em78p131a ELAN Microelectronics Corp, em78p131a Datasheet - Page 29

• Manufacturer Part Number: em78p131a
• Description: 8-bit Microcontroller With Otp Rom
• Manufacturer: ELAN Microelectronics Corp

Product Specification (V1.2) 09.16.2009
(This specification is subject to change without further notice)
The following table provides the recommended values of C1 and C2. Since each
resonator has its own attribute, 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.
Table 6 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonator
Note: The values of Capacitors C1 and C2 are for reference only
5.7.3 External RC Oscillator Mode
For some applications that do not require a very precise timing calculation, the RC
oscillator (Fig 5-13) offers a cost-effective oscillator configuration. 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 not be
less than 20pF, and the value of Rext should not be greater than 1 MΩ. If they cannot
be kept in this 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, will affect the system frequency.
Ceramic Resonators
Crystal Oscillator
Oscillator Type
Frequency Mode
HXT
HXT
LXT
Frequency
32.768kHz
2.0 MHz
4.0 MHz
1.0 MHz
2.0 MHz
4.0 MHz
455kHz
100kHz
200kHz
455kHz
8-Bit Microcontroller with OTP ROM
100~150
C1 (pF)
20~40
10~30
20~40
15~30
25
25
25
15
15
EM78P131A
100~150
C2 (pF)
20~150
20~40
10~30
15~30
15
25
25
15
15
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