EM78F651NAMSSLASHNAMJ datasheet(41/58 Pages) EMC

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EM78F651N

8-Bit Microcontroller

Product Specification (V1.1) 10.20.2006

• 37

(This specification is subject to change without further notice)

OSCI

EM78F651N

7404

4.7K

10K

XTAL

7404

Vdd

10K

Fig. 6-13 Circuit for Crystal/Resonator-Parallel Mode

6.9.3 External RC Oscillator Mode

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

oscillator (Fig 6-14 ) offers a cost-effective oscillator configuration.. 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 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

Ω. 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

since the NMOS cannot discharge correctly the current of the capacitance.

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, and

the PCB layout, will affect the system frequency.

OSCI

EM78F651N

Vcc

Rext

Cext

Fig. 6-14 Circuit for External RC Oscillator Mode

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