Rabbit Rabbit 4000, User Manual

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180 Rabbit 4000 Microprocessor User’s Manual
When a DMA transfer is occurring, normal code execution will not occur until the transfer
is completed. To prevent DMA transfers from excessively blocking interrupts or otherwise
interfering with normal code execution, two options can be set in DMTCR. First, the max-
imum limit of a DMA transfer can be set from 1 to 64 bytes, which sets an upper limit on
interrupt latency arising from a DMA transfer. Second, the minimum number of clocks
before the DMA can be active again can be set from 12 to 512 clocks, guaranteeing
processing time for the application.
The values providing roughly equal access to the memory bus for both the processor and
the DMA is eight bytes per burst and 64 clocks between bursts.
When starting up, the DMA requires several cycles of overhead. This overhead comes
about because the DMA actually uses part of the processor to perform the data transfers,
and consists of one instruction fetch time plus three clock cycles. The byte fetched during
the instruction fetch time is discarded, and will be refetched at the completion of the DMA
burst. At the end of the DMA burst, two clock cycles are required before this first instruc-
tion fetch starts. An individual DMA channel transfers data without any overhead between
bytes, but there is always one clock cycle of dead time when switching between DMA
channels. Table 19-3 shows the number of clock cycles required per burst, assuming a sin-
gle DMA channel transfer and no wait states.
The total number of clocks listed in Table 19-3 is related to the number of bystes per burst
by the following formula.
Total Clocks = 4 × Number of Bytes per Burst + 7 (for overhead)
Table 19-3. Maximum DMA Transfer Rates
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