MOTOROLA
MC68341 USER'S MANUAL
6- 35
When read, this register always contains the next source address. If a bus error terminates
the transfer, this register contains the next source address that would have been run had
the error not occurred.
6.8 DATA PACKING
The internal DHR is a 32-bit register that can serve as a buffer register for the data being
transferred during dual-address DMA cycles. No address is specified since this register can
not be addressed by the programmer. The DHR allows the data to be packed and
unpacked by the DMA during the dual-address transfer. For example, if the source operand
size is byte and the destination operand size is word, then for each DMA request two byte
read cycles occur, followed by one word write cycle (see Figure 6-16). The two bytes of
data are buffered in the DHR until the destination (write) word cycle occurs. The DHR
allows for packing and unpacking of operands for the following sizes: bytes to words, bytes
to long words, words to long words, words to bytes, long words to bytes, and long words to
words.
..
.. .
..
.. .
..
.. .
BYTE0
BYTE1
BYTE0
BYTE1
BYTE2
BYTE3
BYTE0
BYTE2
BYTE1
BYTE3
BYTE0
BYTE1
BYTE2
BYTE3
BYTE0
BYTE1
BYTE2
BYTE3
BYTE0
BYTE1
SOURCE/DESTINATION
DESTINATION/SOURCE
Figure 6-16. Packing and Unpacking of Operands
For normal transfers aligned with the size and address, only two bus cycles are required for
each transfer: a read from the source and a write to the destination.
6.9 DMA CHANNEL INITIALIZATION SEQUENCE
The following paragraphs describe DMA channel initialization and operation. If the DMA
capability of the MC68341 is being used, the initialization steps should be performed during
the part initialization sequence. The mode operation steps should be performed to start a
DMA transfer. The DONE≈ pin requires an external pullup resistor even if operating only in
the internal request mode.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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