PM9311 Datasheet, PDF (342/343 Pages) PMC-Sierra, Inc – Enhanced TT1TM Switch Fabric

RELEASED
Data Sheet
PMC-2000164
ISSUE 3
PMC-Sierra, Inc. PM9311/2/3/5 ETT1™ CHIP SET
ENHANCED TT1™ SWITCH FABRIC
2. Whenever the linecard receives an LCS cell that does not have a grant and F = 1 then reset F to 0
and send an Idle cell.
This very simple algorithm ensures that an Idle cell can be sent whenever the linecard does not have to
respond to a grant. The only requirement here is that the ETT1 port must send a minimum rate of
non-grant cells. This can be achieved in one of two ways:
1. Program the Idle Count register in the EPP to send Idles slightly more frequently than the linecard.
2. Program the NoGrant register in the EPP to restrict the rate of grants coming from the EPP.
NOTE: The maximum number of cells between Idles sent by the linecard, if it follows this algo-
rithm, is then set by the following equation:
max_non_idle = linecard_idle_count + min(EPP_idle_count, EPP_NoGrant_count)
This means that the just setting the Idle count at the linecard is not sufficient to ensure a minimum Idle cell
rate; the rate at which non-grant cells are received must be included in the equation.
The only further complication is that the linecard also sends out requests for new cells. However, it is
assumed that requests (new cells) arrive at the linecard at a slower rate than they can be forwarded to the
ETT1 fabric due to speedup within the fabric. Given this, then the linecard just needs to provide a
one-deep queue for outbound requests, as shown in Figure 96. Given the higher rate at which requests
are sent, then the request FIFO will never overflow.
338
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE