Channel Status Data E Buffer Access; Aes3 User (U) Bit Management; Mode 1: Transmit All Zeros; Mode 2: Block Mode - Cirrus Logic Crystal CS8427 Series Preliminary Product Information

14.1.4 Channel Status Data E Buffer
Access
The E buffer is organized as 24 x 16-bit words. For
each word the MS Byte is the A channel data, and
the LS Byte is the B channel data (see Figure 29).
There are two methods of accessing this memory,
known as one byte mode and two byte mode. The
desired mode is selected via a control register bit.
One Byte mode
In many applications, the channel status blocks for
the A and B channels will be identical. In this situ-
ation, if the user reads a byte from one of the chan-
nel's blocks, the corresponding byte for the other
channel will be the same. Similarly, if the user
wrote a byte to one channel's block, it would be
necessary to write the same byte to the other block.
One byte mode takes advantage of the often identi-
cal nature of A and B channel status data.
When reading data in one byte mode, a single byte
is returned, which can be from channel A or B data,
depending on a register control bit. If a write is be-
ing done, the CS8427 expects a single byte to be in-
put to its control port. This byte will be written to
both the A and B locations in the addressed word.
One byte mode saves the user substantial control
port access time, as it effectively accesses 2 bytes
worth of information in 1 byte's worth of access
time. If the control port's autoincrement addressing
is used in combination with this mode, multi-byte
accesses such as full-block reads or writes can be
done especially efficiently.
Two Byte mode
There are those applications in which the A and B
channel status blocks will not be the same, and the
user is interested in accessing both blocks. In these
situations, two byte mode should be used to access
the E buffer.
DS477PP1
In this mode, a read will cause the CS8427 to out-
put two bytes from its control port. The first byte
out will represent the A channel status data, and the
2nd byte will represent the B channel status data.
Writing is similar, in that two bytes must now be
input to the CS8427's control port. The A channel
status data is first, B channel status data second.

14.2 AES3 User (U) Bit Management

The CS8427 U bit manager has two operating
modes: transmit all zeros and block mode.

14.2.1 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.

14.2.2 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.
CS8427
53