80C186EC/80C188EC Microprocessor User's Manual

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).
T4
or T3
or T2
or T1
or TW
or TI
T4
or T3
or T2
or T1
or TI
T4
or T3
or TW
or TI
4
T4
or TI
T1
of
DMA
Cycle
DRQ
1
2 3
NOTES:
1. T CLIS
: 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.
A1187-0A
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 destinationsynchronized
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 moving
from memory to the disk). The type of synchronization a channel uses is programmable. (See
“Selecting Channel Synchronization” on page 10-23.)
10-4