B-3
COMPATIBILITY WITH THE PC/AT* ARCHITECTURE
Figure B-1. Derivation of AEN Signal in a Typical PC/AT System
For systems based on Intel386 EX processor, the AEN signal could be derived as shown in Figure
B-2. Notice that since the DMA acknowledge signals are used instead of a generic HLDA, there
is no need to incorporate the REFRESH# signal in the logic.
Figure B-2. Derivation of AEN Signal for Intel386™ EX processor-based Systems
In a PC/AT system using the 8237A DMA controller in fly-by mode, the 8237A generates appro-
priate control signals for memory (MEMR# or MEMW#) and for I/O (IOW# and IOR#). The
Intel386 EX processor’s internal DMA, during fly-by transfers, generates control signals (M/IO#
and W/R#) that apply to the memory device. There needs to be some external logic that can detect
the DMA operation (through the AEN signal) and generate a complementary I/O cycle. For ex-
ample, if the DMA is generating a memory read cycle and AEN is active, then the logic should
drive the IOW# signal on the PC/AT bus. Actually, the internal DMA could be programmed in a
two-cycle mode eliminating the need for external logic. This will not have a significant impact
on the performance — of the two cycles required to complete the transfer, the I/O cycle is the long
one (meeting PC/AT timings) while the memory cycle is relatively very quick.
The drive capability and the operating frequency of the Intel386 EX processor signals are differ-
ent from the standard PC/AT bus, which requires 24 mA drive capacity at 200 pF capacitive load.
Most PC/AT systems presently operate in a “quiet bus” mode so that non-ISA cycles are not re-
flected on the ISA bus. In a typical implementation, the address/data buses may change states, but
the control signals are not strobed if a non-ISA cycle is detected. External three-state buffers and
some decoding logic are needed to implement this scheme. The EV386EX (evaluation board for
A2504-01
HLDA
MASTER#
(From PC/AT* Bus)
AEN
Processor
A2503-01
Processor
DACK0#
DACK1#
DACK
MASTER# (From PC/AT* Bus)
AEN
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