NXP Semiconductors MPC5566, Reference Manual

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Enhanced Direct Memory Access (eDMA)
MPC5566 Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor 9-45
9.4.4 DMA Arbitration Mode Considerations
9.4.4.1 Fixed-Group Arbitration and Fixed-Channel Arbitration
In this mode, the channel service request from the highest priority channel in the highest priority group is
selected to execute. If the eDMA is programmed so the channels within one group use ‘fixed’ priorities,
and that group is assigned the highest ‘fixed’ priority of all groups, it is possible for that group to take all
the bandwidth of the eDMA controller; that is, no other groups is serviced if there is always at least one
DMA request pending on a channel in the highest priority group when the controller arbitrates the next
DMA request. The advantage of this scenario is that latency can be small for channels that need to be
serviced quickly. Preemption is available in this scenario only.
9.4.4.2 Round-Robin Group Arbitration, Fixed-Channel Arbitration
The occurrence of one or more DMA requests from one or more groups, the channel with the highest
priority from a specific group is serviced first. Groups are serviced starting with the highest group number
with a service request and rotating through to the lowest group number containing a service request.
After the channel request is serviced, the group round-robin algorithm selects the highest pending request
from the next group in the round-robin sequence. Servicing continues using the round-robin method,
always servicing the highest priority channel in the next group in the sequence, or just skipping a group if
it has no pending requests.
If a channel requests service at a rate that equals or exceeds the round-robin service rate, then that channel
is always serviced before lower priority channels in the same group, and thus the lower priority channels
never are serviced. The advantage of this scenario is that no one group uses all the eDMA bandwidth. The
highest priority channel selection latency is potentially greater than fixed/fixed arbitration. Excessive
request rates on high priority channels can prevent the servicing of lower priority channels in the same
group.
9.4.4.3 Round-Robin Group Arbitration, Round-Robin Channel Arbitration
Groups are serviced as described in Section 9.4.4.2, “Round-Robin Group Arbitration, Fixed-Channel
Arbitration , but this time channels are serviced in channel number order. Only one channel is serviced from
each requesting group for each round-robin pass through the groups.
Within each group, channels are serviced starting with the highest channel number and rotating through to
the lowest channel number without regard to channel priority levels.
Because channels are serviced using a round-robin method, any channel that generates DMA requests
faster than a combination of the group round-robin service rate and the channel service rate for its group
does not prevent the servicing of other channels in its group.
This scenario ensures that all channels are guaranteed service at some point, regardless of the request rates.
However, the potential latency can be quite high. All channels are treated equally. Priority levels are not
used in round-robin/round-robin mode.