Buffer Descriptors and Connection Tables
MPC885 PowerQUICC Family Reference Manual, Rev. 2
37-6
Freescale Semiconductor
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
.
0x00
(cont.)
15
CR
Receive CRC error. Indicates a CRC32 error for AAL5 channels or a CRC10 error for
AAL0 channels. The receiver performs CRC32 checking on the frame for AAL5 channels
and sets the CR bit in the last BD when a CRC error is detected. For AAL0 channels, the
receiver performs CRC10 checking on the cell payload and sets the CR bit if a CRC error
is detected.
0 No error
1 CRC32 error for AAL5 channels; CRC10 failure for AAL0 channels
Note:
CRC10 checking for AAL0 channels can be disabled by setting the channel’s
RCT[NCRC] (no CRC). RxBD[CR] is set only when the received cell includes a CRC10
field in the payload and an error is detected. This check is provided to support OAM
CRC10 according to the ITU specification I.610.
0x02
—
Data length/
CH_CODE
Data length(AAL5). For AAL5 BDs, the data length is the number of octets written by the
CP into the BD’s data buffer. When the buffer is the last buffer in the frame (BD[L] = 1), the
data length contains the total number of frame octets. This field is written by the CP as the
buffer is closed.
Channel code (AAL0 only). This field applies only when filtering is activated
(SRSTATE[MCF] = 1). When not operating in extended channel mode, the CH_CODE field
contains the channel’s RCT address in dual-port RAM. In extended channel mode,
CH_CODE is the actual channel number.
0x04
—
Receive
data buffer
pointer
Receive data buffer pointer. Points to the first location of this BD’s data buffer, which may
reside in either internal or external memory. This pointer must be burst aligned (divisible
by 16).
0x08
—
CPCS-UU
and CPI
CPCS-UU and CPI (AAL5 only). Contains the frame’s CPCS-UU and CPI fields. This field
is taken from the frame trailer, and contains user-to-user (UU) information and common
part indications (CPI). This field is written by the CP and is valid only for the last BD in the
AAL5 frame.
0x0A
—
—
Reserved, should be cleared.
0x0C
—
Cell header
expansion 1,
2, and 3
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).
Table 37-1. ATM RxBD Field Descriptions (continued)
Offset from
RBD_PTR
Bits
Name
Description
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