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CHAPTER 6 PHY FUNCTION
262
The
μ
PD98405 processes the transmit data in the following sequence.
(1) Reception of cell data from ATM layer
The
μ
PD98405 receives cell data from the ATM layer via the ATM layer interface and stores that data into
the transmit FIFO. The transmit FIFO has a capacity of four cells and a buffer function for adjusting the rate
between the ATM layer side and PMD side.
While no cell data is being transmitted from the ATM layer and the amount of transmit data remaining in the
FIFO falls to less than one cell, an idle cell (empty cell) is inserted. Figure 6-3 shows the format of the idle
cell.
Figure 6-3. Format of Idle Cell (Empty Cell)
1
2
3
4
5
1
2
47
48
00H
00H
00H
01H
52H
6AH
6AH
6AH
6AH
(2) Creation of HEC
CRC calculation is performed by using the following polynomial expression with the higher four of the five
bytes of the ATM cell header. A value of "55H" is added to the result of the CRC calculation, and the result
of this addition is inserted into the fifth byte of the ATM header to enable HEC (Header Error Check).
Expression
G (X) = X
8
+ X
2
+ X + 1
(3) Scrambling of ATM cell
The data of the ATM cell is scrambled by using the following polynomial expression. The range of the
scrambling is limited to the payload of the ATM cell.
Expression
G (X) = X
43
+ 1
A user can set scramble stop mode to enable testing. Scramble stop mode is set by using the CRCRM bit
of mode register 3 (MDR3).
Header: 5 bytes
Payload: 48 bytes