SigmaTel, Inc.
Data Sheet
STAC9721
4.4.1 Reset Register (Index 00h)
Writing any value to this register performs a register reset, which causes all registers to
revert to their default values. This register returns the AC’97 ID code of the part when
read.
4.4.2 Play Master Volume Registers (Index 02h, 04h, and 06h)
These registers manage the output signal volumes. Register 02h controls the stereo
master volume (both right and left channels), register 04h controls the optional stereo true
line level out, and register 06h controls the mono volume output. Each step corresponds
to 1.5 dB. The MSB of the register is the mute bit. When this bit is set to 1 the level for
that channel is set at -∞ dB. ML5 through ML0 is for left channel level, MR5 through
MR0 is for the right channel and MM5 through MM0 is for the mono out channel. If
optional bits D13 and D5 of register 02h and 04h, or D5 of register 06h are set to 1, then
the corresponding attenuation is set to 46dB and the register reads will produce 1Fh as a
value for this attenuation/gain block
. Play Master Volume Register
Table 8
Mute
Mx5…Mx0
Function
Range
0
0
1
00 0000
01 1111
xx xxxx
0dB Attenuation
46.5 Attenuation
Req.
Req.
Req.
∞ dB
Attenuation
4.4.3 PC Beep Register (Index 0Ah)
This register controls the level for the PC Beep input. Each step corresponds to
approximately 3 dB of attenuation. The MSB of the register is the mute bit. When this
bit is set to 1 the level for that channel is set at -∞ dB. PC_BEEP supports motherboard
implementations. The intention of routing PC_BEEP through the STAC9721/23 mixer is
to eliminate the requirement for an onboard speaker by guaranteeing a connection to
speakers connected via the output jack. In order for this to be viable the PC_BEEP signal
needs to reach the output jack at all times. NOTE: the PC_BEEP is routed to L & R
Line outputs even when the STAC9721/23 is in a RESET state. This is so that Power On
Self Test (POST) codes can be heard by the user in case of a hardware problem with the
PC. For further PC_BEEP implementation details please refer to the AC'97 Technical
FAQ sheet. The default value of 0000h corresponds to 0 dB attenuation with mute off.
. PC_BEEP Register
Table 9
Mute
PV3…PV0
Function
0
0
1
0000
1111
xxxx
0 dB Attenuation
45 dB Attenuation
∞ dB Attenuation
22
04/07/00
04/07/00
ETC [ ETC ]
