CS8420 datasheet(75/80 Pages) CIRRUS

75 / 80 page

CS8420

DS245PP2

16.2

AES3 User (U) Bit Management

The CS8420 U bit manager has four operating

modes:

Mode 1. Transmit all zeros.

Mode 2. Block mode.

Mode 3. Reserved

Mode 4. IEC Consumer B.

16.2.1

Mode 1: Transmit All Zeros

Mode 1 causes only zeros to be transmitted in the

output U data, regardless of E buffer contents or U

data embedded in an input AES3 data stream. This

mode is intended for the user who does not want to

transceive U data, and simply wants the output U

channel to contain no data.

16.2.2

Mode 2: Block Mode

Mode 2 is very similar to the scheme used to con-

trol the C bits. Entire blocks of U data are buffered

from input to output, using a cascade of 3 block-

sized RAMs to perform the buffering. The user has

access to the second of these 3 buffers, denoted the

E buffer, via the control port. Block mode is de-

signed for use in AES3 in, AES3 out situations in

which input U data is decoded using a microcon-

troller via the control port. It is also the only mode

in which the user can merge his own U data into the

transmitted AES3 data stream.

The U buffer access only operates in two byte

mode, since there is no concept of A and B blocks

for user data. The arrangement of the data in the

each byte is that the MSB is the first received bit

and is the first transmitted bit. The first byte read is

the first byte received, and the first byte sent is the

first byte transmitted.

16.2.3

IEC60958 Recommended U Data

Format For Consumer Applications

Modes (3) and (4) are intended for use in AES3 in,

AES3 out situations, in which the input U data is

formatted as recommended in the “IEC60958 Dig-

ital Audio Interface, part 3: Consumer applica-

tions” document.

In this format, “messages” are formed in the U data

from Information Units or IUs. An IU is 8 bits long,

and the MSB is always 1, and is called the start bit,

or 'P' bit. The remaining 7 bits are called Q R S T U

V & W, and carry the desired data.

A “message” consists of 3 to 129 IUs. Multiple IUs

are considered to be in the same message if they are

separated by zero to eight 0s, denoted here as filler.

A filler sequence of nine or more 0s indicates an in-

ter-message gap. The desired information is nor-

mally carried in the sequence of corresponding bits

in the IUs. For example, the sequential Q bits from

each IU make up the Q sub-code data that is used to

indicate Compact Disk track information. This data

is automatically extracted from the received

IEC60958 stream, and is presented in the control

port register map space.

Where incoming U data is coded in the above for-

mat, and needs to be re-transmitted, the data trans-

fer cannot be done using shift registers, because of

the different Fsi and Fso sampling clocks. Instead,

input data must be buffered in a FIFO structure, and

then read out by the AES3 transmitter at appropri-

ate times.

Each bit of each IU must be transceived; unlike the

audio samples, there can be no sample rate conver-

sion of the U data. Therefore, there are 2 potential

problems:

(1) Message Partitioning

When Fso > Fsi, more data is transmitted than re-

ceived per unit time. The FIFO will frequently be

completely emptied. Sensible behavior must occur

when the FIFO is empty, otherwise, a single incom-

ing message may be erroneously be partitioned into

multiple, smaller, messages.

(2) Overwriting

When Fso < Fsi, more data is received than trans-

mitted per unit time. There is a danger of the FIFO

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