Freescale Semiconductor MPC8260 - Manual (Page 877)

Multi-Channel Controllers (MCCs)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 28-29
28.5 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.
28.5.1 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 28-13 ). The only entries in the
superchannel table which must be initialized are those whose numbers appear in superchannelled entries
in the SIRAM.
0 1 2 9 10 11 12 13 14 15
Field 0 0 Channel Number 0 0 0 0 0 0
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 28-13. Super Channel Table Entry