Справочник Пользователя для IBM EM78P447N

 (V1.1) 03.30.2005

 

 

 

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Table 12 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator  

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 

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.