.
Data Sheet
STAC9721
18
04/07/00
04/07/00
3.2 AC-Link Low Power Mode
The STAC9721/23 AC-Link can be placed in the low power mode by programming register 26h to the
appropriate value. Both BIT_CLK and SDATA_IN will be brought to, and held at a logic low voltage
level. The AC'97 controller can wake up the STAC9721/23 by providing the appropriate reset signals.
Figure 9. STAC9721/23 Powerdown Timing
SYNC
BIT_CLK
SDATA_OUT
per frame
SDATA_IN
slot2
TAG
Write to
Data
slot2
TAG
per frame
0x20
PR4
Note: BIT_CLK not to scale
BIT_CLK and SDATA_IN are transitioned low immediately (within the maximum specified time)
following the decode of the write to the Powerdown Register (26h) with PR4. When the AC'97 controller
driver is at the point where it is ready to program the AC-Link into its low power mode, slots (1 and 2)
are assumed to be the only valid stream in the audio output frame (all sources of audio input have been
neutralized).
The AC'97 controller should also drive SYNC and SDATA_OUT low after programming the
STAC9721/23 to this low power mode.
3.2.1 Waking up the AC-Link
Once the STAC9721/23 has halted BIT_CLK, there are only two ways to “wake up” the
AC-Link. Both methods must be activated by the AC'97 controller.
The AC-Link protocol provides for a “Cold AC'97 Reset”, and a “Warm AC'97 Reset”.
The current power down state would ultimately dictate which form of reset is appropriate.
Unless a “cold” or “register” reset (a write to the Reset register) is performed, wherein
the AC'97 registers are initialized to their default values, registers are required to keep
state during all power down modes. Once powered down, re-activation of the AC-Link
via re-assertion of the SYNC signal must not occur for a minimum of 4 audio frame times
following the frame in which the power down was triggered. When AC-Link powers up
it indicates readiness via the Codec Ready bit (input slot 0, bit 15).
Cold Reset - a cold reset is achieved by asserting RESET# for the minimum specified
time. By driving RESET# low, BIT_CLK, and SDATA_IN will be activated, or re-
activated as the case may be, and all STAC9721/23 control registers will be initialized to
their default power on reset values.
Note: RESET# is an asynchronous input.
# denotes active low
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