CX28344 Conexant, CX28344 Datasheet - Page 99

• Manufacturer Part Number: CX28344
• Description: (CX2834x) Dual / Triple / Quad / Hex / Octal - Enhanced DS3 / E3 Framer
• Manufacturer: Conexant

CX28342/3/4/6/8 Data Sheet
2.2.7.3
28348-DSH-001-B
Near-Full FIFO Event
A Near-Full FIFO event occurs when the number of bytes in the FIFO buffer is equal
to or more than the Near-Full FIFO threshold and the FIFO near-full interrupt is off
(the byte whose writing leads to this event is referred to as the threshold byte). This
results in an interrupt with the indication FIFO near-full. A type (a2) block is marked
in the block type field, block length is set to the number of data octets stored and a
new block is started on the byte following the last data byte stored.
If the first octet to arrive after the threshold byte is a flag sequence (i.e., the threshold
byte was the last byte of the FCS), the new block is a type (a1) block with a length of
0 or a type (b) block. If the first octet to arrive after the threshold byte is an abort
sequence, the new block is a type (b) block. Otherwise, the new block contains the
upcoming bytes of the same message.
If the last octet to arrive before the threshold byte is a flag or abort sequence (i.e., the
threshold byte is the status byte of an upcoming message), the type (a2) block has a
length of 0, and the new block contains the actual data of the new message; i.e., the
system receives a message received interrupt (on the byte before the threshold byte)
and shortly afterwards a FIFO near-full interrupt (on the threshold byte).
FIFO Overrun
If the FIFO buffer is full when another byte has to be written to it and the FIFO
overrun interrupt is off, an overrun error occurs, resulting in an interrupt with the
indication FIFO overrun. If the new byte is a data byte, it is lost (i.e., data bytes
already present in the FIFO buffer are not overwritten), and the data block is
immediately terminated and marked as a (b) block. If the new byte is a status byte, its
writing to the FIFO buffer is postponed until at least one byte has been read from the
FIFO buffer, and it is guaranteed to be a type (b) status byte.
While the RDL is in overrun state (i.e., until the indication is turned off), all incoming
data bytes are discarded and nothing is written to the FIFO buffer except the status
byte of the message where the overrun error occurred. New data is written to the FIFO
buffer only upon detection of a transition from flag sequence to zero-stuffed data
following the clearing of the interrupt.
Reading Messages from the FIFO
The system reads the contents of the FIFO buffer by accessing the Receive Data Link
Message Byte Status register; each access returns the next byte from the FIFO buffer.
The Receive Data Link(i) Status register is read before a new status is read.
This is done to make sure that there are data blocks in the FIFO buffer (RxBlk) and
that the type of the next status byte (a1, a2), or type (b) are fetched from RxGoodBlk.
The FIFO buffer contains zero or more complete data blocks at any time, plus zero or
more bytes of the current incomplete block.
At no time are the bytes of the current incomplete block accessible by the system.
After reading the Receive Data Link(i) Status register, the status itself is read and
understood according to the RxGoodBlk field value.
The data block in FIFO status indication bit (the RxBlk field in the Receive Data Link
Status register) is cleared if there are zero complete blocks, otherwise it is set.
Attempting to read the FIFO buffer when this bit is not set returns an undefined value,
but the FIFO read pointer is not incremented.
Mindspeed Technologies™
Functional Description
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