ELANsat Tech OTP ROM EM78P156N - Manual (Page 28)

EM78P156N
OTP ROM
Table 10 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator
Oscillator Type Frequency Mode Frequency C1(pF) C2(pF)
455 kHz 100~150 100~150
2.0 MHz 20~40 20~40 Ceramic Resonators HXT
4.0 MHz 10~30 10~30
32.768kHz 25 15
100KHz 25 25 LXT
200KHz 25 25
455KHz 20~40 20~150
1.0MHz 15~30 15~30
2.0MHz 15
15
Crystal Oscillator
HXT
4.0MHz 15 15
1. The value of capacitors (C1, C2) is for reference.
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. 28 07.29.2004 (V1.2)
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