DS92UT16TUF Datasheet, PDF (7/86 Pages) National Semiconductor (TI) – UTOPIA-LVDS Bridge for 1.6 Gbps Bi-directional Data Transfers

6.0 Functional Description (Continued)
6.3.1 Cell Rate Decoupling
In the down-bridge direction, the TCS Assembler inserts idle
cells when no valid traffic cells are available from the FIFO
for onward transmission. In the up-bridge direction, the TCS
Disassembler rejects all received idle cells.
6.3.2 Link Transport Container (TC)
The ATM cells received on the UTOPIA interface can be
standard or user-specified cells. Cell length is programmable
from 52 to 64 bytes. These cells are treated as Protocol Data
Units (PDU), which are packaged into Transport Containers
(TC) for transmission over the serial link. In the reverse
direction, the cell PDUs are unpacked from the link TCs
before being passed out on the UTOPIA interface.
This is illustrated in Figure 6.
The PDU fields are configured as shown in Table 1. The total
PDU cell length must be in the range of 52 to 64 bytes. In
addition, variable length fields must be programmed to an
even number of bytes because the DS92UT16 operates with
an internal 16 bit data path. The total number of bytes
defined for User Prepend plus UDF1/2 and User Append
must not exceed 12 bytes to maintain the maximum PDU cell
length of 64 bytes.
TABLE 1. PDU Cell Format Options
Field
User Prepend
Cell Header
UDF1/2
Payload
User Append
Fixed/
Variable
Variable
Fixed
Variable
(On/Off)
Fixed
Variable
Bytes
0, 2, 4, 6, 8, 10, 12
4
2, 0 in 16 bit mode
1, 0 in 8 bit mode
48
0, 2, 4, 6, 8, 10, 12
Although the UDF1/2 bytes will always be present, the
DS92UT16 can be programmed to either transport these
bytes or ignore them. If they are to be ignored, then the TCS
strips them out in the down-bridge direction and the UTOPIA
up-bridge section inserts a HEC byte in UDF 1. Otherwise,
they can be transported transparently the same as any other
PDU byte.
FIGURE 6. PDU and Link Transport Container Format
20031605
Each link TC has an MPHY address byte, two Flow Control
(F) Channel bytes, and a HEC byte in addition to the PDU
cell. The two F1 and F2 bytes per TC constitute the F
Channel, which is used for flow control and OAM purposes
over the link. The TCS uses the HEC byte for container
delineation, frame delineation, and cell header error detec-
tion.
6.3.3 MPHY Tagging and Routing
In the down-bridge direction, the DS92UT16 adds an addi-
tional byte (MPHY byte) to each PDU. It contains the MPHY
port address associated with that PDU, as shown in Table 2.
Bit
Function
TABLE 2. MPHY Byte
76543210
MPHY Port Address 0–31 Reserved
At the other end of the link, this byte is used to route the
incoming PDU from the LVDS interface to the appropriate
MPHY port queue.
6.3.4 Transport Container Delineation and Error
Monitoring
In the down-bridge direction, the device calculates and in-
serts the HEC byte using the CRC-8 polynomial x⁸ + x² + x
+ 1 and optional coset x⁶ + x⁴ + x² + 1 defined in I.432.1 [2.].
The HEC byte is calculated over the preceding 7–19 bytes,
which make up the link TC header. To aid delineation at the
far end, the entire contents of the TC, excluding the HEC,
are scrambled and the HEC is calculated on the scrambled
TC header. A scrambler using the pseudo-random sequence
polynomial x³¹ + x²⁸ + 1 defined in I.432.1 [2.] is used.
In the up-bridge direction, the device determines the cell
delineation within the received data by locking onto the HEC
byte within the transport container, using the algorithm speci-
fied in I.432.1 [2.].
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