em78p134n ELAN Microelectronics Corp, em78p134n Datasheet - Page 47

• Manufacturer Part Number: em78p134n
• Description: 8-bit Microcontroller With Otp Rom
• Manufacturer: ELAN Microelectronics Corp

Product Specification (V1.6) 06.02.2010
(This specification is subject to change without further notice)
Capacitor selection guide for crystal oscillator or ceramic resonators:
5.10.3 External RC Oscillator Mode
For some applications that do not require
precise timing calculation, the RC
oscillator (Figure 5-19) offers effective
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 manufacturing
process variations.
In order to maintain a stable system frequency, the values of the Cext should be no less
than 20pF, and the value of Rext should not be greater than 1MΩ. If the frequency
cannot be kept within this range, it can be affected easily 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 will become
unstable because the NMOS cannot discharge the capacitance current correctly.
Based on the above reasons, it must be kept in mind that all the supply voltage, the
operation temperature, the components of the RC oscillator, the package type, and the
PCB layout, have certain effects on the system frequency.
Ceramic Resonators
Crystal Oscillator
Oscillator Type
Frequency Mode
HXT
HXT
LXT
Frequency
455kHz
2.0 MHz
4.0 MHz
32.768kHz
100kHz
200kHz
455kHz
1.0 MHz
2.0 MHz
4.0 MHz
Figure 5-19 External RC Oscillator Mode
8-Bit Microcontroller with OTP ROM
OSCI
100~150
20~40
10~30
25
25
25
20~40
15~30
15
15
C1 (pF)
Circuit
EM78P134N
Vcc
100~150
20~40
10~30
15
25
25
20~150
15~30
15
15
C2 (pF)
Rext
Cext
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