PM5381-BI PMC-Sierra, Inc., PM5381-BI Datasheet - Page 526

• Manufacturer Part Number: PM5381-BI
• Description: ATM UNI, Single Channel ATM PHY Interface for 2488.32Mbps
• Manufacturer: PMC-Sierra, Inc.

13.8
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-2000489, Issue 4
For PL3 FIFOs, BT[4:0] should be set according to the rules specified in 13.12. For ATM FIFOs,
this value should be at least 3 and the upstream device should set its maximum burst size to be
equal to one ATM cell.
RXSDQ and TXSDQ Data Available and Burst-Size Operation
In the RXSDQ and TXSDQ blocks, the FIFO indicates whether or not data is available for
reading by asserting the Data Available line (DA). The DA line is asserted by the RXSDQ or
TXSDQ under two conditions:
·
·
The DA line gets de-asserted when the data in the FIFO falls below the threshold and there are no
EOPs in the FIFO. For the RXSDQ, the DA state is reflected by the RCA output signal on the
Utopia L3 interface. When using the POS-PHY L3 interface, the RXPHY polls the DA signals
using its Calendar Sequence to determine if there is any data in the PHY to output.
For the RXSDQ, the BURST_SIZE[3:0] setting in the RXPHY Indirect Burst Size register is
closely tied to the DT[7:0] setting in the RXSDQ FIFO Indirect Data Available Threshold
register. BURST_SIZE[3:0] must be set to a value less than or equal to (DT[7:0]). This will
prevent the RXPHY from initiating a burst until there is sufficient data in the RXSDQ.
For transfer of ATM cells, the BURST_SIZE[3:0] and DT[7:0] must both be set to the value 3.
This makes the threshold equal to 4 blocks which will contain one ATM cell.
It is also important to note that larger burst sizes may cause the FIFO in the RXSDQ to reach
higher fill levels because more time is needed for the PHY to complete a burst. Also, since a
burst is terminated when an EOP signal is detected, small packets will cause the PHY to be
penalized in net throughput across the POS-PHY L3 interface. To maximize fairness, it is
recommended to keep the burst size fairly small – on the order of 4 blocks (BURST_SIZE[3:0] =
0x3).
In the TXSDQ, once the DA signal indicates that enough data is in the TXSDQ to begin a cell or
packet transfer, the downstream TCFP block will begin pulling data from the FIFO. It will
continue pulling regardless of the DA state until the cell or packet is complete. At this time, it
will start the next cell or packet transfer from the TXSDQ only after the DA signal is again
asserted.
The value of DT[7:0] in the TXSDQ Indirect Data and Buffer Available Thresholds register must
be set so that:
(DT[7:0] + 1) + (BT[4:0] + 1) ≤ FIFO size
If the queue depth of the FIFO is greater than a user configured Data Available Threshold
(DT), i.e. if there is more data than specified by the threshold.
If at least one EOP is stored in the FIFO, i.e. the tail end of at least one packet is available in
the threshold.
S/UNI-2488 Telecom Standard Product Datasheet
Released
526