LM4549B datasheet(17/28 Pages) NSC

17 / 28 page

Functional Description (Continued)

to the RESET register, 00h. It resets all registers to their

default state and will modify circuit configurations accord-

ingly but does not reset any other internal circuits.

AC Link Serial Interface Protocol

AC LINK OUTPUT FRAME:

SDATA_OUT, CONTROLLER OUTPUT TO LM4549B INPUT

The AC Link Output Frame carries control and PCM data to

the LM4549B 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

LM4549B 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 LM4549B 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 LM4549B 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

LM4549B always samples SDATA_OUT on the next falling

edge. SYNC is sampled with the falling edge of BIT_CLK.

The LM4549B 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 LM4549B expects to receive data MSB first, in an MSB

justified format.

20123504

FIGURE 3. AC Link Bidirectional Audio Frame

20123506

FIGURE 4. AC Link Output Frame

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