mt90224 Zarlink Semiconductor, mt90224 Datasheet - Page 49

• Manufacturer Part Number: mt90224
• Description: 8-port Ima/tc Phy Device
• Manufacturer: Zarlink Semiconductor

3.2.13.1
The ICP Cell from each link of the same IMA Group is used to determine the external SRAM read and write pointers.
The distance between the read and write pointers is referred to as the recombiner delay. Setting the recombiner
delay to the maximum acceptable delay results in a fixed recombiner delay that is not optimum. For example, setting
recombiner delay to 25 msec when the worst case delay is 12 msec results in an additional, unnecessary delay of
13 msec.
The minimum recombiner delay would be the current worst case differential delay. In the example above, the
recombiner delay would be set to 12 msec. In this case, a link with larger transport delay than the current worst value
cannot be added to an existing IMA group: the cells from this slower link have not arrived when the cells sequence
is rebuilt, as the read pointer was set using the previous worst case link. If this slower link is to be added, then the
recombiner process has to stop for the time required to receive the cells on the slower link and then the recombiner
process can resume. This causes disruption in the operation of the recombiner and will affect the Cell Delay Variation
(CDV).
To provide an optimal recombiner delay, the MT90222/3/4 adds a guardband delay to the current worst case
recombination delay when the IMA Group is first started up. Guardband delay is programmable and minimizes the
number of disruptions that would otherwise occur in accommodating link delays exceeding the current worst case.
The guardband delay value is specified for each IMA group by writing to the RX Guardband/Delta Delay
(0x0287-0x028E) registers. It should be the smallest value possible consistent with minimizing the disruptions (the
smallest allowed value is 4). When operational, the value of the guardband delay corresponds to the delay value of
the link having the greater transport delay (the link where the data is the last to arrive to the MT90222/3/4).
3.2.13.2
The various delays on links of the same IMA Group are measured and compared to the programmed ’maximum
allowable value’ stored in the RX Maximum Operational Delay (0x029A-0x02A1) registers for the IMA Group.
This value corresponds to the worst delay value that is expected. This value cannot be larger than the number of
cells that can be stored in the external memory. The smallest ’maximum allowable value’ is four cells. These values
are independently established for each of the four IMA Groups.
3.2.13.3
If a link to be added is slower and cannot be accommodated by the present guardband, an LODS signal is
generated and the link delay value is reported negative. The value reported is with respect to the read pointer and
represents the minimum number of cells that has to be added to the present guardband before adding the link in the
IMA group. See paragraph 3.2.13.6 Incrementing/Decrementing the Recombiner Delay for more details.
If a link to be added is faster and would cause its write pointer to be set beyond the RX Maximum Operational
Delay (0x029A-0x02A1) programmed value, then the link is reported to be faulty through an LODS condition. The
recombination process will not be affected as long as the amount of delay is not larger than the total number of cells
in the external memory.
LODS will also be reported if, during operation, the delay of a link changes to exhibit higher or lower delay resulting
in a negative delay value or a value beyond the RX Maximum Operating Delay value.
LODS events are reported by the IRQ Link Status (0x0435) register and investigated by selecting the current
maximum delay using the RX Delay Select (0x02AA), RX Delay Register (0x0285) and RX Delay Link Number
(0x0286) register. Link Out of Delay Sychronization (LODS) and end of LODS can optionally generate an interrupt
(IRQ).
RX Maximum Operational Delay Value
RX Recombiner Delay Value
Link Out of Delay Synchronization (LODS)
Zarlink Semiconductor Inc.
MT90222/3/4
49
Data Sheet