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place in one bus cycle, where only the memory is explicitly addressed. The DMA bus
cycle may be either a read or a write cycle. The DMA provides the address and control
signals required for the operation. The requesting device either sends or receives data to
or from the specified address. Only external requests can be used to start a transfer when
the single-address mode is selected. An external device uses DREQ≈ to request a
transfer.
Each DMA channel can be independently programmed to provide single-address
transfers. The CCR ECO bit controls whether a source read or a destination write cycle
occurs on the data bus. If the ECO bit is set, the external handshake signals are used with
the source operand and a single-address source read occurs. If the ECO bit is cleared,
the external handshake signals are used with the destination operand, and a single-
address destination write occurs. The channel can be programmed to operate in either
burst transfer mode or cycle steal mode. See 6.7 Register Description for more
information.
If external 32-bit devices and a 32-bit bus are used with the MC68340, the DMA can
control 32-bit transfers between devices that use the 32-bit bus in single-address mode
only. External logic is required to complete a 32-bit (long-word) transfer. If both byte and
word devices are used on an external bus, then an external multiplexer must be used to
correctly transfer data. The SIZx and A0 signals can be used to control this external
multiplexer.
6.4.1.1 SINGLE-ADDRESS READ. During the single-address source (read) cycle, the
DMA controls the transfer of data from memory to a device. The memory selected by the
address specified in the source address register (SAR), the source function codes in the
function code register (FCR), and the source size in the CCR provides the data and
control signals on the data bus. This bus cycle operates like a normal read bus cycle. The
DMA control signals (DACK≈ and DONE≈) are asserted in the source (read) cycle. See
Figures 6-5 and 6-6 for timing diagrams single-address read for external burst and cycle
steal modes.
MOTOROLA
MC68340 USER’S MANUAL
6- 7
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