45111 Datasheet, PDF (178/184 Pages) List of Unclassifed Manufacturers – 14-DAY MONEY BACK GUARANTEE

15 Appendix E: SX Data Sheet
Figure 39 - SX with External System Clock
Clock signal
from external
system
OSC1
SX
Not Connected OSC2
15.6.1.2 External RC Mode
For timing insensitive applications, the RC device option offers additional cost savings. The RC
oscillator frequency is a function of the supply voltage, the resistor (Rext) and capacitor (Cext) values,
and the operating temperature. In addition to this, the oscillator frequency will vary from unit to unit
due to normal process variation. Furthermore, the difference in lead frame capacitance between
package types will also affect the oscillation frequency, especially for low Cext values. Variation due to
tolerance of external R and C components must also be considered.
Figure 40 - External RC Mode
Vdd
Rext
OSC1
Cext
SX
OSC2
Figure 40 - External RC Mode shows the RC connections to the SX. For Rext values below 2.2kΩ, the
oscillator operation may become unstable, or stop completely. For very high Rext values (e.g. 1 MΩ) the
oscillator becomes sensitive to noise, humidity and leakage. The recommended Rext value is 3kΩ to
100kΩ.
Although the oscillator will operate with no external capacitor (Cext = 0 pF), using values above 20 pF is
recommended for noise and stability reasons. With little external capacitance, the oscillation frequency
can vary dramatically due to changes in external capacitance, such as PCB trace capacitance or package
lead frame capacitance.
Page 178 • SX-Key/Blitz Development System Manual 2.0 • Parallax, Inc.