PIC16F5X
FIGURE 4-4:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
(USING XT, HS OR LP
OSCILLATOR MODE)
4.3
External Crystal Oscillator Circuit
Either a pre-packaged oscillator or a simple oscillator
circuit with TTL gates can be used as an external
crystal oscillator circuit. Pre-packaged oscillators
provide a wide operating range and better stability. A
well designed crystal oscillator will provide good perfor-
mance with TTL gates. Two types of crystal oscillator
circuits can be used: one with parallel resonance or one
with series resonance.
To Other
Devices
K
K
330
330
PIC16F5X
74AS04
74AS04
74AS04
CLKIN
0.1 μF
Figure 4-3 shows an implementation example of a
parallel resonant oscillator circuit. The circuit is
designed to use the fundamental frequency of the
crystal. The 74AS04 inverter performs the 180° phase
shift that a parallel oscillator requires. The 4.7 kΩ
resistor provides the negative feedback for stability.
The 10 kΩ potentiometers bias the 74AS04 in the
linear region. This circuit could be used for external
oscillator designs.
Open
OSC2
XTAL
4.4
RC Oscillator
For applications where precise timing is not a require-
ment, the RC oscillator option is available. The
operation and functionality of the RC oscillator is
dependent upon a number of variables. The RC
oscillator frequency is a function of:
FIGURE 4-3:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
(USING XT, HS OR LP
OSCILLATOR MODE)
• Supply voltage
• Resistor (REXT) and capacitor (CEXT) values
• Operating temperature.
+5V
The oscillator frequency will vary from unit to unit due
to normal process parameter variation. The difference
in lead frame capacitance between package types will
also affect the oscillation frequency, especially for low
CEXT values. The user also needs to account for the
tolerance of the external R and C components.
Figure 4-5 shows how the R/C combination is
connected.
To Other
Devices
10k
74AS04
PIC16F5X
4.7k
CLKIN
74AS04
Open
OSC2
10k
XTAL
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.
10k
20 pF
20 pF
FIGURE 4-5:
RC OSCILLATOR MODE
VDD
Figure 4-4 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverters perform a 360°
phase shift in a series resonant oscillator circuit. The
330 kΩ resistors provide the negative feedback to bias
the inverters in their linear region.
REXT
Internal
clock
OSC1
N
CEXT
VSS
PIC16F5X
OSC2/CLKOUT
FOSC/4
DS41213D-page 22
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
