Intel 80C186XL, 80C188XL user manual External Requests

DIRECT MEMORY ACCESS UNIT

10.1.4 External Requests

External DMA requests are asserted on the DRQ pins. The DRQ pins are sampled on the falling edge of CLKOUT. It takes a minimum of four clocks before the DMA cycle is initiated by the BIU (see Figure 10-2). The DMA request is cleared four clocks before the end of the DMA cycle (effectively re-arming the DRQ input).

				T1
T1 or	T2 or	T3 or		of
TW or	TW or	TW or	T4 or	DMA
TI	TI	TI	TI	Cycle
		4

DRQ 1

2 3

NOTES:

1.TINVCL : DMA request to clock low.

2.Synchronizer resolution time.

3.DMA unit priority arbitration and overhead.

4.Bus interface unit latches DMA request and decides to run DMA cycle.

A1528-0A

Figure 10-2. DMA Request Minimum Response Time

External requests (and the resulting DMA transfer) are classified as either source-synchronized or destination-synchronized. A source-synchronized request originates from the peripheral that is sending data. For example, a disk controller in the process of reading data from a disk would use a source-synchronized request (data would be moving from the disk to memory). A destination- synchronized request originates from the peripheral that is receiving data. If a disk controller were writing data to a disk, it would use a destination-synchronized request (data would be mov- ing from memory to the disk). The type of synchronization a channel uses is programmable. (See “Selecting Channel Synchronization” on page 10-18.)

10-4