wm9707scft-v Wolfson Microelectronics plc, wm9707scft-v Datasheet - Page 19

• Manufacturer Part Number: wm9707scft-v
• Description: Ac?97 Revision 2.1 Audio Codec With Spdif Output
• Manufacturer: Wolfson Microelectronics plc

WM9707
AC-LINK AUDIO OUTPUT FRAME (SDATAOUT)
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The audio output frame data streams correspond to the multiplexed bundles of all digital output data
targeting the WM9707’s DAC inputs, and control registers. As briefly mentioned earlier, each audio
output frame supports up to 12 20-bit outgoing data time slots. Slot 0 is a special reserved time slot
containing 16-bits, which are used for AC-link protocol infrastructure.
OUTPUT TAG SLOT (16-BITS)
Within slot 0 the first bit is a global bit (SDATAOUT slot 0, bit 15) which flags the validity for the entire
audio frame. If the Valid Frame bit is a 1, this indicates that the current audio frame contains at least
one time slot of valid data. The next 12-bit positions sampled by the WM9707 indicate which of the
corresponding 12 time slots contain valid data.
In this way data streams of differing sample rates can be transmitted across AC-link at its fixed
48kHz audio frame rate. Figure 12 illustrates the time slot based AC-link protocol.
When the CODEC is a slave device, bits 14 and 13 are not used to validate data in slots 1 and 2.
Instead, if the message ID bits (1:0) match the CODEC ID then the address is valid and bit 19 from
slot 1 then indicates if slot 2 is valid.
Figure 13 Start of an Audio Output Frame
A new audio output frame begins with a low to high transition of SYNC as shown in Figure 13. SYNC
is synchronous to the rising edge of BITCLK. On the immediately following falling edge of BITCLK,
the WM9707 samples the assertion of SYNC. This falling edge marks the time when both sides of
AC-link are aware of the start of a new audio frame. On the next rising edge of BITCLK, AC’97
transitions SDATAOUT into the first bit position of slot 0 (Valid Frame bit). Each new bit position is
presented to AC-link on a rising edge of BITCLK, and subsequently sampled by the WM9707 on the
following falling edge of BITCLK. This sequence ensures that data transitions and subsequent
sample points for both incoming and outgoing data streams are time aligned.
Baseline AC’97 specified audio functionality MUST ALWAYS sample rate convert to and from a fixed
48ks/s on the AC’97 controller. This requirement is necessary to ensure that interoperability between
the AC’97 controller and the WM9707, among other things, can be guaranteed by definition for
baseline specified AC’97 features.
SDATAOUT’s composite stream is MSB justified (MSB first) with all non-valid slot bit positions
stuffed with 0s by the AC’97 controller.
In the event that there are less than 20 valid bits within an assigned and valid time slot, the AC’97
controller always stuffs all trailing non-valid bit positions of the 20-bit slot with 0s.
Bit (15)
Bit (14)
Bit (13)
Bit (12:3)
Bit 2
Bit (1:0)
SDATAOUT
BITCLK
SYNC
END OF PREVIOUS AUDIO FRAME
Frame Valid
Slot 1 Valid Command Address bit
Slot 2 Valid Command Data bit
Slot 3-12 Valid bits as defined by AC’97
Reserved
2-bit Message ID field
WM9707 SAMPLES
SYNC ASSERTION HERE
WM9707 SAMPLES
FIRST SDATAOUT
BIT OF FRAME HERE
FRAME
VALID
SLOT (1)
SLOT (2)
(Primary CODEC only)
(Set to 0)
(00 reserved for Primary; 01
indicates Secondary)
(Primary CODEC only)
PD, Rev 4.4, March 2009
Production Data
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