Freescale Semiconductor MPC8260 - Manual (Page 1124)

Inverse Multiplexing for ATM (IMA)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
33-22 Freescale Semiconductor
The states are described as follows:
• Group Unassigned—This corresponds to a link which is known to be IMA, but for which no
information is known (e.g. IMA group number, IMA frame size). In this state, the receive task only
screens the incoming cells for ICP cells and discards all others. ICP cells in which an SCCI change
is noted are passed to a user-defined receive channel, where software must interpret their content
in order to initialize the link and add it to the group. The link transitions to the ‘Link IFSM
Unsynchronized’ state when the link’s ‘Group Assigned’ flag is set by software.
• Link IFSM Unsynchronized—In this state, the link state machine has not yet found or validated
the frame boundary for this link. This state is initially entered after software defines the link’s M
value and expected ICP cell format, then sets the ‘Group Assigned’ flag. This state can be
subsequently re-entered as the result of a link defect. In this state, the link searches for an ICP cell,
then looks for ICP cells in the expected position of subsequent frames in order to validate frame
synchronization. Cells other than ICP cells are discarded. When the IFSM becomes synchronized,
an interrupt to the host is generated. The link transitions to the ‘Link Delay Unsynchronized’ state
when the link’s group and link synchronization flags are set appropriately by software.
• Link Delay Unsynchronized—This state contains two separate operations, depending on the state
of the group to which the link belongs. If the link enters this state as the result of group start-up, it
performs the “new-group link delay synchronization algorithm”. If the link enters this state while
the group is already activated (per the link addition/slow recovery (LASR) procedure of the IMA
standard), it performs the “added-link delay synchronization algorithm”. As the result of either
algorithm, an interrupt will be generated to the host when delay synchronization is achieved.
• No Link Defect—This is the state in which normal receive operation occurs after the link has been
established and the link-state has been communicated appropriately to the far end. If the link is not
inhibited at the group or link level, reception of ATM cells will occur. If the link or group is
inhibited, non-ICP received cells will be replaced with filler cells. In the “no link defect” state, cells
received (or their replacements) are written to the link’s delay compensation buffer. The link will
transition out of this state only as the result of an error or if reset by host software.
33.3.3.2 Cell Processing Activation Function
The cell processing activation function operates in two modes. The first, simplest mode is for on-demand
cell processing. The second mode is for cell processing as determined by the reconstructed IMA data cell
rate, as determined from the recovered receive data clock of the timing reference link (TRL) of the IMA
group.
33.3.3.2.1 On-Demand Cell Processing
In this mode, the cell processing activation function is null. The cell processing task is triggered directly
by the cell reception task if a cell is written to a delay compensation buffer.
This mode is strictly demand-driven; there is no attempt to reconstruct an IMA Data Cell Rate (IDCR)
from the IMA group at which to process incoming cells. Per the IMA specification, this is allowable under
the following conditions:
• The IMA receiver is directly built into end equipment that directly terminates the ATM layer (i.e.
terminates all ATM connections), and