em78f644nso28s ELAN Microelectronics Corp, em78f644nso28s Datasheet - Page 162

• Manufacturer Part Number: em78f644nso28s
• Description: Flash Series 8-bit Microcontroller
• Manufacturer: ELAN Microelectronics Corp

EM78F648/644/642/641/548/544/542/541N
8-Bit Microcontroller
152 •
6.15.3 External RC Oscillator Mode
For some applications that do not require a very precise timing calculation, the use of
RC oscillator (see figure below) offers a lot of cost savings. Nevertheless, it should
be noted that the frequency of the RC oscillator is influenced by such factors as
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 variations.
In order to maintain a stable system frequency, the values of the Cext should be more
than 20pF, and that the value of Rext should be less than 1 MΩ. If the values cannot
be kept within the prescribed range, the frequency can be easily affected by noise,
humidity, and leakage.
The smaller the Rext in the RC oscillator is, 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 of the supply voltage, the
operation temperature, the components of the RC oscillator, the package types, the
way the PCB is layout, could affect the system frequency.
Ceramic Resonators
Crystal Oscillator
Oscillator Type
Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator:
Figure 6-28 Circuit for External RC Oscillator Mode
Frequency
Mode
HXT
HXT
LXT
OSCI
(This specification is subject to change without further notice)
Frequency
455 kHz
2.0 MHz
4.0 MHz
32.768kHz
100KHz
200KHz
455KHz
1.0MHz
2.0MHz
4.0MHz
Product Specification (V1.0) 05.05.2010
Vcc
Rext
Cext
100~150
20~40
10~30
25
25
25
20~40
15~30
15
15
C1(pF)
100~150
20~40
10~30
15
25
25
20~150
15~30
15
15
C2(pF)