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LM4549A Datasheet(PDF) 19 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) 19 Page - National Semiconductor (TI)

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AC Link Serial Interface Protocol
SDATA_OUT: Slots 5 to 12 – Reserved
These slots are not used by the LM4549A and should all be
stuffed with zeros by the AC ’97 Controller.
The AC Link Input Frame contains status and PCM data from
the LM4549A control registers and stereo ADC. Input
Frames are carried on the SDATA_IN signal which is an
input to the AC ’97 Digital Audio Controller and an output
from the LM4549A codec. As shown in Figure 3, Input
Frames are constructed from thirteen time slots: one Tag
Slot followed by twelve Data Slots. The Tag Slot, Slot 0,
contains 16 bits of which 5 are used by the LM4549A. One is
used to indicate that the AC Link interface is fully operational
and the other 4 to indicate the validity of the data in the four
of the twelve following Data Slots that are used by the
LM4549A. Each Frame consists of 256 bits with each of the
twelve data slots containing 20 bits.
A new Input 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 6 and
Figure 7, the first tag bit in the Frame (“Codec Ready”) is
clocked from the LM4549A by the next rising edge of BIT-
_CLK. The LM4549A always clocks data to SDATA_IN on a
rising edge of BIT_CLK and the controller is expected to
sample SDATA_IN on the next falling edge. The LM4549A
samples SYNC on the rising edge of BIT_CLK.
Input and Output Frames are aligned to the same SYNC
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 an old Frame
then the new Frame is ignored i.e. no valid data is sent on
SDATA_IN until a valid new Frame is detected.
The LM4549A transmits data MSB first, in a MSB justified
format. All reserved bits and slots are stuffed with "0"s by the
SDATA_IN: Slot 0 – Codec/Slot Status Bits
The first bit (bit 15, “Codec Ready”) of slot 0 in the AC Link
Input Frame indicates when the codec’s AC Link digital
interface and its status/control registers are fully operational.
The digital controller is then able to read the LSBs from the
Powerdown Control/Stat register (26h) to determine the sta-
tus of the four main analog subsections. It is important to
check the status of these subsections after Initialization,
Cold Reset or the use of the powerdown modes in order to
minimize the risk of distorting analog signals passed before
the subsections are ready.
The 4 bits 14, 13, 12 and 11 indicate that the data in slots 1,
2, 3 and 4, respectively, are valid.
FIGURE 6. AC Link Input Frame
FIGURE 7. Start of AC Link Input Frame

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