EM78P257
OTP ROM
Table 17 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonators
Oscillator Type
Frequency Mode
Frequency
C1(pF)
C2(pF)
455 kHz
2.0 MHz
4.0 MHz
32.768kHz
100KHz
200KHz
455KHz
1.0MHz
2.0MHz
4.0MHz
100~150
20~40
10~30
25
25
25
20~40
15~30
15
100~150
20~40
10~30
15
25
25
20~150
15~30
15
Ceramic Resonators
HXT
LXT
HXT
Crystal Oscillator
15
15
3. External RC Oscillator Mode
For some applications that do not need to have its timing to be calculated precisely, the RC oscillator (IV.12.3-1)
offers a lot of 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 1 M ohm. 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 KW, the oscillator becomes unstable because the NMOS cannot discharge the current of the
capacitance correctly.
Based on the reasons above, 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
Vcc
Rext
OSCI
Cext
EM78P257A/B
Fig. 20 Circuit for External RC Oscillator Mode
This specification is subject to change without prior notice.
48
07.27.2004 (V1.4)
ELAN [ ELAN MICROELECTRONICS CORP ]
