CS8420 datasheet(76/80 Pages) CIRRUS

76 / 80 page

CS8420

DS245PP2

becoming completely full, allowing incoming data

to overwrite data that has not yet been output

through the AES3 transmitter.

16.2.4

Mode (3): Reserved

This mode has been removed. Use IEC Consumer

mode B.

16.2.5

Mode (4): IEC Consumer B

In this mode, the partitioning problem is solved by

buffering an entire message before starting to trans-

mit it. In this scheme, zero-segments between mes-

sages will be expanded when Fso > Fsi, but the

integrity of individual messages is preserved.

The overwriting problem (when Fso < Fsi) is

solved by only storing a portion of the input U data

in the FIFO. Specifically, only the IUs themselves

are stored (and not the zeroes that provide inter-IU

and inter-message “filler”). An inter-IU filler seg-

ment of fixed length (OF) will be added back to the

messages at the FIFO output, where the length of

OF is equal to the shortest observed input filler seg-

ment (IF).

Storing only IUs (and not filler) within the FIFO

makes it possible for the slower AES3 transmitter

to “catch up” to the faster AES3 receiver as data is

read out of the FIFO. This is because nothing is

written into the FIFO when long strings of zeroes

are input to the AES-EBU receiver. During this

time of no writing, the transmitter can read out data

that had previously accumulated, allowing the

FIFO to empty out. If the FIFO becomes complete-

ly empty, zeroes are transmitted until a complete

message is written into the FIFO.

Mode 4 is not fail-safe; the FIFO can still get com-

pletely full if there isn't enough “zero-padding” be-

tween incoming messages. It is up to the user to

provide proper padding, as defined below:

Minimum padding

= (Fsi/Fso - 1)*[8N + (N-1)*IF +9] + 9

where N is the number of IUs in the message, IF is

the number of filler bits between each IU, and Fso

≤ Fsi.

Example 1: Fsi/Fso = 2, N=4, IF=1: minimum

proper padding is 53 bits.

Example 2: Fsi/Fso = 1, N=4, IF=7: min proper

padding is 9 bits.

The CS8420 detects when an overwrite has oc-

curred in the FIFO, and synchronously resets the

entire FIFO structure to prevent corrupted U data

from being merged into the transmitted AES3 data

stream. The CS8420 can be configured to generate

an interrupt when this occurs.

Mode 4 is recommended for properly formatted U

data where mode 3 cannot provide acceptable per-

formance, either because of a too-extreme Fsi/Fso

ratio, or because it's unacceptable to change the

lengths of filler segments. Mode 4 provides error-

free performance over the complete range of

Fsi/Fso ratios (provided that the input messages are

properly zero-padded for Fsi > Fso).

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