PM5380-BI PMC-Sierra, Inc., PM5380-BI Datasheet - Page 319

• Manufacturer Part Number: PM5380-BI
• Description: Network Interface, SATURN User Network Interface (8x155) Telecom Standard Product
• Manufacturer: PMC-Sierra, Inc.

Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC- 2010299, Issue 2
Register 0x1016: TUL3 POS Level 3 FIFO High Water Mark
HWM[5:0]:
The HWM[5:0] bits determine the channel FIFO depth at which PTPA and STPA deassert in
POS-PHY Level 3 operation. FIFO fill level control may be important when the layer
device latency is large.
When the channel FIFO is filled to the specified depth, the transmit packet available signals
PTPA and STPA are deasserted. TMOD[1:0] is ignored when calculating FIFO depth.
Therefore, PTPA and STPA may deassert when end of a packet is within 4 bytes of HWM.
PTPA and STPA are asserted based on the FIFO level specified by LWM[5:0]. Together
with LWM[5:0], HWM[5:0] provides hysteresis in the toggling of the transmit packet
available signal.
HWM[5:0] specifies the FIFO level in the number of double-words (4 bytes). Therefore,
the high water mark may be configured from 0 bytes to 255 bytes and must be less than the
maximum size of the FIFO. For proper operation, the high water mark level must be
greater than the low water mark level.
Note that when sending ATM cells over the POS-PHY Level 3 interface, HWM[5:0]
together with LWM[5:0] do not directly translate to the number of writeable locations in the
FIFO as the ATM cells are internally stored at 64 byte entities. In general, the HWM[5:0]
should be programmed less than 240.
Bit
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Type
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Function
Reserved
Reserved
HWM[5]
HWM[4]
HWM[3]
HWM[2]
HWM[1]
HWM[0]
S/UNI®-8x155 ASSP Telecom Standard Product Data Sheet
Default
0
0
1
1
1
0
0
0
Released
319