EM78P257AM datasheet(48/91 Pages) EMC

48 / 91 page

EM78P257

OTP ROM

This specification is subject to change without prior notice.

07.27.2004 (V1.4)

Table 17 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonators

Oscillator Type

Frequency Mode

Frequency

C1(pF)

C2(pF)

455 kHz

100~150

2.0 MHz

20~40

Ceramic Resonators

HXT

4.0 MHz

10~30

32.768kHz

100KHz

LXT

200KHz

455KHz

20~40

20~150

1.0MHz

15~30

2.0MHz

Crystal Oscillator

HXT

4.0MHz

3. External RC Oscillator Mode

For some applications that do not need to have its timing to be calculated precisely, the RC oscillator (IV.12.3-1)

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 t he 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 reasons above, 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

OSCI

EM78P257A/B

Vcc

Rext

Cext

Fig. 20 Circuit for External RC Oscillator Mode

EM78P257AM Datasheet(PDF) 48 Page - ELAN Microelectronics Corp

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