ATM Overview
MPC885 PowerQUICC Family Reference Manual, Rev. 2
36-8
Freescale Semiconductor
compression tables in external RAM, or an external content-addressable memory (CAM). A cell header
that has no match is treated as an AAL0 cell and is passed to the global raw cell queue (usually defined by
convention to be channel number 0). If the cell header is matched to an active channel, the payload (48
bytes) is copied to the current buffer, the CRC is calculated (optional for AAL0), and the RCT parameters
are updated. If, however, the current buffer is not empty, a busy interrupt is optionally generated and the
cell is discarded.
For AAL5, when a cell with an end-of-frame marker (indicated by PTI[1]) is received, the receiver
separates the trailer of the CPCS-PDU from the user data. The pads are removed as required, the length
field is checked against the length which was calculated during the frame receive, the CPCS-UU and CPI
are copied to the RxBD, and the buffer is closed. An interrupt is optionally generated to declare the end of
received frame. If a CRC or length error occurs, it is marked in the RxBD and an interrupt is optionally
generated.
For AAL0, the ATM controller copies the cell (except the HEC) from the UTOPIA interface to the
channel’s current buffer and optionally performs a CRC10 check on the cell payload. The CRC10 option
is used to support OAM cell checking (by host software) according to the ITU specification I.610.
Note that the received HEC is not checked by the ATM controller in UTOPIA mode; it is the responsibility
of the PHY to check the HEC and discard cells with an incorrect HEC.
36.6.3
Expanded Cells
An option for supporting ATM cells larger than the standard 53 bytes (4-byte header, 1-byte HEC and
48-byte payload) is available when operating in UTOPIA mode.
The MPC885 supports cells up to 65 bytes in length (referred to as expanded cells) that use extra header
fields for internal information in switching applications. Expanded cells consist of an expanded header of
0 to 12 bytes, a cell header (4 bytes), and a payload (48 bytes); the HEC can be optionally included.
Figure 36-2
shows the structure of an expanded cell.
Figure 36-2. Expanded Cell Structure
For transmitting, the expanded header of each cell is taken from the expansion field in the TxBD and sent
ahead of the cell header and payload. For receiving, the expanded header of the last cell of the current
connection is copied to the expansion field in the connection’s RxBD.
Expanded Header
Payload
Header
(0-12 bytes)
(4 bytes)
(48 bytes)
HEC - Optional (1 byte)
Содержание PowerQUICC MPC870
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Страница 118: ...MPC885 Overview MPC885 PowerQUICC Family Reference Manual Rev 2 1 20 Freescale Semiconductor ...
Страница 158: ...The MPC8xx Core MPC885 PowerQUICC Family Reference Manual Rev 2 3 18 Freescale Semiconductor ...
Страница 192: ...MPC885 Instruction Set MPC885 PowerQUICC Family Reference Manual Rev 2 5 22 Freescale Semiconductor ...
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Страница 606: ...SDMA Channels and IDMA Emulation MPC885 PowerQUICC Family Reference Manual Rev 2 19 20 Freescale Semiconductor ...
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Страница 780: ...SCC Ethernet Mode MPC885 PowerQUICC Family Reference Manual Rev 2 27 24 Freescale Semiconductor ...
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Страница 882: ...Universal Serial Bus USB MPC885 PowerQUICC Family Reference Manual Rev 2 31 34 Freescale Semiconductor ...
Страница 944: ...Parallel I O Ports MPC885 PowerQUICC Family Reference Manual Rev 2 34 26 Freescale Semiconductor ...
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Страница 1004: ...Buffer Descriptors and Connection Tables MPC885 PowerQUICC Family Reference Manual Rev 2 37 30 Freescale Semiconductor ...
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Страница 1068: ...ATM Pace Control MPC885 PowerQUICC Family Reference Manual Rev 2 40 22 Freescale Semiconductor ...
Страница 1090: ...UTOPIA Interface MPC885 PowerQUICC Family Reference Manual Rev 2 43 8 Freescale Semiconductor ...
Страница 1120: ...AAL2 Implementation MPC885 PowerQUICC Family Reference Manual Rev 2 44 30 Freescale Semiconductor ...
Страница 1162: ...Fast Ethernet Controller FEC MPC885 PowerQUICC Family Reference Manual Rev 2 45 40 Freescale Semiconductor ...
Страница 1172: ...SEC Lite Overview MPC885 PowerQUICC Family Reference Manual Rev 2 46 8 Freescale Semiconductor ...
Страница 1176: ...SEC Lite Address Map MPC885 PowerQUICC Family Reference Manual Rev 2 47 4 Freescale Semiconductor ...
Страница 1214: ...SEC Lite Execution Units MPC885 PowerQUICC Family Reference Manual Rev 2 48 38 Freescale Semiconductor ...
Страница 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
Страница 1313: ...MPC885 PowerQUICC Family Reference Manual Rev 2 Freescale Semiconductor B 1 Appendix B Serial Communications Performance TBD ...
Страница 1314: ...Serial Communications Performance MPC885 PowerQUICC Family Reference Manual Rev 2 B 2 Freescale Semiconductor ...
Страница 1320: ...Register Quick Reference Guide MPC885 PowerQUICC Family Reference Manual Rev 2 C 6 Freescale Semiconductor ...
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Страница 1386: ...Revision History MPC885 PowerQUICC Family Reference Manual Rev 2 I 2 Freescale Semiconductor ...