External Rc Oscillator Mode - Elan EM78P458 User Manual

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.
Table 14 RC Oscillator Frequencies
Cext
20 pF
This specification is subject to change without prior notice.
OSCI
EM78P458
EM78P459
Fig. 22 Circuit for External RC Oscillator Mode
Rext Average Fosc 5V,25°C
3.3k 3.57 MHz
5.1k 2.63MHz
10k 1.30 MHz
100k 150 KHz
Vcc
Rext
Cext
Average Fosc 3V,25°C
48
EM78P458/459
OTP ROM
2.94 MHz
1.92 MHz
1.22 MHz
153 KHz
07.01.2003 (V1.3)