Typewriter User Manual

MOTOROLA MC68340 USER’S MANUAL 6- 29

DSIZE—Destination Size Control Field

This field controls the size of the destination (write) bus cycle that the DMA channel is

running. Table 6-3 defines these bits.

Table 6-3. DSIZEx Encoding

Bit 7 Bit 6 Definition

0 0 Long Word*

0 1 Byte

1 0 Word

1 1 Not Used

*External logic is required to complete a long-

word transfer.

REQ—Request Generation Field

This field controls the mode of operation the DMA channel uses to make an operand

transfer request. Table 6-4 defines these bits.

Table 6-4. REQx Encoding

Bit 5 Bit 4 Definition

0 0 Internal Request at Programmable Rate

0 1 Reserved

1 0 External Request Burst Transfer Mode

1 1 External Request Cycle Steal

BB—Bus Bandwidth Field

This field controls the percentage of 1024 clock periods of the IMB that the DMA

channel can use during internal requests only. Table 6-5 defines these bits.

Table 6-5. BBx Encoding and Bus Bandwidth

REQ Field BB Field Bus Bandwidth

Bit 5 Bit 4 Bit 3 Bit 2 Definition (Clock Periods)

0 0 0 0 25% 256

0 0 0 1 50% 512

0 0 1 0 75% 768

0 0 1 1 100% 1024

Frees

cale Semiconductor,

Freescale Semiconductor, Inc.

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