PC87591L datasheet(90/437 Pages) NSC

90 / 437 page

Embedded Controller Modules (Continued)

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Revision 1.07

4.2.5

Bus Policy

Intermittent Operation Mode

Figure 30. DMAC Direct Bus Cycles in Intermittent Mode, DMRQ Asserted Constantly.

When BPC bit in DMACNTLn is 0, channel n is in Intermittent mode. In this mode, the DMAC channel relinquishes the bus

after each transaction, regardless of the state of its DMA request input. In this way, the DMAC gives the core (and other

DMA channels) a chance to use the bus, even if a DMA device needs the bus for multiple transfers.

Continuous Operation Mode

Figure 31. DMAC Direct Bus Cycles in Continuous Mode - External Bus

When BPC bit in DMACNTLn register is 1, channel n is in Continuous mode. In this mode, a DMAC channel uses the bus

continuously, as long as its request is active and BLTCn > 0. This allows the channel to utilize the full bandwidth of the bus.

The activity of this channel cannot be interrupted by any other internal bus master, including higher priority DMAC channels.

It is the system designer’s responsibility to limit the duration of the DMA request to prevent bus starvation.

CLK

DMRQi

ADDR

DMACKi

Tidle

Bus State

ADCA

Tidle

Core Cycle Address

CLK

DMRQi

ADDR

DMACKi

Tidle

Bus State

ADCA

Core Cycle Address

ADCA

PC87591L 数据表(PDF) 90 Page - National Semiconductor (TI)