參數(shù)資料
型號： PM73488
廠商： PMC-SIERRA INC
元件分類： 數(shù)字傳輸電路
英文描述： 5 Gbit/s ATM Switch Fabric Element
中文描述： ATM SWITCHING CIRCUIT, PBGA596
封裝： EPBGA-596
文件頁數(shù)： 24/135頁
文件大?。?/td> 1735K
代理商： PM73488
PMC-Sierra, Inc.
PM73488 QSE
L
PMC-980616
Issue 3
5 Gbit/s ATMSwitch Fabric Element
Released
Datasheet
26
2.1
Phase aligners aid in constructing large systems. Clock information is recovered from the data sent to each QSE
switch fabric port. Phase aligners are used on the BP_ACK_IN(31:0), SE_SOC_IN(31:0), and SE_D_IN(31:0, 3:0)
signal lines. Since there is no setup or hold time requirements on these signals, the overall clock distribution scheme
within the system can be simplified. However, overall system jitter and skew between signals on the same switch fab-
ric data port must still be managed.
Phase Aligners
2.2
Another aid to constructing large systems is an elastic store at each QSE input data port. The data elastic store allows
data arriving from different ports to be offset by up to a maximum of eight clock cycles. The internally generated and
software programmable local CELL_START signal marks the end of an 8-clock-period window within which the
SOC marker on each of the SE_SOC_IN(31:0) lines must arrive.
Data Drivers
2.3
Each of the 32 nibble-wide inputs is connected to an output by a crossbar. This crossbar is transparently controlled by
the
cell’s routing tag, which specifies the input-to-output connection. In the event of a conflict for an output port,
higher priority cells are given preference over lower priority cells. There are three unicast cell priorities: high,
medium, and low.
Unicast Routing and Distribution
The gang of 32, also known as distribution mode, is a special unicast routing mode in which incoming unicast cells
are routed to outputs using PMC’s patented congestion-minimization (Evil Twin Switching) algorithm. In this mode,
no routing information is used from the cell’s routing tag.
Depending on the gang mode, the QSE will need a number of routing bits to determine the output gang of a unicast
cell. For example, in gang mode of four, there are eight output gangs, thus three routing bits are required for selecting
the QSE. However, in distribution mode no routing bits are needed. The routing bits are taken from the head of the
routing tag and are then shifted back in at the tail (which preserves header parity). This allows the next set of routing
bits to be always accessible at the same spot in the tag, namely the head. The cell routing tag is broken into eight nib-
bles, namely TAG_0 through TAG_7.
Figure 14 on page 27
shows the tag rotation for gang mode of four (three routing bits are used by the QSE from
TAG_0 and then shifted back in at the tail of TAG_7). TAG_0 is broken up and part of it ended up at the end of
TAG_7 (shown by the white area in
Figure 14 on page 27
). As a result, all the other tags (TAG_1 through TAG_7)
also get broken up and shifted (as shown by the light and dark gray areas of
Figure 14 on page 27
).
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