PIC16C55X(A)
7.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
7.2.4
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) and 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 will also affect the oscillation frequency,
especially for low Cext values. The user also needs to
take into account variation due to tolerance of external
R and C components used. Figure 7-6 shows how the
R/C combination is connected to the PIC16C55X. For
Rext values below 2.2 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 to keep Rext between 3 kΩ and 100 kΩ.
Either a pre-packaged oscillator can be used or a sim-
ple 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 can be
used; one with series resonance, or one with parallel
resonance.
Figure 7-4 shows implementation 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 could be used for external oscillator designs.
FIGURE 7-4: EXTERNAL PARALLEL
RESONANT CRYSTAL
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 no or
small external capacitance, the oscillation frequency
can vary dramatically due to changes in external
capacitances, such as PCB trace capacitance or
package lead frame capacitance.
OSCILLATOR CIRCUIT
+5V
To other
Devices
PIC16C55X(A)
10k
74AS04
4.7k
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 (Figure 3-2 for
waveform).
CLKIN
74AS04
10k
XTAL
FIGURE 7-6: RC OSCILLATOR MODE
10k
VDD
PIC16C55X(A)
Rext
20 pF
20 pF
OSC1
Figure 7-5 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a 180°
phase shift in a series resonant oscillator circuit. The
330 Ω resistors provide the negative feedback to bias
the inverters in their linear region.
Internal Clock
Cext
VDD
OSC2/CLKOUT
Fosc/4
FIGURE 7-5: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
To other
Devices
330 Ω
330 Ω
PIC16C55X(A)
74AS04
74AS04
74AS04
CLKIN
0.1 µF
XTAL
DS40143B-page 38
Preliminary
1997 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
