Intel 80386

I/O INTERFACING
logic must use HLDA to disable the output buffers of the 80386 bus controller, data bus,
and address bus, and enable the output buffers of the 82258 and its bus controller, data bus,
and address bus.
The wait-state generator must be started with the ALE output of the 82288 Bus Controller.
Although the 82258 can share wait-state generator logic with the 80386, the logic must be
modified to support longer wait states for 82258 cycles. The 82258 divides its CLK input by
two internally, so that its wait state requires two CLK cycles rather than one. In addition,
the READY # output must meet the 82258 input hold time requirement of 25 nanoseconds.
When the 82258 completes its operation, its HOLD request goes inactive (low), disabling
the 82258 output buffers and re-enabling the 80386 and its output buffer.
8.6.6.2 82258 AS PERIPHERAL
Although most of the communication between the 80386 and the 82258 is achieved indirectly
through memory, the 80386 occasionally performs bus cycles to the 82258. For example, the
80386 performs a direct access to set the general mode register during initialization. The
access occurs asynchronously over the slave mode interface of the 82258 that is shown in
Figure 8-15. The interface consists of the following pins:
• Chip Select (CS#)-enables the slave mode interface
• Control (RD#, WR#)-indicates bus cycle type (asynchronous interface)
• Register Address (A7-AO)-selects an 82258 internal register
• Data (D15-DO)-transfers data to and from the 82258
With the 82258,asynchronous cycles must be used to allow time for the conversion of 80386
status signals to 80286-compatible inputs. The SO# and S1# inputs of the 82258 must there-
fore be inactive (high) during slave mode operations. Asynchronous cycles require an address
hold time after a read command or write command, so the address outputs from the 80386
ADMAEN# cs#
A7-
FROM ADDRESS
AO
DECODER
HIGH 50#
80386
BHE#
LOCAL
HIGH
51#
BUS
FROM
80386
{ RD#
RD#
BUS
WR#
WR#
CONTROL
LOGIC
D15-DO
82258
G30107
Figure 8-15. 82258 Slave Mode Interface
8-26