EM78P458AP datasheet(47/76 Pages) EMC

47 / 76 page

EM78P458/459

OTP ROM

offer users with an effective 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 1M ohm. If they cannot be kept in this range, the

frequency can be affected easily 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 above reasons, it must be kept in mind that all supply voltage, the operation

temperature, the components of the RC oscillator, the package types, and the way the PCB is layout,

have certain effect on the system frequency.

OSCI

Vcc

Rext

Cext

EM78P458

EM78P459

Fig. 20 Circuit for External RC Oscillator Mode

Table 14 RC Oscillator Frequencies

Cext

Rext

Average Fosc 5V,25

°C

Average Fosc 3V,25

°C

3.3k

3.57 MHz

2.94 MHz

5.1k

2.63MHz

1.92 MHz

10k

1.30 MHz

1.22 MHz

20 pF

100k

150 KHz

153 KHz

3.3k

1.43 MHz

1.35 MHz

5.1k

980 KHz

877 KHz

10k

520 KHz

465 KHz

100 pF

100k

57 KHz

54 KHz

This specification is subject to change without prior notice.

06.25.2004 (V1.4)

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