MPC8280 datasheet(918/1386 Pages) FREESCALE

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Multi-Channel Controllers (MCCs)

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

29-28

Freescale Semiconductor

29.5

Superchannels

A TDM may not be programmed to contiguously transmit more than one byte of data from the same MCC

channel. This is true whether the user wants to program more than one byte in the same SI entry or have

back-to-back SI entries for the same channel. Instead, superchannelling is used to achieve sending multiple

back-to-back bytes from the same MCC channel. Refer to Section 15.4.3, “Programming SIx RAM

Entries,” for information about how to program SIRAM entries as superchannelled timeslots.

A single MCC channel is the combination of one MCC FIFO and one set of related channel parameters

and buffer descriptors. A superchannel is the combination of multiple MCC TX channels’ FIFOs and one

set of an MCC channel’s parameters and buffer descriptors. In this case, the one set of parameters and

buffer descriptors is used to manage this group of FIFOs. In effect, this provides the ability to construct a

larger overall FIFO than that of a single normal MCC TX channel.

MCC TX channels whose numbers appear in superchannelled SIRAM entries are dedicating their TX

FIFOs to that superchannel and may not be used for any other purpose (i.e. cannot also be used elsewhere

in a TDM as a normal channel). The FIFO of the channel whose parameters are used to control a

superchannel may still be used as part of the superchannel.

Although a timeslot for a normal MCC channel may be of any length up to 8 bits, a superchannelled

timeslot must always be 8 bits.

Although a normal MCC transmit FIFO is 4 bytes, one that is used as part of a superchannel is 2 bytes.

Note that an MCC channel whose FIFO is in superchannel mode consumes twice as much CPM bandwidth

as a normal channel.

29.5.1

Superchannel Table

When the SI encounters an SIRAM entry that is programmed to be part of a superchannel, the channel

number in that SIRAM entry represents which MCC channel’s FIFO is to be used during that timeslot.

Later, when that FIFO requires service from the CPM, a lookup occurs using the Superchannel Table

(SCT).

The SCT serves as a mapping between the FIFOs being used as part of a superchannel (as programmed in

SIRAM) and which channel’s parameters are being used to manage that superchannel. The MCC channel

FIFO number used in the MCSEL field of the superchannelled SIRAM entry is used to calculate an offset

to a superchannel table entry that contains the MCC channel number whose parameters and buffer

descriptors are being used to control that superchannel (see Figure 29-13). The only entries in the

superchannel table which must be initialized are those whose numbers appear in superchannelled entries

in the SIRAM.

Field

Channel Number

Addr

DPRAM_base_address+SCTPBASE+2*MCC_FIFO_number

(MCC_FIFO_number is the number written in the MCSEL field of the corresponding SI RAM entry)

Figure 29-13. Super Channel Table Entry

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