Intel 80C186XL, User Manual

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10-5
DIRECT MEMORY ACCESS UNIT
10.1.4.1 Source Synchronization
A typical source-synchronized transfer is shown in Figure 10-3. Most DMA-driven peripherals
deassert their DRQ line only after the DMA transfer has begun. The DRQ signal must be deas-
serted at least four clocks before the end of the DMA transfer (at the T1 state of the deposit phase)
to prevent another DMA cycle from occurring. A source-synchronized transfer provides the
source device at least three clock cycles from the time it is accessed (acknowledged) to deassert
its request line if further transfers are not required.
Figure 10-3. Source-Synchronized Transfers
10.1.4.2 Destination Synchronization
A destination-synchronized transfer differs from a source-synchronized transfer by the addition
of two idle states at the end of the deposit cycle (Figure 10-4). The two idle states extend the DMA
cycle to allow the destination device to deassert its DRQ pin four clocks before the end of the
cycle. If the two idle states were not inserted, the destination device would not be able to deassert
its request in time to prevent another DMA cycle from occurring.
The insertion of two idle states at the end of a destination synchronization transfer has an impor-
tant side effect. A destination-synchronized DMA channel gives up the bus during the idle
states, allowing any other bus master to gain ownership. This includes the CPU, the Refresh
Control Unit, an external bus master or another DMA channel.
T1 T2 T3 T4
CLKOUT
DRQ (Case 1)
T1 T2 T3 T4
DRQ (Case 2)
Fetch Cycle Deposit Cycle
NOTES:
1. Current source synchronized transfer will not be immediately
followed by another DMA transfer.
2. Current source synchronized transfer will be immediately
followed by another DMA transfer.
2
1
A1188-0A