Atm Transmit Buffer Descriptors (Txbds) - Freescale Semiconductor MPC850 User Manual

Table 37-1. ATM RxBD Field Descriptions (Continued)
Offset
from Bits
RBD_PTR
0x0A — —
0x0C — Cell header
expansion 1,
2, and 3

37.1.4 ATM Transmit Buffer Descriptors (TxBDs)

The format of the ATM transmit buffer descriptor (TxBD) applies to both UTOPIA and
serial ATM modes. ATM TxBDs are 12 bytes, as shown in Figure 37-5.
0 1
OFFSET + 0 R —
OFFSET + 2
OFFSET + 4
OFFSET + 8
OFFSET + A
For UTOPIA operation, a global option to support expanded cells is available. ATM TxBDs
in expanded cell mode are 24 bytes, as shown in Figure 37-6.
0 1
OFFSET + 0 R —
OFFSET + 2
OFFSET + 4
OFFSET + 8
OFFSET + A
OFFSET + C
OFFSET + 10
OFFSET + 14
Figure 37-6. ATM TxBD in Expanded Cell Mode (UTOPIA Only)
Name
Reserved, should be cleared.
Cell header expansion 1, 2, and 3 (UTOPIA expanded cell mode only). These
fields are added to the BD when expanded cells are enabled (SRSTATE[EC]
is set in the UTOPIA parameter RAM). The size of the cell header expansion
is programmable and is determined by ECSIZE in the UTOPIA parameter
RAM.
When the last BD of an AAL5 frame is closed, the ATM controller copies the
expanded cell header into these fields. For AAL0 channels, the expanded cell
headers are copied into these fields for every RxBD.
Cell header expansion 1 is the first word to be received; cell header
expansion 3 is the last. The bytes are ordered according to the DMA byte
ordering (programmed through SRFCR in the parameter RAM).
2 3 4 5
W I L —
Figure 37-5. ATM TxBD
2 3 4 5
W I L —
Chapter 37. Buffer Descriptors and Connection Tables
Description
6 7 8 9
CM
— — —
DA TA LENGTH
TX DA TA BUFFER POINTER
CPCS-UU+CPI/HEADER_L
RESERVED/HEADER_H
6 7 8 9
CM
— — —
DA TA LENGTH
TX DA TA BUFFER POINTER
CPCS-UU+CPI/HEADER_L
RESERVED/HEADER_H
CELL HEADER EXPANSION 1
CELL HEADER EXPANSION 2
CELL HEADER EXPANSION 3
ATM Buffer Descriptors (BDs)
10 11 12 13 14
— — — —
10 11 12 13 14
— — — —
15
— —
15
— —