Table 9 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator
3. External RC Oscillator Mode
For some applications that do not need a very precise timing calculation, the RC oscillator (Fig. 15)
offers a lot of cost savings. Nevertheless, it should be noted that the frequency of the RC oscillator
is influenced by the supply voltage, the values of the resistor (Rext), the capacitor (Cext), and even
by the operation temperature. Moreover, the frequency also changes slightly from one chip to
another due to the manufacturing process variation.
In order to maintain a stable system frequency, the values of the Cext should not be less than 20pF,
and that the value of Rext should not be greater than 1 M ohm. If they cannot be kept in this range,
the frequency is easily affected by noise, humidity, and leakage.
The smaller the Rext in the RC oscillator, the faster its frequency will be. On the contrary, for very
low Rext values, for instance, 1 K
Ω, the oscillator becomes unstable because the NMOS cannot
discharge the current of the capacitance correctly.
Based on the above reasons, it must be kept in mind that all of the supply voltage, the operation
temperature, the components of the RC oscillator, the package types, the way the PCB is layout,
will affect the system frequency.
This specification is subject to change without prior notice.