MPC8280 datasheet(1190/1386 Pages) FREESCALE

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Inverse Multiplexing for ATM (IMA)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

34-44

Freescale Semiconductor

34.4.6.1

Transmit Queues

Transmit queues are allocated on a per-link basis. They are circular queues of 64-byte buffers, defined by

their start and end pointers. For the transmit queue of the timing reference link (TRL), the queue must

consist of a minimum of four buffers (although it may consist of five buffers, if consistency of data

structures is desired). For the transmit queues of non-TRL links, the queues must consist of five buffers.

Queue fill and extract pointers must be initialized by software to the start of the queue; thereafter, these

pointers are managed by the microcode. The queue pointers must be on a 64-byte aligned boundary.

Figure 34-26. IMA Transmit Queue

34.4.6.2

Delay Compensation Buffers (DCB)

Cells received on a link are initially stored in a delay compensation buffer (DCB). DCBs are allocated on

a per-link basis. They are of user-definable length, thereby providing a programmable maximum

synchronizable delay. Note that DCBs of links within a group must all have the same size.

DCBs consist of a circular queue of 64-byte cell buffers. These cell buffers contain the received cell

followed by 12 bytes of header/status information.

Figure 34-27. Cell Buffer in Delay Compensation Buffer

The length of a DCB is defined by the link’s DCBSP and DCBEP parameters. The length of the DCB

(defined by (DCBEP-DCBSP) x 16) must be an integer multiple of the IMA frame length in bytes, M x 64.

Furthermore, the DCBSP must be aligned on a M x 64-byte boundary. For example, if M = 64, then DCBSP

must be on a 4KB boundary. To ensure group delay synchronization, the minimum length of the DCB

should be 2(Mx64).

IMA External Structure Region

Transmit

IMAEXTBASE

Queue

ITQSP

bits 0-11

bits 12-27

0000

bits 28-31

IMAEXTBASE

ITQEP

0000

IMAEXTBASE

64-byte boundary

ITQSP + TQ_SIZE - 4

64 bytes

12 bytes HEADER/STATUS

52 bytes RX CELL

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