DS3172 Maxim Integrated, DS3172 Datasheet - Page 97

• Manufacturer Part Number: DS3172
• Description: Network Controller & Processor ICs Dual DS3/E3 Single Chip Transceiver
• Manufacturer: Maxim Integrated

Specifications of DS3172

• Part # Aliases: 90-31720-000

DS3171/DS3172/DS3173/DS3174
10.7.3 Transmit FIFO
The Transmit FIFO block contains memory for 256 bytes of data with data status information and controller circuitry
for reading and writing the memory. The Transmit FIFO controller functions include filling the memory, tracking the
memory fill level, maintaining the memory read and write pointers, and detecting memory overflow and underflow
conditions. The Transmit FIFO receives data and status from the microprocessor interface, and stores the data
along with the data status information in memory. The Transmit Packet Processor reads the data and data status
information from the Transmit FIFO. The Transmit FIFO also outputs FIFO fill status (empty/data storage
available/full) via the microprocessor interface. All operations are byte based. The Transmit FIFO is considered
empty when its memory does not contain any data. The Transmit FIFO is considered to have data storage
available when its memory has a programmable number of bytes or more available for storage. The Transmit FIFO
is considered full when it does not have any space available for storage. The Transmit FIFO accepts data from the
register interface until full. If the Transmit FIFO is written to while the FIFO is full, the write is ignored, and a FIFO
overflow condition is declared. The Transmit Packet Processor reads the Transmit FIFO. If the Transmit Packet
Processor attempts to read the Transmit FIFO while it is empty, a FIFO underflow condition is declared.
10.7.4 Transmit HDLC Overhead Processor
The Transmit HDLC Overhead Processor accepts data from the Transmit FIFO, performs bit reordering, FCS
processing, stuffing, packet abort sequence insertion, and inter-frame padding.
A byte is read from the Transmit FIFO with a packet end status. When a byte is marked with a packet end
indication, the output data stream will be padded with FFh and marked with a FIFO empty indication if the Transmit
FIFO contains less than two bytes or transmit packet start is disabled. Transmit packet start is programmable (on
or off). When the Transmit Packet Processor reads the Transmit FIFO while it is empty, the output data stream is
marked with an abort indication. Once the Transmit FIFO is empty, the output data stream will be padded with
interframe fill until the Transmit FIFO contains two or more bytes of data and transmit packet start is enabled.
Bit reordering changes the bit order of each byte. If bit reordering is disabled, the outgoing 8-bit data stream
DT[1:8] with DT[1] being the MSB and DT[8] being the LSB is input from the Transmit FIFO with the MSB in TFD[0]
and the LSB in TFD[7] of the transmit FIFO data TFD[7:0]. If bit reordering is enabled, the outgoing 8-bit data
stream DT[1:8] is input from the Transmit FIFO with the MSB in TFD[7] and the LSB in TFD[0] of the transmit FIFO
data TFD[7:0]. DT[1] is the first bit transmitted on the outgoing data stream.
FCS processing calculates an FCS and appends it to the packet. FCS calculation is a CRC-16 calculation over the
16
12
5
entire packet. The polynomial used for the CRC-16 is x
+ x
+ x
+ 1. The CRC-16 is inverted after calculation,
and appended to the packet. For diagnostic purposes, an FCS error can be inserted. This is accomplished by
appending the calculated CRC-16 without inverting it. FCS error insertion is programmable (on or off). When FCS
processing is disabled, the packet is output without appending an FCS. FCS processing is programmable (on or
off).
Stuffing inserts control data into the packet to prevent packet data from mimicking flags. Stuffing is halted during
FIFO empty periods. The 8-bit parallel data stream is multiplexed into a serial data stream, and bit stuffing is
performed. Bit stuffing consists of inserting a '0' directly following any five contiguous '1's. Stuffing is performed
from a packet start until a packet end.
Inter-frame padding inserts inter-frame fill between the packet start and end flags when the FIFO is empty. The
inter-frame fill can be flags or '1's. If the inter-frame fill is flags, flags (minimum two) are inserted until a packet start
is received. If the inter-frame fill is all '1's, an end flag is inserted, ‘1’s are inserted until a packet start is received,
and a start flag is inserted after the ‘1’s. The number of '1's between the end flag and start flag may not be an
integer number of bytes, however, the inter-frame fill will be at least 15 consecutive '1's. If the FIFO is not empty
between a packet end and a packet start, then two flags are inserted between the packet end and packet start. The
inter-frame padding type is programmable (flags or ‘1’s).
Packet abort insertion inserts a packet abort sequences as necessary. If a packet abort indication is detected, a
packet abort sequence is inserted and inter-frame padding is done until a packet start is detected. The abort
sequence is FFh.
Once all packet processing has been completed, the datastream is inserted into the DS3/E3 datastream at the
proper locations. If transmit data inversion is enabled, the outgoing data is inverted after packet processing is
performed. Transmit data inversion is programmable (on or off).
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