EM78P152S datasheet(28/52 Pages) ETC1

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EM78P152/3S

8-Bit Microcontroller with OTP ROM

Table 5-6 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator

Oscillator Type

Frequency Mode

Frequency

C1 (pF)

C2 (pF)

455kHz

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

1.0 MHz

15~30

2.0 MHz

Crystal Oscillator

HXT

4.0 MHz

Note:

The values of capacitors C1 and C2 are for reference only

5.7.3 External RC Oscillator Mode

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

oscillator (Figure 5-13) 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 variations.

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 can be easily affected by noise, humidity,

and leakage.

The smaller the Rext in the RC oscillator is, 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 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.

OSCI

EM 78P153S

Vcc

Rext

Cext

Figure 5-13 External RC Oscillator Mode Circuit

•

Product Specification (V1.8) 09.08.2009

(This specification is subject to change without further notice)

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