MC68HC08XL36 Datasheet, PDF (119/362 Pages) Freescale Semiconductor, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Direct Memory Access Module (DMA)
Functional Description
DMA
Source/Destination
Address
Calculation
Three 16-bit buses connect the 16-bit DMA arithmetic/logic unit (ALU) to
the DMA channel registers. During a DMA transfer, the DMA ALU:
• Calculates the transfer source and transfer destination addresses.
• Increments the DMA byte count register for each byte transferred.
• Determines when a block or loop transfer is complete by
comparing the DMA byte count register with the value
programmed in the DMA block length register.
The DMA source address register and destination address register
contain the base addresses for a DMA transfer. The DMA ALU uses
these address registers as base pointers when it starts the transfer. The
DMA byte count register contains the number of bytes transferred in the
current DMA operation. The DMA ALU uses the source/destination
address registers and the byte count register to calculate the actual
source and destination addresses in the following manner:
• When an address is configured to increment, the DMA ALU adds
the byte count register to the base address.
• When an address is configured to decrement, the DMA ALU
subtracts the byte counter register from the base address.
• When an address is configured to remain static, the DMA ALU
uses the base address as is.
The DMA can be programmed to:
• Stop the transfer after a number of bytes is transferred or
• After a number of bytes is transferred, loop back to the base
addresses and continue the transfer.
Figure 5 through Figure 13 show how the DMA calculates source and
destination addresses.
13-dma_b
MOTOROLA
Direct Memory Access Module (DMA)
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MC68HC08XL36
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