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Rabbit 6000 User’s Manual
digi.com
259
The total number of clocks listed in Table 24-4 is related to the number of bytes per burst by the following
formula.
Total Clocks = 4 × Number of Bytes per Burst + 7 (for overhead)
24.3.4 DMA Channel Priority
It is possible to control the priority between separate DMA channels. There are three channel-priority
options in the Rabbit 6000. The first is fixed priority after every byte where the priority of each channel is
equal to its number, i.e., if both DMA Channels 3 and 4 have a pending transfer request, DMA Channel 4
will always be enabled first. If at any point a channel with higher priority than the one currently transfer-
ring has a DMA request pending, the current transfer will be suspended and the new channel’s transfer will
start. With this setting, DMA Channel 15 will always have priority over all other channels, and DMA
Channel 0 will transfer only if no other channels have pending requests.
The other two settings rotate the priority among channels as shown in Table 24-5; after the fifteenth rota-
tion, the priority sequence restarts at the top of the table. One option is to rotate priority after every byte
analogous to the fixed-priority setting. The priority list is updated after each byte transferred, and if a
higher priority channel has a pending request, the current transfer will be suspended and the new channel
transfer will start. The other option is to rotate after every burst; this will guarantee that reasonable
amounts of data are transferred by each channel before a switchover occurs.
There is a separate priority setting for both the bus-interleaving and bus-sharing modes, since both DMA
modes may be occurring depending on the hardware setup.
Table 24-4. Maximum DMA Transfer Rates (Bus Sharing Mode)
Setting
Total Clocks
Clocks per Byte
Transferred
1 byte per burst
11 clocks
11
2 bytes per burst
15 clocks
7.5
3 bytes per burst
19 clocks
6.3
4 bytes per burst
23 clocks
5.8
8 bytes per burst
39 clocks
4.9
16 bytes per burst
71 clocks
4.4
32 bytes per burst
135 clocks
4.2
64 bytes per burst
263 clocks
4.1
Table 24-5. Example of Rotating DMA Channel Priority
Rotation
Channel Priority, High to Low
Initial
15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0
First
14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 15
Second
13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 15, 14
Third
12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13
Содержание 6000
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