PM8621 PMC-Sierra, Inc., PM8621 Datasheet - Page 137

• Manufacturer Part Number: PM8621
• Description: NSE-8G Standard Product Data Sheet Preliminary
• Manufacturer: PMC-Sierra, Inc.

12.1.4
12.2
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-2010850, Issue 1
PCB Design Notes
To maintain flexibility, all unused LVDS inputs and outputs should be left floating. This will
prevent accidental damage caused by firmware enabling outputs, or releasing resets of inactive
ports.
“C1” Synchronization
Any NSE/SBS fabric can be viewed as a collection of five “columns” of devices: column 0
consists of the ingress flow from the load devices (e.g., some SBI device); column 1 consists of
the ingress flow through the SBS devices; column 2 consists of the NSE-8G device; column 3
consists of the egress flow through the SBS devices; and column 4 consists of the egress flow
through the load devices (e.g. some SBI device). Note that the devices in columns 0 and 4 are
SBI bus devices while columns 1 and 3 are SBS or SBS-lite devices. The dual column references
refer to their two separate simplex flows. Path-aligned STS-12 frames are pipelined through this
structure in a regular fashion, under control of a single clock source and frame pulse. There are
latencies between these columns, and these latencies may vary from path to path. The following
design is used to accommodate these latencies.
A timing pulse for SBI frames (2kHz, 500= s) is generated and fed to each device in the fabric.
Each chip has a FrameDelay register (RC1DLY) which contains the count of 77.76 MHz clock
ticks that device should delay from the reference timing pulse before expecting the C1 characters
of the ingress STS-12 frames to have arrived. The base timing pulse is called t . The delays from t
based on the settings of the RC1DLY registers in the successive columns of the devices are called
t 0 , … t 4 . The first signal, t
used to instruct the ingress load devices (column 0) to start emitting an STS-12 frame (with its
special “C1” control character) at that time. t i is determined by the customer, based on device and
wiring delays to be approximately the earliest time that all “C1” characters will have arrived in
the ingress FIFOs of the t i column of devices. t i is selected to provide assurance that all “C1”
characters have arrived at the i
emission of the STS-12 frames. The ingress FIFOs permit a variable latency in C1 arrival of up to
16 clock cycles.
Note: The SBS device, being a memory switch adds a latency of one complete frame or row plus
a few clock ticks to the data.
1
(equal to t
th
column. The i
0
), determines the start of an STS-12 frame; this signal is
th
column of devices use the t i signal to synchronize
NSE-8G™ Standard Product Data Sheet
Preliminary
136