em78p257 ELAN Microelectronics Corp, em78p257 Datasheet - Page 48

• Manufacturer Part Number: em78p257
• Description: 8-bit Microcontroller
• Manufacturer: ELAN Microelectronics Corp

This specification is subject to change without prior notice.
3. External RC Oscillator Mode
Table 17 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonators
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 K , 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
Ceramic Resonators
Crystal Oscillator
Oscillator Type
Fig. 20 Circuit for External RC Oscillator Mode
Frequency Mode
EM78P257A/B
HXT
HXT
LXT
OSCI
48
Frequency
32.768kHz
2.0 MHz
4.0 MHz
455 kHz
100KHz
200KHz
455KHz
1.0MHz
2.0MHz
4.0MHz
Cext
Vcc
Rext
100~150
C1(pF)
20~40
10~30
20~40
15~30
25
25
25
15
15
EM78P257
07.27.2004 (V1.4)
OTP ROM
100~150
20~150
C2(pF)
20~40
10~30
15~30
15
25
25
15
15