Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
sion (these bits are for future use and currently are required to be “00”). In the case of a backpres-
sure packet, the next bit is the backpressure bit on for low priority multicast cells, followed by one
code extension bit.
The background is reset to four zeros and four ones after transmission of each packet.
The QRT and QSE allow back-to-back acknowledgment and backpressure packets. In the case of
back-to-back acknowledgment and backpressure packets, the receiving device may see an
inverted bit (a “1”) followed by the rest of the packet instead of a reset background pattern.
One backpressure packet and either one or none acknowledgment packet are expected to be
received during a cell time. The receipt of multiple acknowledgment or backpressure packets is a
failure condition.
Table 1 describes the backpressure and acknowledgment encodings.
Table 1. Backpressure and Acknowledgment Encodings
Code
Ext 0
Mode
Data 2
Data 1
Data 0
Description
0
1 = Backpressure
on high priority
multicast cell.
1 = Backpressure
on medium prior-
ity multicast cell.
1 = Backpressure
on low priority
multicast cell.
0
Backpressure information.
This signal is present each cell time,
regardless of whether a cell was transmit-
ted or not (on that link).
This signal is withheld if any problem is
detected on the input port.
1
1
1
1
0
0
1
1
0
1
0
1
0
0
0
0
0
0
0
0
Signals no response. Treated as acknowl-
edgment.
Signals Mid-switch Negative ACKnowl-
edgment (MNACK).
Signals Output Negative ACKnowledg-
ment (ONACK).
Signals ACKnowledgment (ACK).
2.2.11 Relation Between External CELL_START and Local CELL_START
Figure 17 shows the relationship between external RX_CELL_START and local CELL_START
signals.
RX_CELL_START Low
CELL_START Delay
Clock Cycle
RX_CELL_START High
SE_CLK
External RX_CELL_START
Delta
Delta
Local CELL_START
Figure 17. QRT Cell-Level Timing
17
PMC [ PMC-SIERRA, INC ]
