Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 970

Powerquicc family

Table of Contents

ATM Overview

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

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.

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.

shows the structure of an expanded cell.

Expanded Header

(0-12 bytes)

HEC - Optional (1 byte)

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.

MPC885 PowerQUICC Family Reference Manual, Rev. 2

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