Specifications
Élan™SC310 Microcontroller Data Sheet 85
PRELIMINARY
frequency. It is therefore required that the load capaci-
tance in the oscillator circuit is duplicated as closely as
possible to the manufacturer’s load capacitance speci-
fication.
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
– (C
O
+
C
STRAY
), where C
L
is the crystal manufacturer’s load ca-
pacitance specification.
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 43.
The series combination of C
D
and C
G
=
C(
D
C
G
×)
C
D
C
G
)+(
---------------------------
Table 43. Recommended Oscillator Component
Value Limits
Minimum Maximum
R
F
14 MΩ 18 MΩ
R
B
0 Ω 10 kΩ
C
D
10 pF 30 pF
C
G
10 pF 30 pF
ÉlanSC310 Microcontroller
X32IN (201) X32OUT (202)
R
F
R
B
L
1
C
1
R
1
C
O
C
D
C
G
A
B
XTAL
C
STRAY
AB
Notes:
For board layout suggestions, refer to the
ÉlanSC310 Microcontroller Evaluation Board User’s Manual
available in PDF format
on the AMD web site.
Figure 29. X32 Oscillator Circuit