AN2168 Freescale Semiconductor / Motorola, AN2168 Datasheet - Page 10

• Manufacturer Part Number: AN2168
• Description: ColdFire Microprocessor DMA Controller Application Note
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Continuous and Cycle Steal Modes
Request Modes
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In continuous mode when the internal or external DMA request is
received and the DMA transfer starts, the DMA will continue to transfer
data until:
In cycle steal mode, there is one DMA transfer per request. Instead of
running multiple bus cycles until the entire byte count is transferred,
there is just one read and one write phase for each request (only one
read or write phase for single address mode).
For example, transferring 32 bytes of data from a longword port to a byte
port using cycle steal mode requires eight requests. For each request a
longword is read from the source, and then the same longword (four
bytes) is written to the destination. Since each transfer decrements the
BCR by four, it will take eight transfers to move the entire 32 bytes
(32/4 = 8). When the eighth transfer completes, the BCR will be cleared
and the DONE bit in the DSR will be set to indicate the completion of the
entire DMA transfer.
The DMA controller has two different types of requests that can be used
to start a DMA transfer:
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A multiple of the BWC value is reached — The DMA will release
the bus when the BCR reaches a multiple of the value
programmed into the BWC.
The transfer completes — The BCR reaches zero, the DMA
terminates with an error, or the DONE bit is set by software
(internal request only).
Internal requests
External requests
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AN2168