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LM4549A Datasheet(PDF) 17 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM4549A
Description  AC ’97 Rev 2.1 Codec with Sample Rate Conversion and National 3D Sound
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com

LM4549A Datasheet(HTML) 17 Page - National Semiconductor (TI)

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AC Link Serial Interface Protocol
The AC Link Output Frame carries control and PCM data to
the LM4549A control registers and stereo DAC. Output
Frames are carried on the SDATA_OUT signal which is an
output from the AC ’97 Digital Controller and an input to the
LM4549A codec. As shown in Figure 3, Output Frames are
constructed from thirteen time slots: one Tag Slot followed by
twelve Data Slots. Each Frame consists of 256 bits with each
of the twelve Data Slots containing 20 bits. Input and Output
Frames are aligned to the same SYNC transition. Note that
since the LM4549A is a two channel codec, it only accepts
data in 4 of the twelve Data Slots – 2 for control, one each for
PCM data to the left and right channel DACs. Data Slot 3 &
4 are used to stream data to the stereo DAC for all modes
selected by the Identity pins ID1#, ID0#.
A new Output Frame is signaled with a low-to-high transition
of SYNC. SYNC should be clocked from the controller on a
rising edge of BIT_CLK and, as shown in Figure 4 and
Figure 5, the first tag bit in the Frame (“Valid Frame”) should
be clocked from the controller by the next rising edge of
BIT_CLK and sampled by the LM4549A on the following
falling edge. The AC ’97 Controller should always clock data
to SDATA_OUT on a rising edge of BIT_CLK and the
LM4549A always samples SDATA_OUT on the next falling
edge. SYNC is sampled with the rising edge of BIT_CLK.
The LM4549A checks each Frame to ensure 256 bits are
received. If a new Frame is detected (a low-to-high transition
on SYNC) before 256 bits are received from the old Frame
then the new Frame is ignored i.e. the data on SDATA_OUT
is discarded until a valid new Frame is detected.
The LM4549A expects to receive data MSB first, in an MSB
justified format.
SDATA_OUT: Slot 0 – Tag Phase
The first bit of Slot 0 is designated the "Valid Frame" bit. If
this bit is 1, it indicates that the current Output Frame con-
tains at least one slot of valid data and the LM4549A will
check further tag bits for valid data in the expected Data
Slots. With the codec in Primary mode, a controller will
indicate valid data in a slot by setting the associated tag bit
equal to 1. Since it is a two channel codec the LM4549A can
only receive data from four slots in a given frame and so only
FIGURE 3. AC Link Bidirectional Audio Frame
FIGURE 4. AC Link Output Frame

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