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| 型號(hào)： | 16F84A |
| 廠(chǎng)商： | Microchip Technology Inc. |
| 英文描述： | CAT 5E CROSSOVER PATCH CORD CABLE BLUE 20 FT |
| 中文描述： | 18引腳閃存/ EEPROM的8位微控制器 |
| 文件頁(yè)數(shù)： | 40/124頁(yè) |
| 文件大?。?/td> | 1322K |
| 代理商： | 16F84A |
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PIC16F8X
DS30430C-page 40
1998 Microchip Technology Inc.
8.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built.
Prepackaged oscillators provide a wide operating
range and better stability. A well-designed crystal
oscillator will provide good performance with TTL
gates. Two types of crystal oscillator circuits are
available; one with series resonance, and one with
parallel resonance.
Figure 8-5 shows a parallel resonant oscillator circuit.
The circuit is designed to use the fundamental
frequency of the crystal. The 74AS04 inverter performs
the 180-degree phase shift that a parallel oscillator
requires. The 4.7 k
resistor provides negative
feedback for stability. The 10 k
potentiometer biases
the 74AS04 in the linear region. This could be used for
external oscillator designs.
FIGURE 8-5:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 8-6 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a
180-degree phase shift. The 330 k
resistors provide
the negative feedback to bias the inverters in their
linear region.
FIGURE 8-6:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
8.2.4
RC OSCILLATOR
For timing insensitive applications the RC device option
offers additional cost savings. The RC oscillator
frequency is a function of the supply voltage, the
resistor (Rext) values, capacitor (Cext) values, and the
operating temperature. In addition to this, the oscillator
frequency will vary from unit to unit due to normal
process
parameter
variation.
difference in lead frame capacitance between package
types also affects the oscillation frequency, especially
for low Cext values. The user needs to take into
account variation due to tolerance of the external
R and C components. Figure 8-7 shows how an R/C
combination is connected to the PIC16F8X. For Rext
values below 4 k
, the oscillator operation may
become unstable, or stop completely. For very high
Rext values (e.g., 1 M
), the oscillator becomes
sensitive to noise, humidity and leakage. Thus, we
recommend keeping Rext between 5 k
and 100 k
.
Although the oscillator will operate with no external
capacitor (Cext = 0 pF), we recommend using values
above 20 pF for noise and stability reasons. With little
or no external capacitance, the oscillation frequency
can vary dramatically due to changes in external
capacitances, such as PCB trace capacitance or
package lead frame capacitance.
See the electrical specification section for RC
frequency variation from part to part due to normal
process variation. The variation is larger for larger R
(since leakage current variation will affect RC
frequency more for large R) and for smaller C (since
variation of input capacitance has a greater affect on
RC frequency).
See the electrical specification section for variation of
oscillator frequency due to V
DD
for given Rext/Cext
values as well as frequency variation due to
operating temperature.
The oscillator frequency, divided by 4, is available on
the OSC2/CLKOUT pin, and can be used for test
purposes or to synchronize other logic (see Figure 3-2
for waveform).
Furthermore,
the
FIGURE 8-7:
V
DD
RC OSCILLATOR MODE
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC16FXX
CLKIN
To Other
330 k
74AS04
74AS04
PIC16FXX
CLKIN
To Other
XTAL
330 k
74AS04
0.1
μ
F
Note:
When the device oscillator is in RC mode,
do not drive the OSC1 pin with an external
clock or you may damage the device.
OSC2/CLKOUT
Cext
Rext
PIC16FXX
OSC1
Fosc/4
Internal
clock
V
SS
Recommended values:
5 k
≤
Rext
≤
100 k
Cext > 20pF
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