EM78P156 Ver la hoja de datos (PDF) - Elan Microelectronics Corp
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EM78P156 Datasheet PDF : 28 Pages
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EM78P156E
Table 8 Capacitor Selection Guide for Crystal Oscillator or Ceramic Resonators
Oscillator Type
Frequency Mode
Frequency
C1 (pF)
Ceramic Resonator
Crystal Oscillator
HXT
LXT
HXT
455 KHz
1.00 MHz
2.0 MHz
4.0 MHz
32.768 KHz
100 KHz
200 KHz
455 KHz
1.0 MHz
2.0 MHz
4.0 MHz
100~150
40~80
20~40
10~30
25
25
25
20~40
15~30
15
15
C2 (pF)
100~150
40~80
20~40
10~30
15
25
25
20~150
15~30
15
15
3. ERC Oscillator Mode
For some applications whose timing need not be calculated precisely, the RC oscillator (Fig.13) offers a lot of cost
savings. Nevertheless, it should be aware that the frequency of the RC oscillator is the function of the supply voltage,
the values of the resistor (Rext), the capacitor (Cext), and even the operation temperature.
Moreover to this, the frequency also changes slightly from one chip to another due to the process variation.
In order to maintain a stable system frequency, the values of the Cext should not be less than 20pF as well as the
value of Rext should not be greater than 1M ohm. If they can not be kept in this range, the frequency is affected easily
by noise, humidity and leakage.
The smaller Rext the RC oscillator has, the faster frequency it gets. On the contrary, for very low Rext values,
for instance, 1KΩ, the oscillator becomes unstable because the NMOS can not discharge the current of the
capacitance correctly.
On a basis of 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 and the ways of PCB layout will effect the system frequency.
VCC
Rext
OSCI
EM78P156E
Cext
Fig. 13 Circuit for External RC Oscillator Mode
* This specification is subject to be changed without notice. 8.11.1999
B3-19
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