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RC
CIRCUIT
LOCAL BUS INTERFACE
30pF
HO:
'"~
200 pI
5pH
X,
X,
~
RES#
RESET
~
82384
F/C#
CLK2
r-
j"OO' j'"
TO
SUPPORT
PERIPHERALS
RESET
CLK2
Figure 3-15. Connecting 82384 to 80386
ADS#
-
80386
G30107
The phase of the CLK output of the 82384 is useful for determining the beginning of a bus
cycle. Because each CLK2 cycle is 31.25 nanoseconds (at CLK2
=
32 MHz), and bus
status signal delays may be as much as 35 nanoseconds, it is impossible to tell from these
status signals alone which CLK2 cycle begins the bus cycle, and therefore when to expect
valid address signals. The phase of CLK can be used to make this determination. ADS#
should be sampled on rising CLK2 transitions when CLK is high, i.e., at the end of phase 2
(see Figure 3-16).
3.3.3 Crystal Oscillator Clock Generator
Figure 3-17 shows a clock generator circuit for the 80386.
It
is implemented with TTL and
CMOS components and is functionally similar to the 82384 clock generator. This circuit
replaces the 82384 and is recommended for new designs.
It
provides the CLK2, CLK, and
RESET signals needed by the 80386 and local bus controller. The CLK2 iiignal provides the
fundamental timing for the 80386 and is generated by a CMOS crystal oscillator. This oscil-
lator must have a CMOS output buffer guaranteed compatible with the 80386 CLK2 input.
3-27
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