P R E L I M I N A R Y
model components for a given crystal (i.e., L
1
, C
1
, R
1
,
and C
O
). In addition to these parameters, the manufac-
turer will provide a load capacitance specification usu-
al l y de s ig na te d as C
L
. T h e l o ad c ap ac i ta n ce
specification is the capacitive load at which the manu-
facturer has tuned the crystal for the specified fre-
quenc y. It is ther efor e req uir ed that the load
capacitance in the oscillator circuit is duplicated as
closely as possible to the manufacturer鈥檚 load capaci-
tance specification.
The crystal load capacitance in the circuit consists of
the capacitor network C
O
, C
STRAY
, C
D
, and C
G
. This net-
work reduces to (C
O
+ C
STRAY
) in parallel with the series
combination of C
D
and C
G
. Therefore, the desired se-
ries combination of C
D
and C
G
is equal to C
L
鈥?(C
O
+
C
STRAY
), where C
L
is the crystal manufacturer鈥檚 load ca-
pacitance specification.
The series combination of C
D
and C
G
=
(
C
D
脳
C
G
)
铮?/div>
--------------------------
铮?/div>
-
铮?
C
D
+ C
G
) 铮?/div>
C
STRAY
is typically difficult to determine. Some value
can be assumed and experimentation will determine
the optimal value for C
D
and C
G
. In determining the ex-
ternal component values to provide the optimal operat-
ing frequency, there are some recommended limits to
ensure a reasonable start-up time for the oscillator cir-
cuit. These limits are shown in Table 49.
Table 49. Recommended Oscillator Component
Value Limits
Minimum
R
F
R
B
C
D
C
G
14 M鈩?/div>
0
鈩?/div>
10 pF
10 pF
Maximum
18 M鈩?/div>
10 k鈩?/div>
30 pF
30 pF
脡lanSC300 Microcontroller
X32IN (201)
X32OUT (202)
R
B
R
F
XTAL
A
B
A
C
O
L
1
C
1
R
1
B
C
D
C
STRAY
C
G
Notes:
For board layout suggestions, refer to the 脡lanSC300 Microcontroller Evaluation Board User鈥檚 Manual available in PDF for-
mat on the AMD web site.
Figure 29. X32 Oscillator Circuit
96
脡lan鈩C300 Microcontroller Data Sheet
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