Freescale Semiconductor MPC5553, Reference Manual

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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 9-43
to take all the bandwidth of the eDMA controller; that is, no other groups will be 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.
9.5.4.2 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 will be 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 will select the highest pending
request from the next group in the round robin sequence. Servicing continues round robin, 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
will always be serviced before lower priority channels in the same group, and thus the lower priority
channels will never be serviced. The advantage of this scenario is that no one group will consume all the
eDMA bandwidth. The highest priority channel selection latency is potentially greater than fixed/fixed
arbitration. Excessive request rates on high priority channels could prevent the servicing of lower priority
channels in the same group.
9.5.4.3 Round Robin Group Arbitration, Round Robin Channel Arbitration
Groups will be serviced as described in Section 9.5.4.2, but this time channels will be 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 in round robin manner, 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 will not
prevent the servicing of other channels in its group. Any DMA requests that are not serviced are simply
lost, but at least one channel will be serviced.
This scenario ensures that all channels will be guaranteed service at some point, regardless of the request
rates. However, the potential latency could be quite high. All channels are treated equally. Priority levels
are not used in round robin/round robin mode.
9.5.4.4 Fixed Group Arbitration, Round Robin Channel Arbitration
The highest priority group with a request will be serviced. Lower priority groups will be serviced if no
pending requests exist in the higher priority groups.
Within each group, channels are serviced starting with the highest channel number and rotating through to
the lowest channel number without regard to the channel priority levels assigned within the group.
This scenario could cause the same bandwidth consumption problem as indicated in Section 9.5.4.1, but
all the channels in the highest priority group will get serviced. Service latency will be short on the highest
priority group, but could potentially get very much longer and longer as the group priority decreases.