EM78156E
8-Bit Microcontroller with MASK ROM
In the most applications, Pin OSCI and Pin OSCO can be connected with a crystal or
ceramic resonator to generate oscillation. Fig. 12 depicts such circuit. The same thing
applies whether it is in the HXT mode or in the LXT mode. Table 10 provides the
recommended values of C1 and C2. Since each resonator has its own attribute, user
should refer to its specification for appropriate values of C1 and C2. RS, a serial
resistor, may be necessary for AT strip cut crystal or low frequency mode.
C1
OSCI
EM78156E
XTAL
OSCO
C2
RS
Fig. 12 Circuit for Crystal/Resonator
Table 10 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator
Oscillator Type
Frequency Mode Frequency
C1(pF)
100~150
20~40
10~30
25
C2(pF)
100~150
20~40
10~30
15
455 kHz
Ceramic Resonators
HXT
LXT
2.0 MHz
4.0 MHz
32.768kHz
100KHz
200KHz
455KHz
1.0MHz
2.0MHz
4.0MHz
25
25
25
25
Crystal Oscillator
20~40
15~30
15
20~150
15~30
15
HXT
15
15
4.7.3 External RC Oscillator Mode
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.
Product Specification (V1.3) 07.29.2004
• 23
(This specification is subject to change without further notice)
ELAN [ ELAN MICROELECTRONICS CORP ]
