© 1999 Scenix Semiconductor, Inc. All rights reserved.
- 21 -
www.scenix.com
SX18AC / SX20AC / SX28AC
Note: Resistor Rf is not required for the Version 2.1 device
(both crystal and resonator circuits.)
9.2 External RC Mode
The external RC oscillator mode provides a cost-effective
approach for applications that do not require a precise
operating frequency. In this mode, the RC oscillator fre-
quency is a function of the supply voltage, the resistor (R)
and capacitor (C) values, and the operating temperature.
In addition, the oscillator frequency will vary from unit to
unit due to normal manufacturing process variations. Fur-
thermore, the difference in lead frame capacitance
between package types also affects the oscillation fre-
quency, especially for low C values. The external R and
C component tolerances contribute to oscillator fre-
quency variation as well.
Figure 9-3 shows the external RC connection diagram.
The recommended R value is from 3kW to 100kW. For R
values below 2.2kW, the oscillator may become unstable,
or may stop completely. For very high R values (such as
1 MW), the oscillator becomes sensitive to noise, humid-
ity, and leakage.
Although the oscillator will operate with no external
capacitor (C = 0pF), it is recommended that you use val-
ues above 20 pF for noise immunity and stability. With no
or small external capacitance, the oscillation frequency
can vary significantly due to variation in PCB trace or
package lead frame capacitances.
In the external RC mode, the OSC2/CLKOUT pin pro-
vides an output frequency, which the input frequency
divided by four.
9.3 Internal RC Mode
The internal RC mode uses an internal oscillator, so the
device does not need any external components. At 4
MHz, the internal oscillator provides +/–8% accuracy
over the allowed temperature range. The internal clock
frequency can be divided down to provide one of eight
lower-frequency choices by selecting the desired value in
the FUSE Word register. The frequency range is from
31.25 KHz to 4 MHz. The default operating frequency of
the internal RC oscillator may not be 4 MHz. This is due
to the fact that the SX device requires trimmimg to obtain
4 MHz operation. The parts shipped out of the factory are
not trimmed. The device relies on the programming tool
provided by the third party vendors to support trimmimg.
Table 9-2. Component Selection for
Crystal Oscillator
Osc
Type
Resonator
Frequency
C1
(pF)
C2
(pF)
RF
XT
4
20
47
1M
HS
8
20
47
1M
HS
12
20
47
1M
HS
16
15
30
1M
HS
20
15
30
1M
HS
25
5
20
10k
HS
30
5
20
4.7k
HS
36
5
15
3.3k
HS
40
5
15
3.3k
HS
50 (Fundemental)
15
15
220K
Figure 9-3. RC Oscillator Mode
Vdd
R
C
Internal
Circuitry
OSC2
OSC1
N
¸ 4
~
SX Device
