Motorola MC68340 manual DSIZEx Encoding, REQx Encoding, BBx Encoding and Bus Bandwidth

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

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

MOTOROLAMC68340 USER’S MANUAL6- 29

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