EM78P156N

OTP ROM

Table 10 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator

Oscillator Type

Frequency Mode

Frequency

C1(pF)

C2(pF)

 

 

 

 

 

Ceramic Resonators

HXT

455 kHz

100~150

100~150

2.0 MHz

20~40

20~40

 

 

4.0 MHz

10~30

10~30

 

LXT

32.768kHz

25

15

 

100KHz

25

25

Crystal Oscillator

 

200KHz

25

25

 

455KHz

20~40

20~150

 

HXT

1.0MHz

15~30

15~30

 

2.0MHz

15

15

 

 

 

 

4.0MHz

15

15

<Note> 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 KΩ, 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)