Intel 80C188XL, 80C186XL user manual Module

Models: 80C186XL 80C188XL

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Module

DIRECT MEMORY ACCESS UNIT

The last point is extremely important when the two channels use different synchronization. For example, consider the case in which channel 1 is programmed for high priority and destination synchronization and channel 0 is programmed for low priority and source synchronization. If a DMA request occurs for both channels simultaneously, channel 1 performs the first transfer. At the end of channel 1’s deposit cycle, two idle states are inserted (thus releasing the bus). With the bus released, channel 0 is free to perform its transfer even though the higher-priority channel has not completed all of its transfers. Channel 1 regains the bus at the end of channel 0’s trans- fer. The transfers will alternate as long as both requests remain active.

 

Module

Timer 2

DMA Request

Internal - DMA

Inter-module

Request

Arbitration

Multiplexer

Logic

Timer 2

Timer 2 Request

Request

Source Pointer

Source Pointer

Destination Pointer

Destination Pointer

Channel 0

Channel 1

Control Logic

Control Logic

DRQ Pin

DRQ Pin

 

A1540-01

Figure 10-5. Two-Channel DMA Module

A higher-priority DMA channel will interrupt the transfers of a lower-priority channel. Figure 10-6 shows several transfers with different combinations of channel priority and synchronization.

10-9

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Intel 80C188XL, 80C186XL user manual Module