ELAN Home Systems EM78P459AK, EM78P459AM, EM78P458AM, EM78P458AP manual External RC Oscillator Mode

EM78P458/459

OTP ROM

3. External RC Oscillator Mode

For some applications that do not require precise timing calculation, the RC oscillator (Fig. 22) could 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 KΩ, 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

EM78P458

EM78P459

Vcc

Rext

Cext

		Fig. 22	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
	20 pF	5.1k	2.63MHz		1.92 MHz
		10k	1.30 MHz		1.22 MHz
		100k	150 KHz		153 KHz
This specification is subject to change without prior notice.				48	07.01.2003 (V1.3)