MC68340 datasheet(262/441 Pages) FREESCALE

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6- 12

MC68340 USER’S MANUAL

MOTOROLA

6.4.2 Dual-Address Mode

The dual-address DMA bus cycle transfers data between a device or memory and the

DMA internal holding register (DHR). In this mode, any operand transfer takes place in

two DMA bus cycles, one where a device is addressed and one where memory is

addressed. The data transferred during a dual-address operation is either read from the

data bus into the DHR or written from the DHR to the data bus.

Each DMA channel can each be programmed to operate in the dual-address transfer

mode. In this mode, the operand is read from the source address specified in the SAR and

placed in the DHR. The operand read may take up to four bus cycles to complete because

of differences in operand sizes of the source and destination. The operand is then written

to the address specified in the DAR. This transfer may also be up to four bus cycles long.

In this manner, various combinations of peripheral, memory, and operand sizes may be

used. See 6.7 Register Description for more information.

The dual-address transfers can be started by either the internal request mode or by an

external device using the

DREQ

≈ input signal. When the external device uses DREQ≈, the

channel can be programmed to operate in either burst transfer mode or cycle steal mode.

6.4.2.1 DUAL-ADDRESS READ. During the dual-address read cycle, the DMA reads data

from a device or memory into the internal DHR. The device or memory is selected by the

address specified in the SAR, the source function codes in the FCR, and the source size

in the CCR. Data is read from the memory or peripheral and placed in the DHR when the

bus cycle is terminated. When the complete operand has been read, the SAR is

incremented by 0, 1, 2, or 4, according to the size and increment information specified by

the SSIZE and SAPI bits of the CCR. The DMA control signals (

DACK

≈ and DONE≈) are

asserted in the source (read) cycle when the source device makes a request. See Figures

6-9 and 6-10 for timing diagrams of dual-address read for external burst and cycle steal

modes.

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