Serial ATM Scrambling, Reception, and SI Programming
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
H-3
H.2.1
HEC Delineation Mechanism
The ATM controller applies the cell delineation mechanism specified in I.432 and shown in
Figure H-3
to
synchronize to the incoming cell stream. The SDH-based physical layer values for alpha and delta, 7 and
6 respectively, are used by the ATM controller.
At start-up, the hunt mode is entered where the CRC-8 value is calculated on each incoming longword.
When this matches the next received byte, it is assumed that the HEC is found and the state machine
advances to the presync state. Because the hunt state only finds a HEC field that is aligned to the start of
reception, it can take up to four correct cells to leave the hunt state.
Figure H-3. Cell Delineation State Diagram
In the presync state, the ATM controller has detected cell boundaries and is verifying that they are correct.
The ATM controller leaves the presync state after a single cell with an incorrect HEC is received or the
RESTART
RECEIVE
command is given. When six consecutive cells with valid HECs are received, the ATM
controller jumps to the sync state and begins reception. The state machine advances immediately after
reception of the sixth correct HEC and is not delayed until the end of the cell. This means that five cells
with valid HECs are discarded and the sixth is received into a data buffer.
In the synch state, the ATM controller receives data and control cells and discards incoming empty cells.
When a stream of cells with incorrect HECs are received, the first six consecutive cells are received and
on reception of the seventh incorrect HEC, the ATM controller immediately jumps to the hunt state and
does not receive the cell with the seventh incorrect HEC. The ATM controller also jumps to the hunt state
after the
RESTART
RECEIVE
command.
PRESYNC
Single Correct
HEC
SYNCH
Cell by Cell
Search for
Correct HECs
Cell by Cell
Verification of
HECs
Single
Incorrect
HEC
Delta (6) Consecutive
Correct HECs
Alpha (7)
Consecutive
Incorrect HECs
Initial Entry
Point
HUNT
4 Byte by 4 Byte
Search for
Correct HEC
Restart
Reception
Command
Restart
Reception
Command
Summary of Contents for PowerQUICC MPC870
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