Motorola M68CPU32BUG Manuel d'utilisateur Page 10

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MOTOROLA MC68332TUT/D
10
Figure 5 DC Model of Oscillator Circuit
2.5.2.3 Layout and Strange Behavior
Oscillator layout is just as important as a good quality crystal and cleanliness in manufacturing the printed
circuit board. The best possible solution is to use a multi-layer board with a separate ground plane. The rules
for oscillator layout are quite simple. First, locate the crystal and all associated external components as
close to the oscillator pins as possible. Second, do not under any circumstance run a high frequency trace
under either the feedback or series resistor or the crystal. Third, if there is no separate ground plane, make
sure that the ground for the bypass capacitors is connected to a solid ground trace.
Figure 6 shows typical
one-layer oscillator layout.
Figure 6 Typical One-Layer Oscillator Layout
Do not run high frequency conductors near, and particularly underneath, the crystal, the feedback resistor
or the series resistor. In Figure 6, only a ground trace runs underneath these components. Also note that,
in Figure 6, the ground trace is tied to the ground pin nearest to the oscillator pins. This helps prevent large
loop currents in the vicinity of the crystal. It is also very important to tie the ground pin to the most solid
ground in the system. The trace that connects the oscillator and the ground plane should not connect to any
other circuit element as the injection of current into this trace tends to make the oscillator unstable.
332TUT XTAL RF/RD CONN
XTAL EXTAL
R
f
TO +5 OR GND
R
d
A
B
C
INTERNAL AMPLIFIER
10 M
10 M
332TUT XTAL PCB LAYOUT
XTAL
R
s
C
bypass
C
bypass
GROUND
PLANE
R
f
EXTAL GND XTAL
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