PM7384 FREEDM-84P672
DATA SHEET
PMC-1990445
ISSUE 5
FRAME ENGINE AND DATA LINK MANAGER 84P672
Figure 15 – Partial Packet Buffer Structure
Partial Packet
Buffer RAM
Block
Pointer RAM
16 bytes
16 bytes
16 bytes
16 bytes
XX
0x03
XX
Block 0
Block 1
Block 2
Block 3
Block 0
Block 1
Block 2
Block 3
0xC8
16 bytes
16 bytes
0x01
XX
Block 200
Block 200
Block 2047
Block 2047
The writer and reader determine empty and full FIFO conditions using flags.
Each block in the partial packet buffer has an associated flag. The writer sets
the flag after the block is written and the reader clears the flag after the block is
read. The flags are initialized (cleared) when the block pointers are written using
indirect block writes. The reader declares a channel FIFO under-run whenever it
tries to read data from a block without a set flag.
The FIFO algorithm of the partial packet buffer processor is based on per-
channel software programmable transfer size and free space trigger level.
Instead of tracking the number of full blocks in a channel FIFO, the processor
tracks the number of empty blocks, called free space, as well as the number of
end of packets stored in the FIFO. Recording the number of empty blocks
instead of the number of full blocks reduces the amount of information the
roamer must store in its state RAM.
The partial packet roamer records the FIFO free space and end-of-packet count
for all channel FIFOs. When the reader signals that a block has been read, the
roamer increments the FIFO free space and sets a per-channel request flag if
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
75
PMC [ PMC-SIERRA, INC ]
