12-25
DMA CONTROLLER
12.2.8 Cascade Mode
Cascade mode allows an external 8237A or another DMA-type device to gain bus control. A cas-
caded device requests bus control by holding a channel’s request input (DRQn) active. Once
granted bus control, the cascaded device remains bus master unt il it relinquis hes bus control b y
deactivating DRQn.
If a refresh request occurs while a cascaded device has bus control, the cascaded device must
deassert its request or the refresh cycle will be missed. The following steps take place in response
to a refresh request.
1. The chan nel deasse rts its acknowledge signal (DACK n#) to t he cascaded device.
—At this point, the cascaded device should relinquish bus control by removing DRQn.
2. As soon as DRQn is removed, the refresh cycle is started.
—At this p oint, if the cas caded devic e wants to regai n bus control a fter the refres h cycle,
it must reassert DRQn.
3. If the cascade d device has reasserted DRQn when the refresh cycle is complete, the
channel reasserts DACKn#, giving bus control back to the cascaded de vice without bus
priority arbitration.
The following flowchart (Figure 12-17) shows this process flow.