B3-19

EM78P156E

Table 8 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonators

Oscillator Type

Frequency Mode

Frequency

C1 (pF)

C2 (pF)

455 KHz

100~150

100~150

1.00 MHz

40~80

40~80

Ceramic Resonator

HXT

2.0 MHz

20~40

20~40

4.0 MHz

10~30

10~30

32.768 KHz

25

15

LXT

100 KHz

25

25

200 KHz

25

25

Crystal Oscillator

455 KHz

20~40

20~150

HXT

1.0 MHz

15~30

15~30

2.0 MHz

15

15

4.0 MHz

15

15

3.

ERC Oscillator Mode

For some applications whose timing need not be calculated precisely, the RC oscillator (Fig.13) offers a lot of cost

savings. Nevertheless, it should be aware that the frequency of the RC oscillator is the function of the supply voltage,

the values of the resistor (Rext), the capacitor (Cext), and even the operation temperature.

Moreover to this, the frequency also changes slightly from one chip to another due to the process variation.

In order to maintain a stable system frequency, the values of the Cext should not be less than 20pF as well as the

value of Rext should not be greater than 1M ohm. If they can not be kept in this range, the frequency is affected easily

by noise, humidity and leakage.

The smaller Rext the RC oscillator has, the faster frequency it gets. On the contrary, for very low Rext values,

for instance, 1K

Ω, the oscillator becomes unstable because the NMOS can not discharge the current of the

capacitance correctly.

On a basis of 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 and the ways of PCB layout will effect the system frequency.

Rext

Cext

OSCI

EM78P156E

Fig. 13 Circuit for External RC Oscillator Mode