PIC16CR84T-10I/SO View Datasheet(PDF) - Microchip Technology
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PIC16CR84T-10I/SO Datasheet PDF : 124 Pages
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PIC16F8X
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 kΩ resistor provides negative
feedback for stability. The 10 kΩ 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
+5V
10k
4.7k
74AS04
To Other
Devices
74AS04
PIC16FXX
CLKIN
10k
XTAL
10k
20 pF 20 pF
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 kΩ resistors provide
the negative feedback to bias the inverters in their
linear region.
FIGURE 8-6:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
330 kΩ
74AS04
330 kΩ
74AS04
0.1 µF
XTAL
To Other
Devices
PIC16FXX
74AS04
CLKIN
DS30430C-page 40
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. Furthermore, the
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 kΩ, the oscillator operation may
become unstable, or stop completely. For very high
Rext values (e.g., 1 MΩ), the oscillator becomes
sensitive to noise, humidity and leakage. Thus, we
recommend keeping Rext between 5 kΩ and 100 kΩ.
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 VDD 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).
FIGURE 8-7: RC OSCILLATOR MODE
VDD
Rext
OSC1
Internal
clock
Cext
PIC16FXX
VSS
OSC2/CLKOUT
Fosc/4
Recommended values: 5 kΩ ≤ Rext ≤ 100 kΩ
Cext > 20pF
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.
© 1998 Microchip Technology Inc.
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