Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
1.5.4.1 TDM Sync Driven by the EPP
The EPP can be programmed to send a Suggested_TDM_Sync to the Scheduler once every N+1 celltimes
by writing N to the EPP’s TDM Sync Generation Control register and writing 1 to the Enable TDM Sync
Generation register. The TDM table select is also a field of the TDM Sync Generation Control register. The
Scheduler is programmed to listen to the Suggested_TDM_Sync from a designated port.
The advantage of this method over the third method is that the only exposure to jitter (uncertainties in
delay) on the TDM_Sync is after the EPP sends the TDM Control Packet to the linecard, due to crossing
asynchronous clock domains (and subport-ordering delay in subport mode). The advantage of this method
over the second method is that when using two Schedulers for fault tolerance, the second method requires
synchronous OOB writes to program the Schedulers at exactly the same time; this method provides a
synchronous Suggested_TDM_Sync to the two Schedulers instead.
A possible disadvantage of this method is that TDM Sync is synchronous to the ETT1’s core clock, not to
the linecard’s clock or any other external synchronization signal.
1.5.4.2 TDM Sync Driven by the Scheduler
The Scheduler can be programmed to generate its own TDM_Sync signals once every N+1 celltimes using
the Scheduler’s TDM Control register. The TDM table select is also controlled via the TDM Control register.
The Scheduler ignores Suggested_TDM_Sync from all ports.
The advantages of this method are simplicity and low jitter, similar to the first method.
The disadvantage of this method is that when using two Schedulers for fault tolerance, this method
requires synchronous OOB writes to program the Schedulers at exactly the same time; if the Schedulers’
TDM Control registers are enabled to send TDM Sync at different times, then all EPPs will report
mismatching frames from the two Schedulers. Scheduler refresh will not synchronize Scheduler-generated
TDM Syncs.
1.5.4.3 TDM Sync Driven by a Designated Linecard
Any linecard can send a Suggested_TDM_Sync to its ETT1 port, using a LCS TDM Control Packet, every
TDM_Period cells. The TDM Control Packet also specifies which of the two TDM tables should be used.
When the EPP receives a TDM Control Packet it sends a Suggested_TDM_Sync signal to the Scheduler.
The Scheduler is programmed to listen to the Suggested_TDM_Sync from a designated port.
The advantage of this method is that an external synchronization source can be used.
The disadvantage of this method is that there is much more exposure to jitter in TDM_Sync. Asynchronous
clock domain crossings (and subport-ordering delays in subport mode) will cause delay variations at
multiple stages: the LCS request for the Suggested_TDM_Sync Control Packet, the LCS grant, the
delivery of the Suggested_TDM_Sync Control Packet payload, and finally the delivery of the TDM_Sync
Control Packet from the ETT1 to the linecards. Additionally, the Scheduler’s internal digital TDM_Sync PLL
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
61
PMC [ PMC-SIERRA, INC ]
