EM78P451SWM datasheet(44/54 Pages) EMC

44 / 54 page

EM78P451S

8-Bit Microcontroller with OTP ROM

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Product Specification (V1.0) 06.01.2004

(This specification is subject to change without further notice)

4.9.3 RC Oscillator Mode

For some applications that do not need a very precise timing calculation, the RC

oscillator (Fig 22 & Fig 23) 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 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.

OS C I

E M 78 P 451 S

Vc c

R ext

Ce x t

Fig. 20 Circuit for External RC Oscillator Mode

OS C I

E M 78 P 451 S

Vc c

R ext

Fig. 21 Circuit for External R, Internal C Oscillator Mode

EM78P451SWM Datasheet(PDF) 44 Page - ELAN Microelectronics Corp