MT90220
3.3.1.2 Link Information
Link Status register. Refer to 6.2.2 IRQ Link Status
and IRQ Link Enable Registers for more details.
All required verification and link validation
information is extracted from the ICP received cells.
The IMA ID, Link ID (LID), Reference Link Number,
ICP Cell Offset and Frame Length can be read and
validated before enabling an IMA Group link.
Software obtains this information by writing to the RX
Load Values register to select a link and by reading
the RX Link IMA ID, RX Link ICP Offset, RX Link
ID and Reference and RX State registers. This
information can also be obtained by collecting all the
received ICP cells in the RX ICP Cell Buffer and then
processing the content of the ICP cell (i.e., writing to
the RX ICP Cell Type RAM register and then
reading from the RX ICP cell buffer).
The LIF status bit is reporting the current condition of
the IMA Frame State Machine at the time it is read
and cannot not be programmed to generate an
interrupt when exiting the LIF condition. The
software has to poll the LIF status bit to determine
when the condition is cleared.
3.3.4 Filler Cell Handling
The MT90220 scans each incoming cell received for
the Filler Cell Indication code. Filler cells are written
to external RAM to keep the IMA frame aligned. They
are automatically discarded after being read from the
external RAM by the recombiner.
The contents of the link information registers should
be read after enabling the RX PCM link in the RX
PCM Link Control register and before enabling the
IMA mode. The link information can be accessed
when a link is either in UNI or IMA mode.
3.3.5 Stuff Cell Handling
Each incoming ICP cell received is scanned for the
Stuff Indication Code. Stuff cells are inserted at the
transmit end as two identical and consecutive ICP
cells with the Link Stuff Indication Bits set as defined
in the IMA specification. The MT90220 automatically
discards one of the two Stuff cells without storing it in
external RAM. The other is kept and processed as a
regular ICP cell. IMA Frame synchronization is
maintained for all cases (except case 7, O-19
optional requirements) as described in Figure 20 of
the IMA Specification.
3.3.1.3 RX OAM Label
The RX OAM Label is treated differently than the
other link’s parameters. Four registers, the RX OAM
Label registers, 1 per RX IMA Group, are used to
defined the RX OAM Label. Its value is written by the
software and can be changed at any point in time.
However, the RX OAM Label has to match the value
contained in the RX ICP cell for the IMA Frame State
Machine to reach the ACTIVE state.
3.3.6 Received ICP Cell Buffer
An internal buffer is implemented to collect cells from
the RX PCM links for analysis by the software. This
storage unit is a circular buffer for each link and
contains up to three cells per link. The buffer can
selectively collect:
3.3.2 Out of IMA Frame (OIF) Condition
Status bits in the RX OIF Status register, one bit per
link, is reporting OIF conditions. The status bit is
latching an OIF condition which corresponds to a
transition of the IFSM from SYNC to HUNT. The OIF
condition is reported as a status bit only and cannot
generate an interrupt. The status bit is cleared by
writing a 0 to the corresponding bit.
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all valid cells coming on a RX PCM port
all valid ICP cells
all valid ICP cells which contain new
information (as indicated by the SCCI field,
valid only when the link is in IMA mode).
There are 8 OIF counters, one per link. For each OIF
transition, the 8-bit counter associated with the link is
incremented by one. The counter can be read with
indirect access when issuing a load command with
the RX Load Values register. The counter can be
cleared by writing to the RX OIF Counter Clear
Command register.
The type of cells collected is defined in the RX ICP
Cell Type RAM registers. A status bit and a
maskable IRQ alerts the software when a new cell is
waiting for processing in a specific link. These are
found in the IRQ Link Status and Enable registers.
Valid ICP cells when a link is in UNI mode is
determined by a valid HEC byte. When in IMA mode,
a valid ICP cell must meet the criteria defined in
Table 16 of the ATM IMA spec.
3.3.3 Link Out Of IMA Frame (LIF)
Synchronization
A
link is declared out of IMA Frame (LIF)
Software can directly access the cells in the RX
buffer through a two-cell-wide access window. This
access window can be advanced, one cell at a time,
by issuing a command to move the internal pointer to
the next cells. Since the window accesses two cells,
synchronization state when the IFSM goes in HUNT
mode for ’gamma +2’ frames after it was in SYNC
state. This condition is latched in bit 2 of the IRQ
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