Intel PXA255, Developer'S Manual

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5-4 Intel® PXA255 Processor Developer’s Manual
DMA Controller
If all channels request data transfers, the Sets are prioritized in following order:
•
Set zero
•
Set one
•
Set zero
•
Set two
•
Set zero
•
Set one
•
Set zero
•
Set three
The pattern repeats for the next eight channel services. In each set, the channels are given round-
robin priority.
The state machine used to determine the priority of the DMA channels is shown in Table 5-3 . Use
this table to determine the exact sequence the DMA controller gives to each channel when not all
channels are running concurrently.
The channels get a round-robin priority in each set. Out of reset, the state machine state is zero. If a
channel in set zero has a pending request, that channel is serviced. If a channel in set one has a
pending request, that channel is serviced and so on. Once a request is serviced, the state machine
Table 5-2. Channel Priority (if all channels are running concurrently)
Set Channels Priority Number of times served
0 0,1,2,3 Highest 4 / 8
1 4,5,6,7 Higher 2 / 8
2 8,9,10,11 Low 1 / 8
3 12,13,14,15 Low 1 / 8
Table 5-3. Channel Priority
State Machine
State
DMA Set Priority within each State Machine State
0 S0 > S1 > S2 > S3
1 S1 > S0 > S3 > S2
2 S0 > S1 > S2 > S3
3 S2 > S3 > S0 > S1
4 S0 > S1 > S2 > S3
5 S1 > S0 > S3 > S2
6 S0 > S1 > S2 > S3
7 S3 > S2 > S1 > S0