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Multi-Service Access Device For Channelized Interfaces
Telecom Standard Product Data Sheet
Production
Proprietary and Confidential to PMC-Sierra, Inc., and for its Customers’ Internal Use
Document ID: PMC-1990823, Issue 4
583
For ATM FIFOs, BT[4:0] and DT[7:0] must be set equal to the value 3. This sets the threshold to
be 4 blocks which is the space occupied by an ATM cell. The upstream device should also set its
maximum burst size to be equal to one ATM cell. Note that for ATM FIFO’s with a POS-PHY
bus, if the TPAHOLD bit = 1, BT[4:0] should be set to 7 as described in Section 15.8.2.
The TXSDQ and RXSDQ FIFOs are divided up into blocks of 16 bytes each which is the
smallest granularity of the FIFO. Note two packets cannot occupy the same FIFO Block. A 17
byte packet will occupy two full blocks, though the second block will be mostly empty, and
wasted space. This needs to be considered when attempting to predict the available space in the
buffer, and this is the reason why bandwidth tends to be lower for packet sizes that are slightly
larger than an integral number of bytes.
14.16 RXSDQ and TXSDQ Data Available and Burst-Size Operation
In the RXSDQ and TXSDQ blocks, each FIFO indicates whether or not data is available for
reading by asserting the Data Available line (DA[p] for FIFO p). The DA[p] line is asserted by
the RXSDQ or TXSDQ under two conditions:
If the queue depth of FIFO p is greater than a user configured Data Available Threshold
(DT[p]), 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.
The DA[p] 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[p] 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[p]
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 FIFOs in the RXSDQ to reach
higher fill levels because more time is needed per PHY to complete a burst. Also, since a burst is
terminated when an EOP signal is detected, small packets will cause their associated 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[p] signal indicates that enough data is in the TXSDQ to begin a cell
or packet transfer, the downstream TTDP or TCFP blocks will begin pulling data from the
corresponding FIFO. It will continue pulling regardless of the DA[p] 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[p] signal is again asserted.