em78p156n ELAN Microelectronics Corp, em78p156n Datasheet - Page 28

• Manufacturer Part Number: em78p156n
• Description: 8-bit Micro-controller
• Manufacturer: ELAN Microelectronics Corp

For some applications that do not need a very precise timing calculation, the RC oscillator (Fig. 15) 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 above reasons, 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.
This specification is subject to change without prior notice.
3. External RC Oscillator Mode
<Note> 1. The value of capacitors (C1, C2) is for reference.
Ceramic Resonators
Crystal Oscillator
Oscillator Type
Table 10 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator
Frequency Mode
HXT
HXT
LXT
28
32.768kHz
Frequency
2.0 MHz
4.0 MHz
455 kHz
100KHz
200KHz
455KHz
1.0MHz
2.0MHz
4.0MHz
100~150
C1(pF)
20~40
10~30
20~40
15~30
25
25
25
15
15
EM78P156N
OTP ROM
07.29.2004 (V1.2)
100~150
20~150
C2(pF)
20~40
10~30
15~30
15
25
25
15
15