MPC8280 datasheet(1155/1386 Pages) FREESCALE

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Inverse Multiplexing for ATM (IMA)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

34-9

34.3.1.1

User Plane Functions Performed by Microcode

•

ATM cell stream splitting and reconstruction

•

ICP cell insertion/removal

•

Cell rate decoupling (i.e. filler cell insertion/removal)

•

IMA frame synchronization

•

Stuffing

•

Discards cells with bad HECs (available on .25

µm rev B silicon and forward)

34.3.1.2

Plane Management Functions Performed by Microcode

As stated above, most plane management functions must be performed in software. However, certain

statistics are intimately related to the lower-level user plane functions, and are thus best provided by the

microcode. These include the following:

•

ICP violations

•

Transmit stuff events

•

Receive stuff events

34.3.2

Transmit Architecture

This section discusses the behavior of the IMA microcode during transmission, focusing particularly on

the independent transmit clock (ITC) mode of IMA. Differences in behavior when common transmit clock

(CTC) mode is used are discussed at the end of this section.

Only one cell scheduler (known as the ATM pace controller or APC) is used per IMA group. This APC

schedules transmission for the IMA group as a single aggregate channel. The APC hands these cells off to

the IMA Tx microcode, which distributes these scheduled cells to each of the PHYs in the IMA group. To

compensate for clocking differences (jitter and average speed differential), the IMA Tx process distributes

ATM cells into N jitter buffers with a depth of 5 cells. The IMA PHYs take cells from these jitter buffers

and transmit them.

The cell scheduling is triggered by requests from the timing reference link (i.e. assertion of TxClav from

the TRL’s PHY). Requests from non-TRL PHYs only interact with the jitter buffer, and perform the

stuffing function as needed (when the jitter buffer becomes too shallow). Therefore, the microcode tasks

performed in response to TRL PHY requests and non-TRL PHY requests are different.

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